BidVertiser

Sunday, December 9, 2007

CataPault

CATAPULT

For the handheld Y-shaped weapon, see slingshot.

A catapult is any siege engine which uses an arm to hurl a projectile a great distance, though the term is generally understood to mean medieval siege weapons. The name is derived from the Greek κατά (against) and βαλλεν (to hurl (a missile)). (An alternate derivation is from the Greek "katapeltes" meaning "shield piercer," kata (pierce) and pelta (small shield)). Originally, "catapult" referred to a dart-thrower, while "ballista" referred to a stone-thrower, but the two terms swapped meaning sometime in the fourth century AD.

Catapults were usually assembled at the site of a siege. An army would carry a few necessary pieces with them because wood was easily available. Although usually incorrectly depicted with a spoon on the end of the arm (as in the picture to the right) catapults were most often equipped with a sling to hold the projectile.

Types

Catapults may be classified according to the physical concept used to store energy when winched and release the energy when fired required to propel the projectile

Catapult

The earliest documented occurrence of catapults in China was the levered catapult and an eight foot high siege crossbow from the Mozi (Mo Jing), a Mohist text written at about the 4th - 3rd century B.C by followers of Mozi who founded the Mohism school of thought during the late Spring and Autumn Period which is about 6th century BC and 5th century BC and the early Warring States period. Much of what we now know of the siege technology of the time came to us from Books 14 and 15 (Chapters 52 to 71) on Siege Warfare from the Mo Jing. Recorded and preserved on bamboo strips, much of the text is now unfortunately extremely corrupted. However, despite the heavy fragmentation, Mohist diligence and attention to details which set Mo Jing apart from other works, ensured that highly descriptive details of the workings of mechanical devices like Cloud Ladders, Rotating Arcuballistas and Levered Catapults, records of siege techniques and usage of siege weaponry can still be found.[1]

The first European catapult distinct from hand-held launchers (bows, javelins, slings, etc.) was the Greek Gastraphetes, a crossbow so large it was braced against the abdomen rather than being held in the hand, hence the nickname belly-bow. The next step from this was a larger form a crossbow mounted on a stand, including early versions of the oxybeles (Greek for bolt shooter) and the ballista (the Roman version of the oxybeles). The arbalette à tour was a medieval version of the stand-mounted crossbow. These catapults are tensional, in that the energy is stored as tension and compression of the bow. Although similar to a crossbow, a sling on the end of the rope meant these weapons could be used for firing all sorts of projectiles, from rocks to pots of Greek fire.

Subsequently, torsional catapults were developed; those with two torsion powered arms, the later versions of the ballista and oxybeles, and those with one torsion powered arm, the onager, known in medieval times as the mangonel. The bottom end of the throwing arm of the onager and the inner ends of both ballista arms are inserted into rope or fibers that are twisted, providing a torsional store of energy. Torsional ballistas were operationally equivalent to their tensional cousins, except the torsional energy store gave greater power. Onagers have an arm with a bucket, cup, or most often a sling to hold the projectile at one end.

Finally, the last type of catapult is a trebuchet, which used gravity or traction rather than tension or torsion to propel the throwing arm. A falling counterweight, or the effort of the one or more operators, pull down the bottom end of the arm and the projectile is thrown from a sling attached to a rope hanging from the top end of the arm, essentially like a sling attached to a giant see-saw. The counterweight is much heavier than the projectile. More modern trebuchets often replace the counterweight with industrial springs to create tension. (Video).

History

French troops using a catapult to throw hand grenades during World War I.

Improvised catapult made out of leaf spring during the Warsaw Uprising for launching of Molotov cocktails.

In Europe, the first catapults appeared in Greek times around 400 BC-300 BC [citations needed]. According to Greek engineer and inventor Hero of Alexandria, the first types derived from by the earlier gastraphetes ("Belly-bow"), consisting in composite bow mounted transversely on a stock, much like the crossbow. A larger version of this was called an oxybeles and is the precurser to the ballista. Biton attributes the creation of the first crewed catapult to one Zopyrus from Taranto, in southern Italy.

Early adopters of the catapult design were Dionysius of Syracuse (who called it katapeltikon) and Onomarchus of Phocis. Katapaltai are mentioned in the Siegecraft (Poliorkētika) treatise of Aeneas Tacticus, from around 350 BC. It is probable that standard torsion-powered catapults entered in common use in Greek world and Macedon only around 330 BC. Alexander the Great introduced the idea of using them to provide cover on the battlefield in addition to using them during sieges. Projectiles included both arrows and (later) stones.

Romans started to use catapults probably as arms for their wars against Syracuse, Macedon, Sparta and Aetolia (3rd-2nd century BC). Standard use of artillery (ballista and onager) is attested only from the time of Julius Caesar, however.

In the Medieval times, when the trebuchet was introduced a relatively short time before the advent of gunpowder, the catapult became basically obsolete. Cannons soon replaced catapults as the standard siege weapon in Europe in the 14th century.

During medieval times, catapults and related siege machines were the first weapons used for biological warfare. The carcasses of diseased animals or even diseased humans, usually those who had perished from the Black Death, were loaded onto the catapult and then thrown over the castle's walls to infect those barricaded inside. There have even been recorded instances of beehives being catapulted over castle walls.

The last large-scale military use of catapults was during the trench warfare of World War I. During the early stages of the war, catapults were used to throw hand grenades across no man's land into enemy trenches.

Until recently, in England, catapults were used by thrill-seekers as human catapults to experience being catapulted through the air. The practice has been discontinued due to fatalities, when the participants failed to land onto the safety net.

Tuesday, October 23, 2007

Habitats and Adaption

Habitat and Adaptation

Common caiman (Caiman crocodilus), also called Narrow-snouted spectacled caiman. French Guiana.
Common caiman (Caiman crocodilus), also called Narrow-snouted spectacled caiman. French Guiana.
© WWF-Canon / Roger LeGuen



Every organism has a unique ecosystem within which it lives. This ecosystem is its natural habitat. This is where the basic needs of the organism to survive are met: food, water, shelter from the weather and place to breed its young. All organisms need to adapt to their habitat to be able to survive.

This means adapting to be able to survive the climatic conditions of the ecosystem, predators, and other species that compete for the same food and space. An adaptation is a modification or change in the organism's body or behaviour that helps it to survive. Explore the links given here to know more about habitats and how different plants and animals.

An animal may adapt to its habitat in different ways. It may be a physical or structural adaptation, just as the limbs of birds have modified into wings or the way the cheetah is shaped for running at a fast speed.

It may be in the way the body works in circulating and respiration, for instance the gills that fish have enable them to breathe in water. Or it may be the way the animal behaves whether it is hunting for food, or running fast to avoid predators or migrating to other places for food or survival. To know more about different types of adaptations visit the link.

An animal's environment consists of many different things. The climate, the kinds of food plants that grow in it, other animals that may be predators or competitors- the animal must learn to adapt to each of these factors in order to survive. With increasing population growth and human activity that disturbs the natural habitat, animals must learn to adapt to these kind of threats as well.

Animals in the wild can only live in places they are adapted to. They must have the right kind of habitat where they can find the food and space they need. Visit the link for a brief overview of how animals adapt to their habitat.

Did you know that animals camouflage themselves so they can adapt to their environment? Adaptation can protect animals from predators or from harsh weather. Many birds can hide in the tall grass and weeds and insects can change their colour to blend into the surroundings. This makes it difficult for predators to seek them out for food.

Some animals, like the apple snail, can survive in different ecosystems- from swamps, ditches and ponds to lakes and rivers. It has a lung/gills combination that reflects its adaptation to habitats with oxygen poor water. This is often the case in swamps and shallow waters. To know more about how the apple snail can survive in different habitats visit the link.

In the harsh cold climate of Alaska, the animals have learnt to adapt to the weather by storing food in their body and protecting themselves from the cold with thick furs. Human inhabitants in Alaska have also learnt to cope with the environment by building shelters that insulate and hold the heat, and yet do not allow the structure to melt.

Plants Habitates

TQmtn.jpg (364194 bytes)


Where the Flowers Grow

Alaska is the largest of the 50 states. Elevations range from sea level to the top of Mt. McKinley, the highest peak in North America. Because of Alaska's size and varied elevations, growing conditions are very different. The amount of snow, how severe the winter is, and when the snow melts, determines the blooming time of the plants in that particular area. If you take a plant out of its' environment it may not grow. Climate has a lot to do with where a plant grows.

There are three main habitats that plants grow in, although, there is a space of transition in each one where habitat features overlap. For the sake of simplicity, Alpine is defined as above treeline and often having scree slopes, Sub-Alpine, below Alpine with some trees and many low shrubs, and lowlands, which is tree line and below with many varieties of plants and tall trees.

Plants in Alpine are slow growing and low to the ground. They are slow growing because to get to a source of water, they must grow their taproot first. It sometimes takes five years for a flower to grow to the size of a half dollar. By this time the taproot is about 10 to 12 inches long. Flowers in this area grow close to rocks on scree slopes. The rocks protect them from high winds, and are a source of

. In a picture, I saw one flower even turned toward a rock, maybe because it confused it for the sun. You are likely to see the Chocolate Lily, Triangular Leafed Fleabane, Nootka Lupine, Wooly Lousewort, and Mountain Forget-Me-Nots in this habitat.

Shrubs, herbs and small plants grow in Sub-Alpine habitats. You will find Salmonberry, Devil's Club, Wild Currents, Tundra Rose, Pasque Flower, and Prickly Rose here. In areas where the shrubs are thick, the taller ones will block out the much needed sun. In these places you will find plants that grow before the leaves appear on the trees or can grow in the shade. In the woodsy areas, fire is the biggest danger to plants. It takes many years for plants to grow back after a fire.

Below Alpine and Sub-Alpine are the Lowlands. There are thick forests in this region so fire is still a big danger to these plants. Along the coast the plants are hugely affected by the saltwater. Most coastal wetlands have silt, sand, and in restricted areas, gravel. These along with the tide, prevent plants from becoming well rooted. Lowlands also consist of woodlands, meadows, swamps, marshes, and shallow open water. In these areas you will find Pond Lillies, Bluebells of Scotland, Wild Iris and Fireweed to name a few.

regions map 1.jpg (30232 bytes)

The Plant Life Cycle

The Life Cyclegrowing2.gif (614043 bytes)

When a seed falls to the ground it starts what is often referred to as The Life Cycle. Once in the ground the seed needs water and warmth to start the next step in growth called germination. Germination is when the seed swells taking in water and nutrients and starts to grow. After the germination process has started the seed forms a root that will search for food and water in the soil to help the seed grow. With heat and moisture the seed starts to form it's first leaves underground. The sprout needs to get those leaves to the surface to absorb more food and to grow, so the sprout pushes up as the roots grow downward. The root now forms tiny lateral roots. Next the cotyledon forces upwards protecting the tender leaves between them. The leaves are referred to as plumule leaves. When the leaves are up out of the ground they open and start to make food for the plant from oxygen and light. This is called photosynthesis. The cotyledons are not needed any more so they wither and fall off. The leaves grow and the stem starts to stretch upward. This stage of growth takes quite a while in some plants. The root system grows downward and outward to provide a foundation for the growing plant. After a while a bud starts to form. Inside the bud a flower forms. The bud consists of many layers of flower parts. When they are fully formed the flower opens up and the true beauty inside is revealed and their scent is released. This attracts butterflies, bees, flies, and other insects that aid in pollination from flower to flower. Once pollinated, the seeds can ripen and be distributed by the wind and other animals eating them and thus carrying them to other places. Seeds can even explode out of the seed pods! Once the seeds are in different places, the life cycle continues once again.

General Things About Plants

Most people do not recognize the importance of plants, when in fact plants play an influential part in our daily lives. Plants take in carbon dioxide and give out oxygen which we breathe in. Without plants, we would not have clean air to breathe!

Plants grow in every region of the world. Heat, light, and water are basic essential needs for all except the simplest types of plants. Most flowers can't thrive without pollination, one way pollination is done is by insects, mainly by bees, butterflies, and fruitflies. Pollination is when pollen from the anther is transferred to the stigma to make seeds.

Some flowers do not need pollination to produce seeds for reproduction, their roots are designed to reproduce on their own. There are several kinds of roots that reproduce, rhizomes, stolons, bulbs, runners, corms, and tubers. Because of Alaska's short summers a lot of wild flowers reproduce in this way. Rhizomes are underground stems that grow just under the surface of the soil where every so often a flower shoots up. Alaska's blue irises reproduce this way. A corm resembles small underground leaves wrapped around each other similar to an onion, that new corms grow from. A bulb is similar to a corm but reproduces in the same place each year without moving. A tuber is a big group of swollen stems that act as a storage chamber, each of which can form a new plant after the main plant has died. A runner is sent above ground and roots grow out to anchor the new plant. All strawberries can reproduce this way.

Saturday, October 20, 2007

The Lost Space - Black Hole

The Black Hole Spinning

The Effect of Black Hole

Black Hole in Solar System


A black hole is a region of space in which the gravitational field is so powerful that nothing can escape after having fallen past the event horizon. The name comes from the fact that even electromagnetic radiation (e.g. light) is unable to escape, rendering the interior invisible. However, black holes can be detected if they interact with matter outside the event horizon, for example by drawing in gas from an orbiting star. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation in the process

While the idea of an object with gravity strong enough to prevent light from escaping was proposed in the 18th century, black holes as presently understood are described by Einstein's theory of general relativity, developed in 1916. This theory predicts that when a large enough amount of mass is present within a sufficiently small region of space, all paths through space are warped inwards towards the center of the volume, forcing all matter and radiation to fall inward.

While general relativity describes a black hole as a region of empty space with a pointlike singularity at the center and an event horizon at the outer edge, the description changes when the effects of quantum mechanics are taken into account. Research on this subject indicates that, rather than holding captured matter forever, black holes may slowly leak a form of thermal energy called Hawking radiation.[5][6][7] However, the final, correct description of black holes, requiring a theory of quantum gravity, is unknown.


Simulated view of a black hole in front of the Milky Way. The hole has 10 solar masses and is viewed from a distance of 600 km. An acceleration of about 400 million g is necessary to sustain this distance constantly.


Sizes of black holes

Black holes can have any mass. Since gravity increases in inverse proportion to volume, any quantity of matter that is sufficiently compressed will become a black hole. However, when black holes form naturally, only a few mass ranges are realistic.

Black holes can be divided into several size categories:

Astrophysicists expect to find stellar-mass and larger black holes, because a stellar mass black hole is formed by the gravitational collapse of a star of 20 or more solar masses at the end of its life, and can then act as a seed for the formation of a much larger black hole.

Micro black holes might be produced by:

What makes it impossible to escape from black holes?

General relativity describes mass as changing the shape of spacetime, and the shape of spacetime as describing how matter moves through space. For objects much less dense than black holes, this results in something similar to Newton's laws of gravity: objects with mass attract each other, but it's possible to define an escape velocity which allows a test object to leave the gravitational field of any large object. For objects as dense as black holes, this stops being the case. The effort required to leave the hole becomes infinite, with no escape velocity defined.

There are several ways of describing the situation that causes escape to be impossible. The difference between these descriptions is how space and time coordinates are drawn on spacetime (the choice of coordinates depends on the choice of observation point and on additional definitions used). One common description, based on the Schwarzschild description of black holes, is to consider the time axis in spacetime to point inwards towards the center of the black hole once the horizon is crossed.[8] Under these conditions, falling further into the hole is as inevitable as moving forward in time. A related description is to consider the future light cone of a test object near the hole (all possible paths the object or anything emitted by it could take, limited by the speed of light). As the object approaches the event horizon at the boundary of the black hole, the future light cone tilts inwards towards the horizon. When the test object passes the horizon, the cone tilts completely inward, and all possible paths lead into the hole.[9]

Do black holes have "no hair"?

Main article: No hair theorem

The "No hair" theorem states that black holes have only 3 independent internal properties: mass, angular momentum and electric charge. It is impossible to tell the difference between a black hole formed from a highly compressed mass of normal matter and one formed from, say, a highly compressed mass of anti-matter; in other words, any information about infalling matter or energy is destroyed. This is the black hole information paradox.

The theorem only works in some of the types of universe which the equations of general relativity allow, but this includes four-dimensional spacetimes with a zero or positive cosmological constant, which describes our universe at the classical level.

Types of black holes

Despite the uncertainty about whether the "No Hair" theorem applies to our universe, astrophysicists currently classify black holes according to their angular momentum (non-zero angular momentum means the black hole is rotating) and electric charge:

Non-rotating

Rotating

Uncharged

Schwarzschild

Kerr

Charged

Reissner-Nordström

Kerr-Newman

(All black holes have non-zero mass, so mass cannot be used for this type of "yes" / "no" classification)

Physicists do not expect that black holes with a significant electric charge will be formed in nature, because the electromagnetic repulsion which resists the compression of an electrically charged mass is about 40 orders of magnitude greater (about 1040 times greater) than the gravitational attraction which compresses the mass. So this article does not cover charged black holes in detail, but the Reissner-Nordström black hole and Kerr-Newman metric articles provide more information.

On the other hand astrophysicists expect that almost all black holes will rotate, because the stars from which they are formed rotate. In fact most black holes are expected to spin very rapidly, because they retain most of the angular momentum of the stars from which they were formed but concentrated into a much smaller radius. The same laws of angular momentum make skaters spin faster if they pull their arms closer to their bodies.

This article describes non-rotating, uncharged black holes first, because they are the simplest type.






Monday, October 1, 2007

The REPRODUCTIVE SYSTEMS.

The human reproductive system. The changes that take place during adolescence as the reproductive system develops are described, as well as the way that the foetus develops in the uterus. It has these parts in it:

  • The male reproductive system
  • The female reproductive system
  • The menstrual cycle
  • Fertilisation and the development of the foetus
  • Puberty

The male reproductive system

The reproductive system contains the organs needed for reproduction or producing babies. The male reproductive system contains these parts:

  • testes (pronounced "test-eez")
  • glands
  • sperm ducts
  • urethra
  • penis.

Image: the male reproductive system

Male reproductive system

Testes

The two testes (one of them is called a testis) are contained in a bag of skin called the scrotum. They have two functions:

  • to produce millions of male sex cells called sperm
  • to make male sex hormones, which affect the way a man's body develops.

Sperm duct and glands

The sperm can pass through the sperm ducts, and mix with fluids produced by the glands. The fluids provide the sperm cells with nutrients. The mixture of sperm and fluids is called semen.

Penis and urethra

The penis has two functions:

  • to pass urine out of the man's body
  • to pass semen into the vagina of a woman during sexual intercourse.


The urethra is the tube inside the penis that can carry urine or semen. A ring of muscle makes sure that there is no chance of urine and semen getting mixed up.

The female reproductive system

The female reproductive system contains these parts:

  • ovaries
  • egg tubes
  • uterus (pronounced "yoo-ter-russ")
  • cervix
  • vagina.

Image: the female reproductive system

Female reproductive system

Ovaries

The two ovaries contain hundreds of undeveloped female sex cells called egg cells or ova. Women have these cells in their bodies from birth - whereas men produce new sperm continually.

Egg tubes

Each ovary is connected to the uterus by an egg tube. This is sometimes called an oviduct or Fallopian tube. The egg tube is lined with cilia, which are tiny hairs on cells. Every month, an egg develops and becomes mature, and is released from an ovary. The cilia waft the egg along inside the egg tube and into the uterus.

Uterus and cervix

The uterus is also called the womb. It is a muscular bag with a soft lining. The uterus is where a baby develops until its birth.

The cervix is a ring of muscle at the lower end of the uterus. It keeps the baby in place while the woman is pregnant.

Vagina

The vagina is a muscular tube that leads from the cervix to the outside of the woman's body. A man's penis goes into the woman's vagina during sexual intercourse. The opening to the vagina has folds of skin called labia that meet to form a vulva. The urethra also opens into the vulva, but it is separate from the vagina, and is used for passing urine from the body.

The menstrual cycle

The female reproductive system includes a cycle of events called the menstrual cycle. It lasts about 28 days, but it can be slightly less or more than this. The cycle stops while a woman is pregnant. These are the main features of the menstrual cycle.

  • The start of the cycle, day 1, is when bleeding from the vagina begins. This is caused by the loss of the lining of the uterus, with a little blood. This is called menstruation or having a period.
  • By the end of about day 5, the loss of blood stops. The lining of the uterus begins to re-grow and an egg cell starts to mature in one of the ovaries.
  • At about day 14, the mature egg cell is released from the ovary. This is called ovulation. The egg cell travels through the egg tube towards the uterus.
  • If the egg cell does not meet with a sperm cell, the lining of the uterus begins to break down and the cycle repeats.

If the egg cell meets and joins with a sperm cell, it is fertilised. It attaches to the lining of the uterus and the woman becomes pregnant.

Fertilisation and the development of the foetus

Fertilisation

Fertilisation happens when an egg cell meets with a sperm cell and joins with it. This happens after sexual intercourse in which a man puts his penis into the woman's vagina. Sperm cells travel in semen from the penis and into the top of the vagina. They enter the uterus through the cervix and travel to the egg tubes. If a sperm cell meets with an egg cell there, fertilisation can happen.

The fertilised egg divides to form a ball of cells called an embryo. This attaches to the lining of the uterus and begins to develop into a foetus (pronounced "fee-tuss") and finally a baby.

Development of the foetus

The foetus relies upon its mother as it develops. These are some of the things it needs:

  • protection
  • oxygen
  • nutrients (food and water).


It also needs its waste substances removing.

The foetus is protected by the uterus and the amniotic fluid, a liquid contained in a bag called the amnion.

The placenta is responsible for providing oxygen and nutrients, and removing waste substances. It grows into the wall of the uterus and is joined to the foetus by the umbilical cord. The mother's blood does not mix with the foetus's blood, but the placenta lets substances pass between the two blood supplies:

  • oxygen and nutrients diffuse across the placenta from the mother to the foetus
  • waste substances, such as carbon dioxide, diffuse across the placenta from the foetus to the mother.

Birth

After nine months the baby is ready to be born. The cervix relaxes and muscles in the wall of the uterus contract, pushing the baby out of the mother's body.

Puberty

The reproductive system of a child is not mature and needs to change as a boy or girl develops into an adult, so that the system is fully working. These changes begin between the ages of ten and fifteen. The time when the changes happen is called puberty.

The changes happen because of sex hormones produced by the testes in boys and by the ovaries in girls. Some changes happen in boys and girls, while others just happen in boys or girls.

Here are some changes that happen to boys and girls:

  • underarm hair grows
  • pubic hair grows
  • body smell gets stronger.


There are also emotional changes at this time, and a growth spurt. The time when the physical changes and emotional changes happen is called adolescence.

Image: showing graph of heights

Graph of heights

Here are some changes that only happen to boys:

  • voice breaks (gets deeper)
  • testes and penis get bigger
  • testes start to produce sperm cells
  • shoulders get wider
  • hair grows on face and chest.



Here are some changes that only happen to girls:

  • breasts develop
  • ovaries start to release egg cells (periods start)
  • hips get wider.

EARTH HISTORY

The history of Earth covers approximately 4.6 billion years (4,567,000,000 years), from Earth’s formation out of the solar nebula to the present. This article presents a broad overview, summarizing the leading, most current scientific theories.


Origin

Formation and evolution of the Solar System

The Earth formed as part of the birth of the Solar System: what eventually became the solar system initially existed as a large, rotating cloud of dust, rocks, and gas. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements ejected by supernovas. Then, as one theory suggests, about 4.6 billion years ago a nearby star was destroyed in a supernova and the explosion sent a shock wave through the solar nebula, causing it to gain angular momentum. As the cloud began to accelerate its rotation, gravity and inertia flattened it into a protoplanetary disk oriented perpendicularly to its axis of rotation. Most of the mass concentrated in the middle and began to heat up, but small perturbations due to collisions and the angular momentum of other large debris created the means by which protoplanets began to form. The infall of material, increase in rotational speed and the crush of gravity created an enormous amount of kinetic heat at the center. Its inability to transfer that energy away through any other process at a rate capable of relieving the build-up resulted in the disk's center heating up. Ultimately, nuclear fusion of hydrogen into helium began, and eventually, after contraction, a T Tauri star, ignited to create the Sun. Meanwhile, as gravity caused matter to condense around the previously perturbed objects outside of the new sun's gravity grasp, dust particles and the rest of the protoplanetary disk began separating into rings. Successively larger fragments collided with one another and became larger objects, ultimately destined to become protoplanets.[1] These included one collection approximately 150 million kilometers from the center: Earth. The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies.

Moon



Animation (not to scale) of Theia forming in Earth’s L5 point and then, perturbed by gravity, colliding to help form the moon. The animation progresses in one-year steps making Earth appear not to move. The view is of the south pole.




Animation (not to scale) of Theia forming in Earth’s L5 point and then, perturbed by gravity, colliding to help form the moon. The animation progresses in one-year steps making Earth appear not to move. The view is of the south pole.

Main articles: Moon#Origin and geologic evolution and Giant impact hypothesis

The origin of the Moon is still uncertain, although much evidence exists for the giant impact hypothesis. Earth may not have been the only planet forming 150 million kilometers from the Sun. It is hypothesized that another collection occurred 150 million kilometers from both the Sun and the Earth, at their fourth or fifth Lagrangian point. This planet, named Theia, is thought to have been smaller than the current Earth, probably about the size and mass of Mars. Its orbit may at first have been stable, but destabilized as Earth increased its mass by the accretion of more and more material. Theia swung back and forth relative to Earth until, finally, an estimated 4.533 billion years ago,[2] it collided at a low, oblique angle. The low speed and angle were not enough to destroy Earth, but a large portion of its crust was ejected into space. Heavier elements from Theia sank to Earth’s core, while the remaining material and ejecta condensed into a single body within a couple of weeks. Under the influence of its own gravity, this became a more spherical body: the Moon.[3] The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons. (A simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons.) It may also have sped up Earth’s rotation and initiated the planet’s plate tectonics.

The Hadean eon



Main article: Hadean

Volcanic eruptions would have been common in Earth's early days.

Volcanic eruptions would have been common in Earth's early days.

The early Earth, during the very early Hadean eon, was very different from the world known today. There were no oceans and no oxygen in the atmosphere. It was bombarded by planetoids and other material left over from the formation of the solar system. This bombardment, combined with heat from radioactive breakdown, residual heat, and heat from the pressure of contraction, caused the planet at this stage to be fully molten. During the iron catastrophe heavier elements sank to the center while lighter ones rose to the surface producing the layered structure of the Earth and also setting up the formation of Earth's magnetic field. Earth's early atmosphere would have comprised surrounding material from the solar nebula, especially light gases such as hydrogen and helium, but the solar wind and Earth's own heat would have driven off this atmosphere.

This changed when Earth was about 40% its present radius, and gravitational attraction allowed the retention of an atmosphere which included water. Temperatures plummeted and the crust of the planet was accumulated on a solid surface, with areas melted by large impacts on the scale of decades to hundreds of years between impact. Large impacts would have caused localized melting and partial differentiation, with some lighter elements on the surface or released to the moist atmosphere. [4]

The surface cooled quickly, forming the solid crust within 150 million years;[5] although new research[6] suggests that the actual number is 100 million years based on the level of hafnium found in the geology at Jack hills in Western Australia. From 4 to 3.8 billion years ago, Earth underwent a period of heavy asteroidal bombardment.[7] Steam escaped from the crust while more gases were released by volcanoes, completing the second atmosphere. Additional water was imported by bolide collisions, probably from asteroids ejected from the outer asteroid belt under the influence of Jupiter's gravity. The planet cooled. Clouds formed. Rain gave rise to the oceans within 750 million years (3.8 billion years ago), but probably earlier. Recent evidence suggests the oceans may have begun forming by 4.2 billion years ago[8] [9]. The new atmosphere probably contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, as well as smaller amounts of other gases. Any free oxygen would have been bound by hydrogen or minerals on the surface. Volcanic activity was intense and, without an ozone layer to hinder its entry, ultraviolet radiation flooded the surface.

Life



The replicator in virtually all known life is deoxyribonucleic acid. DNA is far more complex than the original replicator and its replication systems are highly elaborate.

The replicator in virtually all known life is deoxyribonucleic acid. DNA is far more complex than the original replicator and its replication systems are highly elaborate.

Main article: Origin of life

The details of the origin of life are unknown, though the broad principles have been established. It has been proposed that life, or at least organic components, may have arrived on Earth from space (see “Panspermia”), while others argue that terrestrial origins are more probable. The mechanisms by which life would initially arise are nevertheless held to be similar.[10] If life arose on Earth, the timing of this event is highly speculative—perhaps it arose around 4 billion years ago.[11] In the energetic chemistry of early Earth, a molecule (or even something else) gained the ability to make copies of itself–the replicator. The nature of this molecule is unknown, its function having long since been superseded by life’s current replicator, DNA. In making copies of itself, the replicator did not always perform accurately: some copies contained an “error.” If the change destroyed the copying ability of the molecule, there could be no more copies, and the line would “die out.” On the other hand, a few rare changes might make the molecule replicate faster or better: those “strains” would become more numerous and “successful.” As choice raw materials (“food”) became depleted, strains which could exploit different materials, or perhaps halt the progress of other strains and steal their resources, became more numerous.[12]

Several different models have been proposed explaining how a replicator might have developed. Different replicators have been posited, including organic chemicals such as modern proteins, nucleic acids, phospholipids, crystals,[13] or even quantum systems.[14] There is currently no method of determining which of these models, if any, closely fits the origin of life on Earth. One of the older theories, and one which has been worked out in some detail, will serve as an example of how this might occur. The high energy from volcanoes, lightning, and ultraviolet radiation could help drive chemical reactions producing more complex molecules from simple compounds such as methane and ammonia.[15] Among these were many of the relatively simple organic compounds that are the building blocks of life. As the amount of this “organic soup” increased, different molecules reacted with one another. Sometimes more complex molecules would result—perhaps clay provided a framework to collect and concentrate organic material.[16] The presence of certain molecules could speed up a chemical reaction. All this continued for a very long time, with reactions occurring more or less at random, until by chance there arose a new molecule: the replicator. This had the bizarre property of promoting the chemical reactions which produced a copy of itself, and evolution began properly. Other theories posit a different replicator. In any case, DNA took over the function of the replicator at some point; all known life (with the exception of some viruses and prions) use DNA as their replicator, in an almost identical manner (see genetic code).

Cells



A small section of a cell membrane. This modern cell membrane is far more sophisticated than the original simple phospholipid bilayer (the small blue spheres with two tails). Proteins and carbohydrates serve various functions in regulating the passage of material through the membrane and in reacting to the environment.

A small section of a cell membrane. This modern cell membrane is far more sophisticated than the original simple phospholipid bilayer (the small blue spheres with two tails). Proteins and carbohydrates serve various functions in regulating the passage of material through the membrane and in reacting to the environment.

Modern life has its replicating material packaged neatly inside a cellular membrane. It is easier to understand the origin of the cell membrane than the origin of the replicator, since the phospholipid molecules that make up a cell membrane will often form a bilayer spontaneously when placed in water. Under certain conditions, many such spheres can be formed (see “The bubble theory”).[17] It is not known whether this process preceded or succeeded the origin of the replicator (or perhaps it was the replicator). The prevailing theory is that the replicator, perhaps RNA by this point (the RNA world hypothesis), along with its replicating apparatus and maybe other biomolecules, had already evolved. Initial protocells may have simply burst when they grew too large; the scattered contents may then have recolonized other “bubbles.” Proteins that stabilized the membrane, or that later assisted in an orderly division, would have promoted the proliferation of those cell lines. RNA is a likely candidate for an early replicator since it can both store genetic information and catalyze reactions. At some point DNA took over the genetic storage role from RNA, and proteins known as enzymes took over the catalysis role, leaving RNA to transfer information and modulate the process. There is increasing belief that these early cells may have evolved in association with underwater volcanic vents known as “black smokers”.[18] or even hot, deep rocks.[19] However, it is believed that out of this multiplicity of cells, or protocells, only one survived. Current evidence suggests that the last universal common ancestor lived during the early Archean eon, perhaps roughly 3.5 billion years ago or earlier.[20],[21] This “LUCA” cell is the ancestor of all cells and hence all life on Earth. It was probably a prokaryote, possessing a cell membrane and probably ribosomes, but lacking a nucleus or membrane-bound organelles such as mitochondria or chloroplasts. Like all modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis, and enzymes to catalyze reactions. Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes in lateral gene transfer.[20]

Photosynthesis and oxygen



The harnessing of the sun’s energy led to several major changes in life on Earth.

The harnessing of the sun’s energy led to several major changes in life on Earth.

It is likely that the initial cells were all heterotrophs, using surrounding organic molecules (including those from other cells) as raw material and an energy source.[22] As the food supply diminished, a new strategy evolved in some cells. Instead of relying on the diminishing amounts of free-existing organic molecules, these cells adopted sunlight as an energy source. Estimates vary, but by about 3 billion years ago[23], something similar to modern photosynthesis had probably developed. This made the sun’s energy available not only to autotrophs but also to the heterotrophs that consumed them. Photosynthesis used the plentiful carbon dioxide and water as raw materials and, with the energy of sunlight, produced energy-rich organic molecules (carbohydrates).

Moreover, oxygen was produced as a waste product of photosynthesis. At first it became bound up with limestone, iron, and other minerals. There is substantial proof of this in iron-oxide rich layers in geological strata that correspond with this time period. The oceans would have turned to a green color while oxygen was reacting with minerals. When the reactions stopped, oxygen could finally enter the atmosphere. Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast period of time transformed Earth’s atmosphere to its current state.[24]

This, then, is Earth’s third atmosphere. Some of the oxygen was stimulated by incoming ultraviolet radiation to form ozone, which collected in a layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through the atmosphere. It allowed cells to colonize the surface of the ocean and ultimately the land:[25] without the ozone layer, ultraviolet radiation bombarding the surface would have caused unsustainable levels of mutation in exposed cells. Besides making large amounts of energy available to life-forms and blocking ultraviolet radiation, the effects of photosynthesis had a third, major, and world-changing impact. Oxygen was toxic; probably much life on Earth died out as its levels rose (the “Oxygen Catastrophe”).[25] Resistant forms survived and thrived, and some developed the ability to use oxygen to enhance their metabolism and derive more energy from the same food.

Endosymbiosis and the three domains of life



Main article: Endosymbiotic theory

Some of the pathways by which the various endosymbionts might have arisen.

Some of the pathways by which the various endosymbionts might have arisen.

Modern taxonomy classifies life into three domains. The time of the origin of these domains are speculative. The Bacteria domain probably first split off from the other forms of life (sometimes called Neomura), but this supposition is controversial. Soon after this, by 2 billion years ago[26], the Neomura split into the Archaea and the Eukarya. Eukaryotic cells (Eukarya) are larger and more complex than prokaryotic cells (Bacteria and Archaea), and the origin of that complexity is only now coming to light. Around this time period a bacterial cell related to today’s Rickettsia[27] entered a larger prokaryotic cell. Perhaps the large cell attempted to ingest the smaller one but failed (maybe due to the evolution of prey defenses). Perhaps the smaller cell attempted to parasitize the larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it was able to metabolize the larger cell’s waste products and derive more energy. Some of this surplus energy was returned to the host. The smaller cell replicated inside the larger one, and soon a stable symbiotic relationship developed. Over time the host cell acquired some of the genes of the smaller cells, and the two kinds became dependent on each other: the larger cell could not survive without the energy produced by the smaller ones, and these in turn could not survive without the raw materials provided by the larger cell. Symbiosis developed between the larger cell and the population of smaller cells inside it to the extent that they are considered to have become a single organism, the smaller cells being classified as organelles called mitochondria. A similar event took place with photosynthetic cyanobacteria[28] entering larger heterotrophic cells and becoming chloroplasts.[29],[30] Probably as a result of these changes, a line of cells capable of photosynthesis split off from the other eukaryotes some time before one billion years ago. There were probably several such inclusion events, as the figure at right suggests. Besides the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, it has been suggested that cells gave rise to peroxisomes, spirochetes gave rise to cilia and flagella, and that perhaps a DNA virus gave rise to the cell nucleus,[31],[32] though none of these theories are generally accepted.[33] During this period, the supercontinent Columbia is believed to have existed, probably from around 1.8 to 1.5 billion years ago; it is the oldest hypothesized supercontinent.[34]

Multicellularity



Volvox aureus is believed to be similar to the first multicellular plants.

Volvox aureus is believed to be similar to the first multicellular plants.

Archaeans, bacteria, and eukaryotes continued to diversify and to become more sophisticated and better adapted to their environments. Each domain repeatedly split into multiple lineages, although little is known about the history of the archaea and bacteria. Around 1.1 billion years ago, the supercontinent Rodinia was assembling.[35] The plant, animal, and fungi lines had all split, though they still existed as solitary cells. Some of these lived in colonies, and gradually some division of labor began to take place; for instance, cells on the periphery might have started to assume different roles from those in the interior. Although the division between a colony with specialized cells and a multicellular organism is not always clear, around 1 billion years ago[36], the first multicellular plants emerged, probably green algae.[37] Possibly by around 900 million years ago,[38] true multicellularity had also evolved in animals. At first it probably somewhat resembled that of today’s sponges, where all cells were totipotent and a disrupted organism could reassemble itself.[39] As the division of labor became more complete in all lines of multicellular organisms, cells became more specialized and more dependent on each other; isolated cells would die. Many scientists believe that a very severe ice age began around 770 million years ago, so severe that the surface of all the oceans completely froze (Snowball Earth). Eventually, after 20 million years, enough carbon dioxide escaped through volcanic outgassing; the resulting greenhouse effect raised global temperatures.[40] By around the same time, 750 million years ago,[41] Rodinia began to break up.

Colonization of land



For most of Earth’s history, there were no multicellular organisms on land. Parts of the surface may have vaguely resembled this view of Mars, one of Earth’s neighboring planets.[citation needed]

For most of Earth’s history, there were no multicellular organisms on land. Parts of the surface may have vaguely resembled this view of Mars, one of Earth’s neighboring planets.[citation needed]

As we have already seen, the accumulation of oxygen in Earth’s atmosphere caused the formation of ozone into a layer that absorbed much of Sun’s ultraviolet radiation. As a result, unicellular organisms that reached land were less likely to die, and prokaryotes began to multiply and become better adapted to survival out of the water. Prokaryotes had likely colonized the land as early as 2.6 billion years ago[42] even before the origin of the eukaryotes. For a long time, the land remained barren of multicellular organisms. The supercontinent Pannotia formed around 600 million years ago and then broke apart a short 50 million years later[43]. Fish, the earliest vertebrates, evolved in the oceans around 530 million years ago[44]. A major extinction event occurred near the end of the Cambrian period,[45] which ended 488 million years ago[46].

Several hundred million years ago, plants (probably resembling algae) and fungi started growing at the edges of the water, and then out of it.[47] The oldest fossils of land fungi and plants date to 480–460 million years ago, though molecular evidence suggests the fungi may have colonized the land as early as 1000 million years ago and the plants 700 million years ago.[48] Initially remaining close to the water’s edge, mutations and variations resulted in further colonization of this new environment. The timing of the first animals to leave the oceans is not precisely known: the oldest clear evidence is of arthropods on land around 450 million years ago[49], perhaps thriving and becoming better adapted due to the vast food source provided by the terrestrial plants. There is also some unconfirmed evidence that arthropods may have appeared on land as early as 530 million years ago[50]. At the end of the Ordovician period, 440 million years ago, additional extinction events occurred, perhaps due to a concurrent ice age.[51] Around 380 to 375 million years ago, the first tetrapods evolved from fish.[52] It is thought that perhaps fins evolved to become limbs which allowed the first tetrapods to lift their heads out of the water to breathe air. This would let them survive in oxygen-poor water or pursue small prey in shallow water.[52] They may have later ventured on land for brief periods. Eventually, some of them became so well adapted to terrestrial life that they spent their adult lives on land, although they hatched in the water and returned to lay their eggs. This was the origin of the amphibians. About 365 million years ago, another period of extinction occurred, perhaps as a result of global cooling.[53] Plants evolved seeds, which dramatically accelerated their spread on land, around this time (by approximately 360 million years ago).[54], [55]

Pangaea, the most recent supercontinent, existed from 300 to 180 million years ago. The outlines of the modern continents and other land masses are indicated on this map.

Pangaea, the most recent supercontinent, existed from 300 to 180 million years ago. The outlines of the modern continents and other land masses are indicated on this map.

Some 20 million years later (340 million years ago[56]), the amniotic egg evolved, which could be laid on land, giving a survival advantage to tetrapod embryos. This resulted in the divergence of amniotes from amphibians. Another 30 million years (310 million years ago[57]) saw the divergence of the synapsids (including mammals) from the sauropsids (including birds and non-avian, non-mammalian reptiles). Other groups of organisms continued to evolve and lines diverged—in fish, insects, bacteria, and so on—but less is known of the details. 300 million years ago, the most recent supercontinent formed, called Pangaea. The most severe extinction event to date took place 250 million years ago, at the boundary of the Permian and Triassic periods; 95% of life on Earth died out,[58] possibly due to the Siberian Traps volcanic event. The discovery of a crater hidden under the East Antarctic Ice Sheet has risen up a new theory that a meteor caused the mass extinction and possibly began the breakup of the Gondwana supercontinent by creating the tectonic rift that pushed Australia northward.[59] But life persevered, and around 230 million years ago [60], dinosaurs split off from their reptilian ancestors. An extinction event between the Triassic and Jurassic periods 200 million years ago spared many of the dinosaurs,[61] and they soon became dominant among the vertebrates. Though some of the mammalian lines began to separate during this period, existing mammals were probably all small animals resembling shrews.[62] By 180 million years ago, Pangaea broke up into Laurasia and Gondwana. The boundary between avian and non-avian dinosaurs is not clear, but Archaeopteryx, traditionally considered one of the first birds, lived around 150 million years ago.[63] The earliest evidence for the angiosperms evolving flowers is during the Cretaceous period, some 20 million years later (132 million years ago)[64] Competition with birds drove many pterosaurs to extinction, and the dinosaurs were probably already in decline for various reasons[65] when, 65 million years ago, a 10-kilometer meteorite likely struck Earth just off the Yucatán Peninsula, ejecting vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. Most large animals, including the non-avian dinosaurs, became extinct.[66], marking the end of the Cretaceous period and Mesozoic era. Thereafter, in the Paleocene epoch, mammals rapidly diversified, grew larger, and became the dominant vertebrates. Perhaps a couple of million years later (around 63 million years ago), the last common ancestor of primates lived.[67] By the late Eocene epoch, 34 million years ago, some terrestrial mammals had returned to the oceans to become animals such as Basilosaurus which later gave rise to dolphins and whales.[68]

Humanity



Australopithecus africanus, an early hominid.

Australopithecus africanus, an early hominid.

Main article: Human evolution

A small African ape living around six million years ago was the last animal whose descendants would include both modern humans and their closest relatives, the bonobos, and chimpanzees.[69] Only two branches of its family tree have surviving descendants. Very soon after the split, for reasons that are still debated, apes in one branch developed the ability to walk upright.[70] Brain size increased rapidly, and by 2 million years ago, the very first animals classified in the genus Homo had appeared.[71] Of course, the line between different species or even genera is rather arbitrary as organisms continuously change over generations. Around the same time, the other branch split into the ancestors of the common chimpanzee and the ancestors of the bonobo as evolution continued simultaneously in all life forms.[69] The ability to control fire likely began in Homo erectus (or Homo ergaster), probably at least 790,000 years ago[72] but perhaps as early as 1.5 million years ago.[73] It is more difficult to establish the origin of language; it is unclear whether Homo erectus could speak or if that capability had not begun until Homo sapiens.[74] As brain size increased, babies were born sooner, before their heads grew too large to pass through the pelvis. As a result, they exhibited more plasticity, and thus possessed an increased capacity to learn and required a longer period of dependence. Social skills became more complex, language became more advanced, and tools became more elaborate. This contributed to further cooperation and brain development.[75] Anatomically modern humans — Homo sapiens — are believed to have originated somewhere around 200,000 years ago or earlier in Africa; the oldest fossils date back to around 160,000 years ago.[76] The first humans to show evidence of spirituality are the Neanderthals (usually classified as a separate species with no surviving descendants); they buried their dead, often apparently with food or tools.[77] However, evidence of more sophisticated beliefs, such as the early Cro-Magnon cave paintings (probably with magical or religious significance)[78] did not appear until some 32,000 years ago.[79] Cro-Magnons also left behind stone figurines such as Venus of Willendorf, probably also signifying religious belief.[78] By 11,000 years ago, Homo sapiens had reached the southern tip of South America, the last of the uninhabited continents.[80] Tool use and language continued to improve; interpersonal relationships became more complex.

Civilization



Main article: History of the world

Throughout more than 90% of its history, Homo sapiens lived in small bands as nomadic hunter-gatherers.[81] As language became more complex, the ability to remember and transmit information resulted in a new sort of replicator: the meme.[82] Ideas could be rapidly exchanged and passed down the generations. Cultural evolution quickly outpaced biological evolution, and history proper began. Somewhere between 8500 and 7000 BC, humans in the Fertile Crescent in Middle East began the systematic husbandry of plants and animals: agriculture.[83] This spread to neighboring regions, and also developed independently elsewhere, until most Homo sapiens lived sedentary lives in permanent settlements as farmers. Not all societies abandoned nomadism, especially those in isolated areas of the globe poor in domesticable plant species, such as Australia.[84] However, among those civilizations that did adopt agriculture, the relative security and increased productivity provided by farming allowed the population to expand. Agriculture had a major impact; humans began to affect the environment as never before. Surplus food allowed a priestly or governing class to arise, followed by increasing division of labor. This led to Earth’s first civilization at Sumer in the Middle East, between 4000 and 3000 BC.[85] Additional civilizations quickly arose in ancient Egypt and the Indus River valley.

Vitruvian Man by Leonardo da Vinci epitomizes the advances in art and science seen during the Renaissance.

Vitruvian Man by Leonardo da Vinci epitomizes the advances in art and science seen during the Renaissance.

Starting around 3000 BC, Hinduism, one of the oldest religions still practiced today, began to take form.[86] Others soon followed. The invention of writing enabled complex societies to arise: record-keeping and libraries served as a storehouse of knowledge and increased the cultural transmission of information. Humans no longer had to spend all their time working for survival—curiosity and education drove the pursuit of knowledge and wisdom. Various disciplines, including science (in a primitive form), arose. New civilizations sprang up, traded with one another, and engaged in war for territory and resources: empires began to form. By around 500 BC, there were empires in the Middle East, Iran, India, China, and Greece, approximately on equal footing; at times one empire expanded, only to decline or be driven back later.[87]

In the fourteenth century, the Renaissance began in Italy with advances in religion, art, and science.[88] Starting around 1500 (0.0096 seconds ago), European civilization began to undergo changes leading to the scientific and industrial revolutions: that continent began to exert political and cultural dominance over human societies around the planet.[89] From 1914 to 1918 and 1939 to 1945, nations around the world were embroiled in world wars. Established following World War I, the League of Nations was a first step toward a world government; after World War II it was replaced by the United Nations. In 1992, several European nations joined together in the European Union. As transportation and communication improved, the economies and political affairs of nations around the world have become increasingly intertwined. This globalization has often produced discord, although increased collaboration has resulted as well.

Recent events



Four and a half billion years after the planet's formation, one of Earth’s life forms broke free of the biosphere. For the first time in history, Earth was viewed first hand from the vantage of space.

Four and a half billion years after the planet's formation, one of Earth’s life forms broke free of the biosphere. For the first time in history, Earth was viewed first hand from the vantage of space.

Change has continued at a rapid pace from the mid-1940s to today. Technological developments include nuclear weapons, computers, genetic engineering, and nanotechnology. Economic globalization spurred by advances in communication and transportation technology has influenced everyday life in many parts of the world. Cultural and institutional forms such as democracy, capitalism, and environmentalism have increased influence. Problems such as disease, war, and poverty are still present. Global warming has also emerged as a new problem.

In 1957, the Soviet Union launched the first artificial satellite into orbit and, soon afterward, Yuri Gagarin became the first human in space. Neil Armstrong, an American, was the first to set foot on another astronomical object, the Earth's Moon. The Soviet Union and the United States of America were the primary early leaders in space exploration in the 20th Century. Five space agencies, representing over fifteen countries,[90] have worked together to build the International Space Station. Aboard it, there has been a continuous human presence in space since 2000.[91]

Internal structure

Main article: Structure of the Earth

Earth cutaway from core to exosphere. Not to scale.

Earth cutaway from core to exosphere. Not to scale.

The interior of the Earth, like that of the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents.[32]

The geologic component layers of the Earth[33] are at the following depths below the surface:[34]

Depth

Layer

Density
g/cm³

Kilometers

Miles

0–60

0–37

Lithosphere (locally varies between 5 and 200 km)

0–35

0–22

... Crust (locally varies between 5 and 70 km)

2.2–2.9

35–60

22–37

... Uppermost part of mantle

3.4–4.4

35–2890

22–1790

Mantle

3.4–5.6

100–700

62–435

... Asthenosphere

2890–5100

1790–3160

Outer core

9.9–12.2

5100–6378

3160–3954

Inner core

12.8–13.1

The internal heat of the planet is most likely produced by the radioactive decay of potassium-40, uranium-238 and thorium-232 isotopes. All three have half-life decay periods of more than a billion years.[35] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.[36] A portion of the core's thermal energy is transported toward the crust by Mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.[37]

Tectonic plates

Main article: Plate tectonics

A map illustrating the Earth's major plates.

A map illustrating the Earth's major plates.

According to plate tectonics theory, the outermost part of the Earth's interior is made up of two layers: the lithosphere, comprising the crust, and the solidified uppermost part of the mantle. Below the lithosphere lies the asthenosphere, which forms the inner part of the mantle. The asthenosphere behaves like a superheated and extremely viscous liquid.[38]

The lithosphere essentially floats on the asthenosphere and is broken up into what are called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: convergent, divergent and transform. The last occurs where two plates move laterally relative to each other, creating a strike-slip fault. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.[39]

The main plates are:[40]

Plate name

Area

Covering

106 km²

106 mi²

African Plate

61.3

23.7

Africa

Antarctic Plate

60.9

23.5

Antarctica

Australian Plate

47.2

18.2

Australia

Eurasian Plate

67.8

26.2

Asia and Europe

North American Plate

75.9

29.3

North America and north-east Siberia

South American Plate

43.6

16.8

South America

Pacific Plate

103.3

39.9

Pacific Ocean

Notable minor plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate actually fused with Indian Plate between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/yr[41] (3.0 in/yr) and the Pacific Plate moving 52–69 mm/yr (2.1–2.7 in/yr). At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr (0.8 in/yr).[42]

Surface

Main articles: Landforms and Extreme points of the world

Present day Earth altimetry and bathymetry. Data from the National Geophysical Data Center's TerrainBase Digital Terrain Model.

Present day Earth altimetry and bathymetry. Data from the National Geophysical Data Center's TerrainBase Digital Terrain Model.

The Earth's terrain varies greatly from place to place. About 70.8%[43] of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[27] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts[44] also act to reshape the landscape.

As the continental plates migrate across the planet, the ocean floor is subducted under the leading edges. At the same time, upwellings of mantle material create a divergent boundary along mid-ocean ridges. The combination of these processes continually recycles the ocean plate material. Most of the ocean floor is less than 100 million years in age. The oldest ocean plate is located in the Western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years.[45][46]

The continental plates consist of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.[47] Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.[48] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine.[49] Common carbonate minerals include calcite (found in limestone), aragonite and dolomite.[50]

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.[51] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3×107 km² of cropland and 3.4×107 km² of pastureland.[52

BidVertiser

DASA

DASA