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Formation of Earth
14 December 1996
From New Scientist Print Edition.
Sue Bowler

Leeds OUR Sun, like other stars, formed when a cloud of dust and gas began to clump together in interstellar space. No one quite knows why or how this happened, but it was the first step in the long and complicated evolution of our rocky planet, with its iron core and oxygen-rich atmosphere. The first stage was to build a planet of the right size, using the right materials. But it produced nothing that we would recognise as "our Earth". Later, Earth became layered, with a core rich in iron, a mantle made of silicate minerals and a thin, rigid crust.

Life began about 3500 million years ago. And since then the planet has continued its relentless evolution to the form we know so well. But change continues. For example, earthquakes and volcanoes are testimony to the constant flux that characterises the Earth's processes. The plates that make up the surface constantly move, continents change shape and the rocks themselves are continually eroding, reforming and recycling. It is these processes that set the Earth apart from all the other planets of the Solar System and blotting out its early history.

The Earth was formed from the Solar Nebula of hydrogen and helium. As the cloud started to condense, there was a balance between the accumulation of material at the centre of the nebula—destined to become the Sun—and the outward movement of gas and dust, the raw materials of the planets. Atoms, molecules and small particles continually came into contact as they swirled around, forming ever-larger lumps. Eventually these lumps were big enough to be called planetesimals, measuring 10 kilometres or so across. More importantly, they became big enough for gravity to play a significant part in their interactions with each other.

Ingredients for Earth

Some rocky recipes

THIS is when collisions between planetesimals began to shape the planets of the future. One way to try to understand the effects of these collisions is to make a model in which there are, say, 1000 of these bodies orbiting the Sun, and track each one, monitoring the effects of each collision. It's like trying to understand how a gas behaves by tracking individual molecules. The sheer number of collisions makes the calculations very time-consuming, even with powerful computers. Researchers got around this problem by treating the bodies not as individuals but as a group. The scientists ran a computer model many times, showing different patterns in each case, then looked at the most common results.

This modelling method showed that once a certain density of planetesimals had formed, small groups tended to clump with other groups to form larger bodies. The result was between three and five bodies of about the same size and orbit spacing as Mercury, Venus, Earth and Mars.

The models predicted more impacts. Indeed, it is probable that some of the large bodies hit each other, perhaps generating enough heat to melt some or all of them. There are also suggestions that impacts between big planetesimals could account for some of the strange features of the Solar System. For example, Venus rotates in the opposite way to its path around the Sun, unlike all the other planets, and Uranus has its magnetic pole perpendicular to its pole of rotation (see Inside Science No 30).

Such an impact provides a possible explanation for the origin of the Moon, when a body the size of Mars struck the early Earth a glancing blow. Such enormous collisions would splatter rock from the bodies out into space, perhaps forming asteroids. The chemical composition of a group of meteorites found in Antarctica suggests that they could have come from Mars, flung into space in another of these huge impacts—these are the bodies that some scientists now claim harbour signs of life on Mars.

As planet Earth evolved, the bombardment continued, however there were fewer impacts. The embryo planets orbited the Sun in the company of many smaller bodies. As they continued to collide, the smaller ones fragmented, until the space around each growing planet held fewer small bodies. Once these barren regions exist, impacts become rarer and involve relatively small lumps of rock—a few kilometres across at most.

On most other planets and moons, the density of impacts provides a timescale for their early evolution: parts of their crusts pitted with more craters are clearly older than less scarred areas. But as so much of our planet's surface is either covered with water or has been recycled many times through plate tectonics (see Box), Earth's early history is a mystery.

Although direct evidence is lacking, some things are clear. The Earth became hot some 4500 million years ago—perhaps reaching 5000 kelvin within a few 100 million years of the Earth's formation. As the planetesimals began to stick together, the embryo planet began to heat up. The impact that probably formed the Moon would have generated enough heat to melt most of the Earth, producing an ocean of magma that might even have reached down to the Earth's core. The Moon was certainly completely molten at this stage in its evolution, probably a consequence of its formation in a big impact. But here the histories of the Moon and the Earth diverge. The Moon's ocean of magma began to crystallise into the thick lunar crust familiar to the Apollo astronauts. But the molten Earth took a different route.

The cooling of the Moon seems to have followed a simple pattern that is also seen in large bodies of molten rock on Earth. The first minerals to form when a body of rock cools tend to grow in recognisable crystal shapes. The types of minerals that grow first also differ according to the temperature and pressure of the molten rock, so identifying them can provide useful information. Many igneous rocks contain minerals called feldspars—white, pink or yellow minerals shaped almost like house bricks. They are made of silicon, aluminium and oxygen, in a framework with calcium, sodium or potassium.

Moon rocks contain a type of feldspar called plagioclase, a variety containing sodium and calcium, in crystals that suggest this was the first mineral to form as the rock cooled. The plagioclase crystals were less dense than the magma as a whole, so they floated and clumped together to make a crust. The underlying magma became denser as a result and eventually, different minerals began to crystallise. This process is called fractional crystallisation. It is common on Earth, where it produces characteristic families of rocks.

Fractional types

First atmospheres

BECAUSE certain unusual elements tend to accumulate in the crystals and others in the magma left behind, the pattern of their distribution in the rocks that form is a sure sign that fractional crystallisation has played a part in their formation.

An example is the element europium. This rare earth element tends to accumulate in plagioclase crystals when they form in this way. Moon rocks show just this pattern, with extra europium in the plagioclase rocks. But early rocks on Earth show no such europium anomaly. Whether or not the Earth practically melted as a result of the giant impact that spawned the Moon, it could not have cooled in the same simple way. The Earth does not have as much calcium and aluminium as the Moon, with the result that plagioclase would not accumulate. In addition, the Earth had more water than the Moon, and that makes a big difference; when plagioclase did form it sank in wet magma rather than floating as on the bone-dry Moon.

Earth's layered structure began to form very early in its history. The core, now a third of the planet's mass, formed within the first 100 million years of the Earth's life. The core is made up mainly of iron, which probably melted and sank to the centre as the planetesimals collided and accumulated. Since then, there seems to have been little mixing between the core and the mantle. But the history of the mantle and the outer layer of the Earth, the crust, is very different. Today, the mantle itself is stratified. Continental crust floats on the fluid mantle beneath. And plate tectonics constantly form and reform the ocean crust.

When and how did the layers that define the mantle and crust today come into existence? Again, evidence is sparse, but it does exist. Much of it comes from the chemistry of the Earth, and from comparisons with other rock from other parts of the Solar System, mostly in the form of meteorites. Most of these rocks have surprisingly similar compositions, so that scientists can work out an average Solar System composition. Researchers then compare notes with the Earth to find out how our planet differs from the rest.

A result of the comparisons between cosmic chemistry and Earth minerals is that we know where most of our atmosphere came from—and it was not left over from the Solar Nebula. Gases such as neon are many millions of times rarer on Earth than in the Solar System as a whole. If our planet started out with the same proportions of these gases as the rest, then any atmosphere there to start with, blew away.

Our atmosphere originated from the interior of the Earth. Active volcanoes now produce a mixture of gases, mainly carbon dioxide, but also sulphur dioxide, carbon monoxide and hydrogen, among others. So the very early atmosphere was probably made of much the same materials, mainly carbon dioxide, and the atmospheric pressure was probably ten times its current level. The oceans began as pools of water condensing from volcanic gases. The volcanoes brought these gases and water from the Earth's mantle, supplying the surface with materials mixed into the Earth when it first formed. Early impacts could also have supplied water and gases adding to our atmosphere and hydrosphere.

Oxygen began to accumulate slowly as energy from the Sun broke down molecules such as carbon dioxide and water. But living things, then as now, seem to have been the key to the growth of the atmosphere. Rocks from Greenland as old as 3800 million years contain traces of carbon that probably came from some simple life form. Blue-green algae were playing a part in the formation of rocky mounds, stromatolites, 3500 million years ago. As plants evolved, consuming carbon dioxide and exhaling oxygen, the atmosphere became more hospitable. Eventually, animals would leave the seas and head for dry land.

But before they could take that big step, there had to be some dry land. And that means the formation of continental crust. The first crust of the Earth was made of basalt, much like the crust that forms today at the mid-ocean ridges. Basalt is a hard, black rock made of crystals too small to pick out with the naked eye. It is the rock that today makes the volcanic islands of Iceland and Hawaii.

Basalt makes the floor of the oceans, beneath the thin layer of sediments deposited there. And like the ocean crust today, little of this early crust would have formed land. For the first billion years or so, the surface of the Earth consisted of seas interrupted by chains of volcanic islands. It would probably have been a steamy, smelly place, as the water and gases—a mixture of hydrogen, hydrogen sulphide, hydrogen chloride, carbon monoxide, carbon dioxide and sulphur dioxide and more—given off by the volcanoes continued to add to the growing atmosphere. The volcanic gases reacted with each other in the light and other radiation from the Sun. The steam that accompanied each eruption would not have been reabsorbed into the rocks, but would have accumulated slowly to make the oceans. As water accumulated in the oceans, more and more of this volcanism would have taken place underwater, as it mostly does today.

The oldest parts of the continents today are made of rocks that were once granites or similar rocks. Granite is a light coloured rock made up of big crystals, easily visible to the naked eye and often a few centimetres across. The faces of the crystals of feldspar in particular often catch the light, which makes the rock useful for decoration. Granites form today at subduction zones—regions where ocean floor slides beneath continents or other oceans. Ocean crust is wet, because the igneous rock forms under the sea at mid-ocean ridges, where sea water circulates through cracks and fissures formed as the lava erupts and cools. When this goes down the subduction zone, the water it contains heats up and rises through the mantle above. The water and other relatively volatile materials make the mantle melt where it otherwise would not.

Continental crust

Oxygen in the air

THIS is how the first true continental crust formed. Granite and its associated rocks are much less dense than basalt, so the continental crust, once formed, floats on the denser rock below. Its low buoyancy makes it difficult to destroy, unlike the ocean crust. Rocks formed 4000 million years ago exist today. Most of the ancient cores of the continents and their crusts formed before about 2600 million years ago. And with the continents came a big change in the atmosphere. As the continents grew, so too did the area of shallow sea surrounding them. Life flourished in these shallow waters, boosting the production of oxygen. Much of the life was algal, resulting, for example in the growth of stromatolites. Algae built mounds of the mineral calcite—calcium carbonate—trapping more carbon dioxide in the hydrosphere rather than allowing it to escape to the atmosphere. The net result was a big increase in the proportion of oxygen in the atmosphere. Before this time, any oxygen had been soaked up by chemical reactions in the sea. Afterwards, there was oxygen left over to build up in the atmosphere.

Enough rocks exist from the second billion years of the Earth's life to sketch a picture of the planet then. The rocks that survive form two main types: granitic rocks together with volcanic rocks and sediments known as greenstone belts—all now metamorphosed. Lava, conglomerate, mudstone and sandstone can all be found in these ancient rocks. And there are fossils of primitive creatures such as algae. Much of the world, including the deep ocean floors, would have looked then much as it does now. The fragments of this early crust that persist today, in places such as Australia, southern Africa and Canada, consist of areas of granitic rock separated by belts of greenstone, like a mosaic. The greenstone belts are also often tightly folded, like a concertina, whereas the granitic rocks around them are less intensely distorted. The pattern is superficially like the arrangement of plates on the surface of the Earth today.

So, did the Earth then work in the same way as it does now? Evidence is sparse, but suggests that there may have been some interaction between plates of crust at this time, but nothing like the full-blown tectonics of the Earth today. There must have been something like subduction from 4000 million years ago because the ancient granites formed then as they do today. But the stable granitic areas are generally smaller than today's plates, and the greenstone belts record a different type of deformation. Mountain belts such as the Himalayas record enormous horizontal movements; their uplift is a result of two continental landmasses converging. India and Asia have moved at least 2000 kilometres together since they began to collide. The greenstone belts show plenty of evidence of uplift and sagging, but few signs of such collisions. Modern mountain belts formed by continental collisions are known as orogenic belts; they exist because of the relative movements of plates.

The oldest recognisable orogenic belts formed about 2000 million years ago. The principal reason for their absence earlier is probably heat. The Earth began its life at around 1000 °C and has been cooling since. For the first 2000 million years or so, the mantle would have been too hot for plates to behave in the rigid way they do today. The rocks would have been too runny to support big horizontal movements. As the planet cooled, the modern pattern of plate tectonics appeared and with it the constant recycling of the bulk of the Earth's crust.

Plate tectonics ensures that the ocean floor is continually being created and destroyed: none of it is more than 200 million years old. The continual subtle change in the relative movements of the plates expand and contract the ocean basins, as the continents split apart and collide. Maps of the Earth's past surface look nothing like the maps of today, yet most of the processes would be familiar to geologists. Volcanoes, earthquakes and erosion by wind and water have been present almost from the start. Mountain ranges have grown and ice ages have come and gone. Despite this continuity, Earth would have seemed an alien place for most of this history, as it lacked plants.

Living things played an enormous part in the evolution of the environment in which we live. The earliest living things known on Earth are algae, preserved as fossils in rocks some 3500 million years old. Life before this was so primitive that it did not even involve photosynthesis. For example, the earliest stages of life on Earth were probably algae and bacteria living around hot-water, or hydrothermal, vents in the deep oceans, near volcanic ridges. Later, as the atmosphere became richer in oxygen, more complex forms of life appeared in the oceans. But life on land was restricted to the simplest forms for billions of years. The first algae could have existed in decidedly hostile settings, much like the species thriving around hot springs and mud pools at Yellowstone National Park. And without plants to stabilise their banks, rivers would have developed many alternative routes across stony, barren plains—as in Alaska.

Plants were an established feature of the landscape from 350 million years ago, when the forests that would later become coal deposits flourished. Some of the trees would look strange to us but many similar species grow today. This was the start of a surface Earth that works in the same way as it does now, becoming home to larger and more complex creatures and eventually to humans. But without the interactions between the biology, physics and chemistry of the Earth, the story would have been very different.

* * *

Plate tectonics

THE surface of the Earth today is made up of half a dozen major plates, huge rafts of rock that are constantly edging past each other. These plates are thousands of kilometres across and between 10 and 100 kilometres thick. They make up segments of the Earth's lithosphere, the Earth's outer rigid layer. At their margins, rock vanishes from the surface to join the asthenosphere below and new crust forms elsewhere at the ridges that run beneath the oceans.

Plate tectonics is the framework behind the constant recycling of surface rocks that characterises the face of the Earth today. There are seven major plates—the Pacific, Antarctic, Eurasian, North American, South American, Indo-Australian and African—and many minor ones, all jockeying for position. Where two plates are moving apart, as happens down the spines of the world's oceans, constant volcanic eruptions produce lava that solidifies into new crust. Where plates slide past each other, earthquakes result. And where two plates converge, one sliding beneath the other, or both crumpling: earthquakes, volcanoes and chains of mountains result. When an ocean plate collides with a continent, as happens today along the west coast of South America, the slab of oceanic lithosphere slides below the thicker continental lithosphere because it is more dense. This structure is called a subduction zone. Worldwide, they are the source of most deep earthquakes.

Subduction can also happen when two ocean plates converge: one or the other descends as a slab. The slab stays cold and relatively rigid as it moves into the hotter mantle. As it descends it bends and eventually stretches, generating earthquakes that are distinctive because of their depth—up to 700 kilometres below the surface. Most rocks at depths of a few hundred kilometres should be too hot and therefore too plastic to fail abruptly and generate an earthquake in this way. Sediments that settled on the ocean floor are scraped into a wedge-shaped mass at the boundary between the two plates, where the additional heat and pressure causes metamorphism, which changes the minerals that the rocks contain.

Some sediment descends into the mantle as part of the ocean lithosphere. Because the sediments and the basalt of the ocean floor are wet, one of the first things that happens is that the more volatile components—water and carbon dioxide in the main—of the rocks in the slab melt. The water and volatile gases move upwards into the wedge of mantle above the slab. The mantle in this region is usually fluid, but not molten, because of the prevailing pressure and temperature. But add some water or carbon dioxide, and the mantle starts to melt. And when it melts, it produces a range of compositions of molten rock, including granite.

This is the mechanism that forms strings of volcanoes above subduction zones such as in the Andes, where the Nazca plate descends below South America. When subduction happens between two ocean plates, a line of volcanic islands forms. This is the origin of island chains such as the Marianas Islands where the Pacific plate is sliding beneath the smaller Philippines plate north of Australia.

Sue Bowler is a geologist working at the University of Leeds.

From issue 2060 of New Scientist magazine, 14 December 1996, page

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