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Recycling the Earth
17 January 1998
From New Scientist Print Edition.
Sue Bowler

For 600 million years, continents have broken up and drifted across the globe, weird and wonderful creatures have come and gone, but the Earth has maintained a calm equilibrium. This does not mean that the processes maintaining the status quo are static—far from it. Just take a look at the changes happening all around us. A biting winter wind carries particles from nearby rock or soil. A muddy river carries sediment towards the sea and waves crashing on a beach grind pebbles to sand, or slowly eat away at cliffs each year. And on the larger scale, volcanoes spew out lava adding to the volume of the continents and creating new ocean floor each year. Ocean floor vanishes, too. And all the time the plates that make up the surface of the Earth are moving a few centimetres each year.

To reconcile the long-term stability of the planet that we can see in the geological record with the short-term changes we see all around us is no easy task. Just think about the different timescales, for a start. We can track flotsam floating across the ocean and create a picture of ocean currents, we can even track these currents across the globe from satellites, but how do we combine such snapshots of ocean circulation with the waxing and waning of the sea level over millions of years?

What gives our planet its distinctive character is the continual recycling of our rocks. A process we can only understand if we study the underlying physical processes, such as erosion and combine these studies with observations of the Earth today, plus a look at the historical and geological records.

Think of the surface of the Earth now. Almost everywhere you look, the rocks are continually eroding away. This is not a trivial process. At today's rate, half the exposed sediments in the world will be destroyed in just 100 million years. But that's a mere blink of an eye in geological terms. Wind and water play their part in the physical wear and tear, but chemical effects speed the disintegration of solid rock. Slightly acid rainwater dissolves limestone by breaking the bonds between its calcium and carbonate ions. And other natural fluids can rapidly break down minerals within rocks.

Erosion dominates in high mountain areas, helped by gravity and often assisted by frost. Erosion enhances cracks and joints in supposedly solid rock at the surface of the Earth. Far underground, the rocks are under great pressure from the weight of rocks above them and the confining pressure of the rocks around them. But as they edge closer to the surface as a result of uplift and wear and tear on the rocks above, the stress in the rock decreases and changes, often becoming weaker in one direction than in others. This creates the fractures where weathering often begins.

The joints and other lines of weakness in the rock trap water that freezes, expands and enlarges the cracks. Rock falls shatter large pieces of rock into smaller fragments, mountain streams smooth and smash boulders into pebbles and the mountains slowly wear away (Figure 2).

Physical erosion, though, is only part of the story. The dramatic erosion of limestone to form deep caverns, gullies and caves, a landscape known as karst, is almost entirely chemical. The process picks up speed as channels in the rock concentrate the water flow. In other types of rock, too, chemical erosion speeds up the growth of cracks and hastens the disintegration of individual mineral grains. Feldspars, for example, can degenerate into clays. Although quartz is the main physical component of rocks to survive into the sea, much of the complex chemical make-up of rocks also gets there.

Once in the sea, waves and currents carry the sediment out onto the shallow sandy shelves beyond the shore zone, the so-called continental shelves. Like avalanches down mountain sides sediment tumbles off the shelves and slides down valleys or canyons, onto the deep ocean floor. One important form of "avalanche" is a turbidity current, in which a mixture of sediment and water moves as a turbulent flow: it is denser than the surrounding seawater and can travel long distances close to the ocean floor. Such currents carry sandy sediment considerable distances, one of the few supplies of relatively coarse material into the quiet world of the deep ocean.

Ancient landscapes

Ripples in the sands

SEDIMENT is accumulating on the deep ocean floor all the time, in a modest and slow way. The many tiny plants and animals that float in the seas constantly rain down fine particles, mainly carbonates and silicates, from their excrement or skeletons. Layers of sediment buildup on top of the basalt of the ocean floor. Carbonates are precipitated in the shallower parts of the deep seafloor, where they dominate. But where the seas are especially deep, the pressure of the water above means that carbonate minerals dissolve in water and no longer accumulate as sediment. Below this level, the so called carbonate compensation depth, even existing carbonate mud becomes soluble. What is left is the siliceous debris, which forms deposits of chert, a flinty, often layered material.

Although these deep-sea sediments accumulate over a large area, they don't often appear in the rock record because they are relatively thin. As we shall see, when tectonic plates run into each other these sediments are mostly carried down beneath the plates. This tends to happen within a few hundred million years.

More common are the sediments of the shallow seas and continental shelves, with deltas, estuaries and large rivers. They leave behind patterns called sedimentary structures which are distinctive enough to build up a picture of the land surface at different times in the past. The basis for understanding sedimentary structures is fluid dynamics: water and wind carry particles of sediment in different ways depending on their speed and turbulence. Ripples on the beach, sand dunes and the shapes of deltas are all a result of different sorts of flow.

Imagine water flowing over a sandy bed, perhaps a stream on a sandy beach. The sand grains jump and bounce (saltation) along with the current. A slightly faster current picks up bigger grains and all the particles are carried forward, until some obstacle intervenes. An irregularity in the bed of the stream creates an eddy in the flow and a small whirlpool develops. The water flow reverses and slows immediately downstream of the bump. Because the water is flowing more slowly some of the particles are now too heavy to be carried along, so they fall to the floor. Sediment continues to build up on the upstream face of the bump, which grows in the direction of the current. Such small irregularities are the beginnings of ripples in the sand.

The size and shape of the ripples depends on the currents that create them. Ripples formed in a current flowing in one direction, such as a stream, have a gently sloping upstream face and a steep downstream face. Ripples on a beach form when the water washes back and forth. They are symmetrical and have a less ordered internal structure. Finding symmetrical ripples in rocks tells a geologist that they formed on a beach or in shallow coastal waters washed by tides. Symmetrical ripples also give clues to the size of the sea in which they formed: the bigger the ripples, the greater the sea swell.

When rivers flow faster, the range of sedimentary structures also increases: sand dunes and sand waves form in similar ways to ripples, for example. Flow speed is not the only variable: the sizes and types of sediment in the river also play their parts. In many lowland rivers in Britain, there is little sand and the sediment is predominantly mud. In many circumstances, mud is cohesive enough to retain the marks made in it by, for example, pebbles bouncing along the riverbed. These are called tool marks, and if you find several of them together in a rock you can tell the direction the river flowed. Rocks on the riverbed can also disrupt the water flow and produce eddies, just as in a sandy stream. But if there is no sand to deposit, the eddies scour depressions in the river-bed; these flute marks, like tool marks, show the direction the river flowed.

Together, these observations of sedimentary structures make it possible for geologists to build up a map of the Earth's surface. And because the rocks preserve a sequence of structures through time, they can trace the waxing and waning of rivers and oceans over millions of years.

The same pattern of sediment accumulation, burial, then uplift and erosion, happens across the world. This is why it is common to find fossils of sea creatures in rocks from the Pennine landscape of England to the top of Mount Everest. They formed under the sea, were buried and became rocks, then rose to the surface, where they are now eroding. The pattern ties in with the large-scale landscape of ocean basins and continents. So it makes sense to look to the framework of the crustal plates to understand the shifting structures of the Earth's surface. These plates are vast sheets of the outer rigid layer of the Earth. They float on the hotter, denser layers below, and constantly shift relative to each other.

Plate movements are a useful framework for understanding these cycles, because they can be measured now and traced back through geological history. The key to this historical jigsaw is the continual formation of the ocean floor. Ranges of volcanic mountains where the Earth's crust splits, or rifts apart, form the spines of the ocean floors. Here volcanoes steadily erupt lava, perhaps a third of a cubic kilometre a year along the Mid-Atlantic Ridge alone. It is like a conveyor belt, always moving from the ridges to the shores of the oceans, but a conveyor belt in which the belt itself is formed at the central point. As the rocks solidify, iron-rich minerals in them acquire magnetism that matches the Earth's magnetic field at the place and time where they form. The movement of the conveyor belt means that the rocks act like a tape recorder as time goes by: when the magnetic field reverses, so that north becomes south, as it has done many times in the past, the magnetism recorded in the rocks changes too (see Inside Science No. 26).

Rocks on the move

Plate tectonics

SO the oceans of the world have "stripes" in their rocks—in some the polarity of the magnetic field is normal and in others it is reversed. The longer the period of normal or reversed polarity, the wider the stripe on the ocean floor. And because the reversals do not take place at regular intervals, a distinctive pattern is left which lets us run our imaginary conveyor belt back to discover the shapes and sizes of the oceans in the past. If you know where the oceans are, you can also reconstruct the positions of the continents.

The pattern of plate movement back through the years reveals cycles in which the continents reassemble to form one huge landmass, Gondwana or Pangaea, for example, then rift and separate again (Figure 4). This separation in itself has major consequences for the volume of the oceans. More mid-ocean ridges in the oceans take up more space, displacing the water onto the continental shelves and the land, creating what we see in the geological record as a high sea level. And when there are many small continents, the shallow seas around them take up a proportionately greater area than when all the continents combine into one supercontinent.

But, such major changes in the configuration of the continents don't just affect the sea. The distribution and position of land masses play a huge part in the climate, affecting air circulation and rainfall.

Plate tectonics is one of the main mechanisms for recycling rocks through time. Although the floor of today's oceans is 200 million years old, we have fossils of sea creatures from long before this. Indeed the Earth and the oceans themselves are hundreds of millions of years older. How can this be? Although the ocean floor has been forming for 2500 million years it is also being destroyed. We know the ocean floor is being recycled because there is no sign that the Earth has had to expand its radius to accommodate the enlarging ocean floor at the mid-ocean rifts.

So where has all the ocean crust gone? Back to where it came from. Some continents ride across the face of the Earth with the oceans, slowly getting further from the central ridge where the crust forms. This is what is happening to Europe and the Americas, on each side of the Atlantic Ocean. But other continental edges seem to swallow the ocean crust as it grows, riding over the moving edge of the ocean floor.

Continental crust is made of rocks that are much less dense than ocean crust and so they float higher on the hotter, denser layer below. As a consequence, when the spreading ocean floor and the continental crust collide, the denser ocean crust slides beneath the continental one into the mantle below (Figure 1). The region where the ocean crust nudges under the edge of the continent is called a subduction zone. This is what is happening on the Pacific coast of South America (see Inside Science No. 96). You can trace the path of the subduction zone by mapping earthquakes that begin under the Andes. The ocean crust is cool when it starts to descend into the hotter mantle. As it moves and bends, it deforms in a series of short sharp shocks, as if it is cracking; this is what we see as earthquakes, and they mark a dipping zone called a Benioff Zone. In some cases these zones reach down 500 to 700 kilometres into the crust. The sense of movement on these faults shows that they generally start from the slab sliding downwards. The earthquakes continue until the slab warms up enough to flow rather than crack. So a fast-moving slab will descend deeper than a slow-moving one. In some cases, the sense of movement shown by deep earthquakes indicates vertical stretching—as if the bottom of the slab were dropping off.

Metamorphic changes

Signs of deep stress

GEOLOGISTS use seismic tomography to make images of the descending slabs. It depends on the seismic properties of the slabs differing from those of the surrounding mantle. It works in much the same way as medical tomography but depends on seismic waves from earthquakes rather than X-rays. After the waves have passed through the area studied, they are picked up by an array of receivers. A computer processes the signals, building up a picture of the slabs descending into the mantle.

Parts of the slabs seem to survive as far down as the boundary between the core and the mantle. The layer is known as the D'' boundary, and has been called a slab graveyard. As to the slabs, are they assimilated into the deep mantle or circulate as discrete chunks? We don't yet know.

What is certain is that not all the material that dives down into a subduction zone goes all the way down. Instead some of it is scraped back into overlapping sheets on the seafloor (inset Figure 1). Some of these wedges of sediment build up to form islands: parts of Japan began in this way.

The sediments that do descend are among the first parts of the slab to respond to the increasing temperature. Volatile materials such as water and carbon dioxide respond to the increasing heat by moving upwards into the mantle rocks. Here they change the composition of these rocks enough to make the mantle melt. And magma or molten rock moves upwards and eventually erupts from volcanoes. Japan's volcanoes are the result of this mechanism, the simplest and most obvious recycling of rocks from sediment to volcano.

There is also a more subtle recycling going on around subduction zones. At these boundaries rocks are being transformed from one type to another by heat or pressure or both. So metamorphism is a sure sign that rocks have been buried to great depths and returned to the surface. In many cases, the combinations of minerals that are stable in different conditions of temperature and pressure can be deduced, either from experiments or from theoretical calculations. Metamorphic rocks provide evidence of otherwise hidden movements deep in the Earth. Certain rocks in the Alps, for example, contain coesite, a type of quartz that forms only under very high pressure.

Metamorphic rocks around the world began life either as igneous rocks, which crystallised from molten rocks, or as sedimentary rocks. Rocks that were once deeply buried are uplifted to form the raw material for sediments once again. There is a continual recycling of rocks from the surface of the Earth into its depths.

So the movement of the plates seems to be driving all the changes we see at the surface: they control where new rocks form, and where the ocean floor is swallowed up. And, on an even longer timescale, plate collisions drive metamorphism and the uplift of mountains. But what drives the plates in the first place?

The Earth is a thermally and dynamically active planet, internally as well as externally. The layered structure of the Earth and the steady movements of the plates are a result of the continual cooling. The plates move as a result of huge convection currents, stirring the mantle and gradually bringing heat from the interior of the planet to the surface. The layering of the Earth insulates the core from the surface, but even so, the planet is cooling. The outer core of the Earth is liquid and the inner core is solid: iron is solidifying on the inner core at the rate of a million kilograms of iron per second. This sounds like a lot, but in fact the core is solidifying by less than a tenth of one per cent of its size every million years. Even at this slow rate, it will be completely solid in 10 000 million years.

When this happens, the whole engine that drives the Earth will grind to a halt. There will be no more heating from below to drive convection in the mantle. Plates will slow and stop. Volcanoes will all become extinct. Earthquakes will continue for longer as the crust cools and becomes thicker. But the Earth will no longer be a hospitable planet when the core solidifies totally. For its magnetic field will decay and the magnetosphere, the area around the Earth where our magnetic field dominates the Sun's, will have gone. We will have no protection from radiation and the charged particles of the solar wind.

But this is the least of our worries. The Sun will probably have changed beyond all recognition in 10 000 million years— it is due to start burning helium instead of hydrogen in about 5000 million years. When that happens our star will expand into a red giant, engulfing the Earth—rocks, plates, core, us and all.

* * *

Water transport and graded deposition

FRAGMENTS of material are carried away from the mountains by the streams and gradually worn down into sediment as they move down the watercourse: only the finest particles reach the sea. Rivers carry this sediment by bouncing particles along their beds, saltation, or in suspension. The faster the river flows, the more material can be carried.

Different minerals from the original rocks respond differently to the hurly-burly of the riverbed. Quartz and feldspar are the two most common minerals in continental rocks. While quartz is made of silicon and oxygen, feldspar also has aluminium and calcium, potassium and/or sodium, and both have crystal structures classified as frameworks.

But this is where the similarity ends. In rivers, quartz wears into rounded pebbles, then to rounded grains that survive in the sediment to reach the sea. But because they have three strong planes of cleavage, feldspar crystals break easily into fragments shaped roughly like bricks. The chunks soon break down into smaller pieces and then into very fine particles and clay which does not travel far in rivers.

In fact, most of the sediment in the sea around the edges of the continents is quartz. And if you go to a beach you will find that much of it is made of those same rounded quartz grains. There will be shell fragments and particles of the nearby cliffs too, but much of the material that reaches the sea has survived from erosion of rocks far inland.

* * *

Changing shapes of deltaic structures

Another key to reconstructing the past world is the arrangement of sediment on a larger scale than individual layers. This often involves building up a picture of the landscape at different times as the rocks formed. Rivers carry and deposit different types of sediment at different stages of their journey from mountains to the sea. Conditions in the sea are a big change from the river. For a start, there is no longer a channel in which to flow: the water and its sediment spreads out and the flow slows down. As it slows, the sediment it carries drops to the seafloor and a delta begins to form where the river meets the sea. Once a wedge-shaped mass of sediment has built up on the seafloor, the delta develops its own geography, depending on the balance between the amount of sediment, how fast the current is flowing in the river and the effects of waves and tides carrying material away.

Most deltas exist in a delicate balance between the sediment deposited by the river and erosion by waves and tides. Often these combine to produce lines of barrier islands offshore, as at Venice. Big deltas are heavy enough to sink under the weight of the sediment: regular seasonal floods from the rivers deposit enough sediment on the top of the delta to maintain the balance. But this balance is easily disrupted. A regular influx of sediment on the lands around a delta make them fertile and valuable for farming. They remain valuable as long as the flooding continues. Attempts to reduce the flooding—often to save lives, when many people live on the delta lands—makes the land less fertile. And without regular deposits of sediment the land gradually sinks.

The changing shapes of deltas can be seen maps of the surface of the Earth and in the record of the rocks below ground which show the history of a delta, followed by its abandonment and subsidence.

Sue Bowler is a geologist working at the University of Leeds and editor of Astronomy & Geophysics.

From issue 2117 of New Scientist magazine, 17 January 1998, page

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