Structure of the Earth and plate tectonics

The Earth as a planet, and the Moon as its satellite

The Earth is one of the planets that orbit the Sun, travelling around it once every 365.25 days at an average distance of about 150 million km. It spins on its axis as it goes, which is why we have day and night, and its axis is tilted, which is why we have seasons. The Sun is a star, the Earth is a planet, and the Moon is a natural satellite: a body that orbits a planet rather than the Sun.

The Moon orbits the Earth once every 27.3 days at a distance of about 384 000 km, and it is roughly a quarter of the Earth's diameter. It has no light of its own; we see it because it reflects sunlight, and the changing angle between the Sun, the Earth and the Moon produces the phases of the Moon.

What holds both orbits together is gravity. Gravity is the force of attraction between any two objects with mass, and it gets weaker with distance. The Sun is enormously more massive than the Earth, so its gravity pulls the Earth towards it; the Earth is much more massive than the Moon, so its gravity holds the Moon. Without that pull each body would travel in a straight line off into space.

Gravity does not pull the Earth into the Sun because the Earth is also moving sideways at about 30 km/s. The Sun's pull bends that straight-line motion into a curve, so the Earth falls around the Sun instead of into it. The same balance works for the Moon: it moves sideways at about 1 km/s while the Earth's gravity pulls it inwards, and the result is a stable orbit. Nothing else is needed - no push, no wind, no current.

• Earth: a planet, orbiting the Sun once every 365.25 days at about 150 million km, spinning on a tilted axis.
• Moon: a natural satellite, orbiting the Earth once every 27.3 days at about 384 000 km, and about a quarter of the Earth's diameter.
• Gravity is the force of attraction between masses; it provides the inward pull that keeps each body in orbit.
• An orbit is a balance: the inward pull of gravity plus the sideways motion of the body. Too little sideways speed and the body would spiral in; too much and it would escape.
• The Sun's gravity keeps the Earth in orbit; the Earth's gravity keeps the Moon in orbit.

Inside the Earth: core, mantle and crust

The Earth has three main layers, and you need the order, the composition and the state of each. At the centre is the core, made of iron with some nickel, and it is about half the Earth's radius. It is in two parts. The inner core is solid, even though it is the hottest part of the planet, because the pressure on it is so great that the iron cannot melt. The outer core is liquid, because the pressure there is lower, and it is this layer of moving liquid iron that produces the Earth's magnetic field.

Around the core is the mantle, about 2 900 km thick and made of solid rock rich in silicate minerals. It is hot, but the pressure keeps most of it solid, so it is not a sea of molten rock. Where the temperature is high enough, or the pressure falls, patches of the mantle melt to form magma, a viscous (thick, sticky) liquid that is less dense than the rock around it and therefore rises.

The outer layer is the crust, a thin shell of solid rock. Under the oceans it is only 5 to 10 km thick and is made of dense, dark basalt; under the continents it is 30 to 70 km thick and is made of lighter granite. Compared with the 6 371 km radius of the Earth, the crust is thinner than the skin of an apple, and it is the only layer we have ever drilled into.

Because the crust is thin, cold and brittle, it is the layer that cracks. It is also the layer that carries the oceans, so anything that happens to the crust happens to the sea floor as well - which is why the structure of the Earth is the starting point for everything else in marine science.

The magnetic field

The iron core creates a magnetic field around the Earth. The field is generated by the outer core, where liquid iron is moving: as the molten metal flows and churns, it carries electric currents with it, and those currents produce a magnetic field. The field behaves as though there were a giant bar magnet inside the Earth, tilted slightly relative to the axis of rotation, with a magnetic north pole near the geographic North Pole.

The field extends far out into space, where it is called the magnetosphere, and it deflects most of the charged particles streaming out from the Sun as the solar wind. Without it, those particles would strip away the atmosphere and expose the surface to radiation. Near the poles, some particles are funnelled down the field lines and collide with gases in the upper atmosphere, producing the aurora.

The field is what a compass needle lines up with, which is how navigators have found direction at sea for centuries. Some marine animals, including sea turtles and some whales, are thought to detect the field and use it to navigate across thousands of kilometres of open ocean.

The field is not permanent. It has reversed many times in the Earth's history, and those reversals are recorded in the iron-rich minerals of the ocean floor as alternating magnetic stripes. Those stripes are one of the strongest pieces of evidence that the sea floor is spreading.

• The field is generated in the liquid outer core, by the movement of molten iron.
• It forms a magnetosphere that deflects the solar wind and protects the atmosphere.
• A compass works because its needle aligns with the field; the field has reversed many times in Earth history.
• Magnetic stripes in ocean-floor rocks record those reversals and support the idea of sea-floor spreading.

Tectonic plates and convection currents

The crust is not one unbroken shell. It is made up of a number of tectonic plates: about a dozen large ones and many smaller fragments, fitted together like the cracked shell of an egg. Each plate is a rigid slab of crust (together with the very top of the mantle beneath it) and each one carries either ocean floor or continent or both. The Pacific plate is the largest and carries most of the Pacific Ocean; the Nazca and Cocos plates lie off the west coast of Central and South America; the Indo-Australian plate carries India and Australia; the Eurasian, African, North American, South American and Antarctic plates carry the other continents; and the Philippine, Caribbean and Arabian plates are smaller ones.

The plates float on the mantle. The rock below them is hot enough to deform and flow extremely slowly, like a very stiff tar, so the rigid plates above can slide across it. They are not fixed: they move a few centimetres a year, about the speed at which a fingernail grows, which is slow enough to be invisible in a human lifetime but fast enough, over millions of years, to move a continent across an ocean.

The theory that explains their movement is that the plates are carried by convection currents in the mantle. Heat from the core, plus heat released by the radioactive decay of rocks in the mantle, warms the rock at depth. Hot rock expands, becomes less dense and rises; as it rises it cools, becomes denser and sinks again. This circulation repeats, and the moving rock drags the base of the plates along with it. Where hot mantle rises it pushes plates apart; where cooler, denser material sinks, it pulls plate material down with it.

The evidence for all this is now overwhelming. GPS stations measure the plates moving at a few centimetres a year, in the directions the theory predicts. Earthquakes and volcanoes are concentrated in narrow belts along the plate edges rather than scattered at random. The ocean floor is youngest at the mid-ocean ridges and gets older towards the trenches, and it carries magnetic stripes that match on either side of a ridge. The oldest ocean floor is only about 180 million years old, while continental rocks are billions of years old, because ocean floor is constantly being created at ridges and destroyed at trenches.

Supercontinents and the map we have now

The present-day continents and oceans are the result of the break-up of supercontinents and the movement of tectonic plates over hundreds of millions of years. A supercontinent is a single landmass made of most or all of the continents joined together. The best known is Pangaea, which existed about 250 million years ago and brought Africa, South America, Antarctica, Australia, India and North America into one block, surrounded by one vast ocean.

Pangaea began to break up about 200 million years ago. Rifts opened along what are now the edges of the continents, magma rose into the gaps, and new ocean floor was created as the pieces moved apart. South America and Africa separated and the South Atlantic opened between them; North America pulled away from Africa and Europe, opening the North Atlantic; India moved north across the ocean and eventually collided with Asia; and Australia and Antarctica parted company. Each of those movements is still going on today, at the same few centimetres a year.

The evidence is the same evidence Alfred Wegener used when he proposed the idea of moving continents in 1912. The coastlines of South America and Africa fit together like two pieces of a jigsaw, and so do their rock sequences and mountain belts. The same fossils are found on continents that are now separated by thousands of kilometres of ocean - fossils of animals and plants that could not have crossed a salt sea, such as the freshwater reptile Mesosaurus and the seed fern Glossopteris. Glacial deposits of the same age occur in what are now tropical regions, and coal seams formed from tropical forests are found in Antarctica. Together these only make sense if the continents were once joined and have since moved apart.

This matters for marine science because ocean basins are young and temporary. The Atlantic is widening by a few centimetres a year, the Pacific is being consumed at its edges and is shrinking, and the Mediterranean is the last remnant of a much larger ocean that has almost closed. Over geological time the map of the oceans is redrawn, and with it the patterns of currents, climate and marine life.

• Supercontinent: a single landmass made of most or all of the continents joined together.
• Pangaea: the supercontinent that existed about 250 million years ago and began to break up about 200 million years ago.
• Evidence: matching coastlines, matching rock sequences, identical fossils (Mesosaurus, Glossopteris) on continents now far apart, and glacial deposits in today's tropics.
• Consequence: ocean basins are geologically young - the oldest ocean floor is about 180 million years old, while continents are billions of years old.

Divergent and convergent plate boundaries

A plate boundary is where two plates meet, and almost everything in this topic follows from which way they are moving. There are three types in this syllabus: divergent, where plates move apart; convergent, where they move together; and transform, where they slide past each other.

At a divergent boundary the two plates move away from each other. The gap that opens is filled by hot mantle rock rising towards the surface, which partly melts as the pressure on it falls and erupts as runny basaltic lava. The lava cools and solidifies to form new crust, so the ocean floor grows outwards from the boundary, a process called sea-floor spreading. Under the ocean this builds a long underwater mountain chain, a mid-ocean ridge; where the same process happens under a continent, the crust stretches, cracks and drops between faults to form a rift valley. The earthquakes are shallow and usually moderate, because the crust is thin and weak and breaks easily, and the volcanoes are quiet and effusive rather than explosive. The Mid-Atlantic Ridge is the classic example, and Iceland sits directly on it.

At a convergent boundary the two plates move towards each other and one is forced down beneath the other. Oceanic crust is denser than continental crust, so an oceanic plate is always the one that sinks, in a process called subduction. As it bends down it forms a deep ocean trench, the deepest parts of the sea floor. It also carries water and sediment down with it, and that water lowers the melting point of the mantle rock above, so the mantle partially melts and magma rises through the plate above to form a line of volcanoes set back from the trench. Where the volcanoes are under the sea they build island arcs; where they rise through continental crust they build high fold mountains, as along the Andes. Because the sinking slab is cold and brittle far down, the earthquakes can be very deep, reaching hundreds of kilometres, and because the magma is sticky and gas-rich, the eruptions are explosive.

The two boundaries are opposites in almost every respect, and the link between the boundary and the style of eruption is one of the most examined ideas in the topic: divergent margins give runny basalt, quiet eruptions, shield volcanoes and shallow earthquakes; convergent margins give sticky, gas-rich magma, explosive eruptions, composite volcanoes, trenches and deep earthquakes.

Transform boundaries, earthquakes and where volcanoes happen

At a transform boundary the two plates slide past each other horizontally, either in opposite directions or in the same direction at different speeds. No crust is created and none is destroyed, nothing is subducted, and no water is carried down to make the mantle melt. No magma is produced, so there are no volcanoes at all. What does happen is friction: the rough edges of the plates catch and lock, the rock on either side bends and stores strain for years or centuries, and then it suddenly snaps or jerks forward, releasing the stored energy as an earthquake. These earthquakes are usually shallow and can be very destructive because they happen right under the surface. The San Andreas Fault in California, where the Pacific and North American plates slide past each other, is the standard example.

That gives three different ways of making an earthquake. At a divergent boundary the thin crust cracks as it is pulled apart, giving shallow quakes. At a convergent boundary the plates lock against each other and the sinking slab snaps, giving earthquakes at depths from shallow to hundreds of kilometres. At a transform boundary the plates stick and slip, giving shallow, often violent quakes. In every case the cause is the same: strain builds up in brittle rock until it breaks, and the sudden release radiates seismic waves.

Volcanoes have a narrower set of causes. Magma forms either because pressure falls as plates move apart (divergent), or because water carried down by a sinking plate lowers the melting point of the mantle (convergent). That is why volcanoes occur only at those two boundary types, and why a map of the world's volcanoes is a map of plate boundaries - with a few exceptions such as Hawaii, which sits over a hot spot in the middle of the Pacific plate.

Put the boundaries on a map and the pattern is obvious. The Pacific Ring of Fire is a horseshoe of trenches, island arcs and explosive volcanoes around the Pacific, from New Zealand through Japan, the Philippines and Indonesia, up through the Aleutians and down the west coast of the Americas. The Mid-Atlantic Ridge runs down the middle of the Atlantic, reaching the surface in Iceland and the Azores. The Alpine-Himalayan belt runs from the Mediterranean through Turkey and Iran to the Himalayas and Indonesia, where continental plates collide and slide past each other, so it produces earthquakes and fold mountains rather than many volcanoes.

• Transform boundary: plates slide past each other; no crust created or destroyed; no magma, so earthquakes but no volcanoes - San Andreas Fault.
• Earthquake mechanism: rock locks, bends and stores strain, then breaks or jerks, releasing energy as seismic waves.
• Volcanoes occur only where magma is made: at divergent boundaries (pressure falls) and convergent boundaries (water lowers the melting point).
• Pacific Ring of Fire: the most active belt on Earth, with trenches, island arcs and explosive volcanoes.
• Mid-Atlantic Ridge: a divergent boundary down the middle of the Atlantic, reaching the surface in Iceland.

How a tsunami is formed

A tsunami is a series of waves caused by a sudden displacement of a very large volume of sea water. It is not a tidal wave and it has nothing to do with tides, wind or storms: it is caused by the sea floor or the water above it moving suddenly, so that the whole depth of the ocean is lifted or dropped.

The commonest cause is a shallow-focus earthquake under the sea, usually at a convergent boundary. The two plates have been locked together for years, storing strain, and when they finally slip the sea floor jerks upwards or downwards by several metres over hundreds of kilometres. That sudden movement pushes the water above it out of place, and gravity then pulls the displaced water back, setting the whole water column in motion. A large submarine landslide, or the collapse of a volcanic island, can displace the water in the same way, and so can a meteorite impact, but earthquakes are by far the most frequent cause.

In the open ocean the wave is almost invisible. It has a very long wavelength - often hundreds of kilometres - and a very small height, usually less than a metre, so a ship in deep water passes over it without noticing. But because the whole depth of water is moving, it travels extremely fast, up to about 800 km/h, as fast as a jet airliner. The wave front spreads out across the ocean basin, so a tsunami generated off one coast can reach another continent hours later.

The wave becomes dangerous only when it reaches the shore. As the water shallows, the front of the wave slows down, the wavelength is compressed and the energy is squeezed into a much smaller volume of water, so the wave grows in height - sometimes to more than 10 metres. This is called shoaling. The water may first be drawn back from the beach, exposing the sea floor, and then arrive as a wall of water and debris that runs inland, and there are usually several waves, the later ones often larger than the first. The time between the earthquake and the arrival of the first wave is the only warning anyone gets.

Effects of tsunamis on marine ecosystems and coastal communities

Effects on marine ecosystems. The wave itself is only part of the damage; the debris it carries and the water it drags back do the rest. Coral reefs are smashed by broken timber, concrete and vehicles hurled across them, and the backwash carries rubble and sand that smothers the surviving coral. Seagrass beds and mangroves are uprooted or buried under sediment, and mangrove forests that had taken decades to grow can be stripped in minutes. Nesting beaches used by sea turtles are eroded away or covered in debris, and the eggs and nests in them are destroyed. The surge of salt water and sediment changes salinity and turbidity for weeks, and the sediment settles over the sea bed, smothering bottom-living organisms and the small fish and invertebrates that other species feed on. The loss of these nursery habitats reduces the numbers of juvenile fish and, in the following years, the catch of the coastal fisheries that depend on them.

Recovery is possible but slow. Coral reefs may take decades to recover, and only if the water quality returns to normal and there are healthy reefs nearby to supply larvae. Mangroves and seagrass can regrow faster, but only where the sediment has stabilised and there is no further disturbance from people. Where a tsunami is followed by rebuilding, dredging, sand mining and new coastal construction, the habitats are damaged a second time.

Effects on human coastal communities. The immediate effect is death and injury, from drowning and from being struck by debris, and the number of deaths depends on how many people were on the coast, whether they had a warning and whether they knew to move to high ground. Homes, schools, hospitals, roads, bridges, harbours and boats are destroyed by the water and the debris. Fishing communities lose their boats, nets, engines and processing buildings in a single morning, so the livelihood goes even where the houses survive. Salt water floods the soil and the wells, so crops fail and drinking water becomes salty for months or years, and the stagnant water left behind breeds disease. People are displaced into temporary shelters, sometimes for years; families lose their documents, savings and possessions; and the cost of rebuilding falls on governments that must also pay for relief.

The effects are not evenly spread, and that is what examiners want you to notice. A mangrove belt or a coral reef in front of a village absorbs energy and reduces the height of the wave reaching the shore, so places that have kept their natural coastal defences often suffer less. A community with a tsunami warning system, rehearsed evacuation routes and buildings on high ground loses far fewer people than one without, even when the wave is the same size. Poverty, poor building quality and the loss of coastal vegetation turn a hazard into a disaster.