Tectonic processes, weathering, tides and ocean currents

The structure of the Earth, and the theory that explains it

The Earth has three layers you must be able to name and describe: a thin outer crust, a thick mantle and a core. The crust is the brittle skin of rock under your feet and under the sea, the mantle is the vast hot layer beneath it, and the core is the metal centre. Almost everything in this topic happens because heat escapes from the core outwards, so start with the layers and with the way that heat moves.

Oceanic crust is 5-10 km thick, made of dark, dense basalt with a density of about 3.0 g cm-3. It is created at mid-ocean ridges and destroyed at trenches, so none of it is older than about 200 million years. Continental crust is 30-70 km thick, made of lighter granite with a density of about 2.7 g cm-3, and it is far older - parts of it are billions of years old. That density difference is the most useful fact in the whole topic: when oceanic and continental crust meet, the oceanic plate is the one that sinks.

The mantle is about 2 900 km of hot rock. It is mostly solid, but at the temperatures and pressures down there it flows extremely slowly, like a stiff tar, and heat from the core plus heat from radioactive decay sets it circulating in convection currents. Hot rock rises, spreads sideways, cools, becomes denser and sinks again. The rigid outer shell of the Earth, the crust together with the top of the mantle, is broken into plates that ride on this slowly moving rock.

The core is iron and nickel. Its outer part is liquid and its swirling motion generates the Earth's magnetic field, which is why a magnetic record is preserved in the rocks of the sea floor; the inner part is solid because the pressure there is immense.

Plate tectonics is the theory that this rigid outer shell is broken into a number of plates that move relative to one another over the mantle, carrying the continents with them. It explained an older idea, continental drift, which had proposed that the continents had once been joined and had since moved apart. Plates move a few centimetres a year, driven by convection in the mantle dragging at their base and by the pull of a dense subducting slab at their edge.

The jigsaw fit of the coastlines was the first clue: South America and Africa fit together, and the fit is even better if you match the edges of the continental shelves rather than today's shoreline. Geological matching is stronger evidence, because rock sequences of the same type and age continue from one continent to another. The Appalachian mountains of North America line up with the Caledonian mountains of Scotland and Scandinavia, and coal seams and glacial deposits of the same age are found in South America, Africa, India, Antarctica and Australia.

Fossils and living organisms show the same pattern. Mesosaurus, a small freshwater reptile, is found in rocks of the same age in Brazil and South Africa, and the seed fern Glossopteris is found across the southern continents. A freshwater reptile cannot cross an ocean and the fern's large seeds could not be carried across one on the wind, so the continents must once have been joined. Related flightless birds and similar freshwater fish and earthworms on separate continents point the same way.

Paleomagnetic stripes are the evidence that settled the argument, because they were predicted by the theory before they were found. As basaltic lava erupts at a ridge and cools, its iron minerals lock in the direction of the Earth's magnetic field at that moment. The field reverses polarity every few hundred thousand years, so the new crust is magnetised in alternating normal and reversed stripes. Because new crust forms at the ridge and is carried away on both sides, the stripes form a symmetrical pattern of matching bands either side of the ridge axis, and dating shows the crust gets older the further it lies from the ridge. The sea floor is a tape recording of its own spreading, and the two sides play the same tape.

• Crust: oceanic 5-10 km of basalt, density about 3.0; continental 30-70 km of granite, density about 2.7.
• Mantle: about 2 900 km of hot silicate rock; solid, but flows over long time scales; convection here moves the plates above.
• Core: iron and nickel; the liquid outer part generates the magnetic field, the inner part is solid under immense pressure.
• Four pieces of evidence for plate tectonics: the jigsaw fit of the coastlines, matching rock formations on different continents, the distribution of similar fossils and living organisms, and paleomagnetic stripes on the ocean floor.

The three plate boundaries and the features they produce

A plate boundary is where two plates meet. The syllabus names three types - convergent, divergent and transform - and what happens at each follows from the direction of movement and from the density of the plates involved.

At a convergent boundary the plates move towards each other and one is destroyed by subduction. Where an oceanic plate meets a continental plate, or another oceanic plate, the denser oceanic plate bends and sinks. The line along which it goes down is marked by an ocean trench, the deepest part of the ocean floor. The sinking slab carries water and sediment with it, and that water lowers the melting temperature of the mantle above, so magma rises and erupts to form a line of volcanoes set back from the trench - a volcanic island arc where both plates are oceanic, a mountain chain such as the Andes where the overriding plate is continental. Earthquakes occur all along the descending slab, from shallow near the trench to very deep beneath the arc. Where two continental plates collide, neither sinks far, and the edges are crumpled upwards into fold mountains instead.

At a divergent boundary the plates move apart. Hot mantle rock rises into the gap, partly melts as the pressure on it falls, and erupts as runny basaltic lava that cools to make new oceanic crust. The result is a mid-ocean ridge: a broad underwater mountain range thousands of kilometres long, with a rift valley along its crest. Eruptions are quiet, earthquakes are shallow and comparatively small, and the new crust is hot and fractured, so seawater circulates through it and emerges at hydrothermal vents. The Mid-Atlantic Ridge is the classic example, and Iceland is the place where it stands above sea level.

At a transform boundary the plates slide past one another horizontally. No crust is created or destroyed and no rock melts, so there are no volcanoes; instead the plates lock, strain builds in the rock on either side, and the release is a shallow earthquake. Transform faults also cut across mid-ocean ridges at right angles, offsetting the ridge into segments.

Abyssal plains are the flat stretches of deep ocean floor, 3-6 km down, that make up much of the ocean basin. They are flat because sediment has buried the original rugged volcanic topography, with fine particles settling from the surface water and mud and sand carried downslope by turbidity currents, and the sediment is thickest closest to the continents.

A tsunami is produced when the sea floor is displaced vertically, usually by a shallow earthquake at a convergent boundary where the overriding plate snaps upwards, or by a submarine landslide. The whole water column is lifted, so in the open ocean the wave has a very long wavelength and a height of only a metre or so, and it crosses the ocean at hundreds of kilometres an hour without anyone noticing. As it reaches shallow water it slows, the wavelength shortens and the height increases, and it arrives as a series of surges that flood low-lying coast.

Hydrothermal vents: the plume and the chimneys

Hydrothermal vents are springs on the sea floor where hot, mineral-rich water flows out of the crust. They are found along mid-ocean ridges and in back-arc basins, wherever the new crust is hot, fractured and thinly covered by sediment. Cold seawater seeps down through the cracks, is heated by the hot rock and magma beneath, and reacts with it on the way.

The water that comes back out is under pressure, hot and rich in dissolved nutrients. At 2 000-3 000 m the pressure is hundreds of times atmospheric pressure, so the water does not boil even at 300-400 °C; it stays liquid and expands, which makes it buoyant and drives it quickly out of the vent. During its passage through the crust it has lost oxygen and sulfate to the rock and picked up hydrogen sulfide, methane, silica and metals such as iron, manganese, copper and zinc, along with nutrients such as nitrate and phosphate.

When the jet of hot fluid meets cold seawater it forms a plume. The dissolved metals react with sulfide and precipitate as fine dark particles, which is why a black smoker looks as though it is smoking, and the plume rises hundreds of metres before spreading sideways and drifting with the current. The plume water is warmer, fresher, more turbid and richer in metals and nutrients than the surrounding seawater, so each of these can be measured as an anomaly against the background.

That is why the effects of a vent can be detected some distance from the site: the plume is carried away and diluted, but its temperature, turbidity, metal and nutrient signatures can still be traced kilometres from the vent. Plume surveys are how oceanographers locate new vent fields, and the same measurements show that vent water exports nutrients and chemicals into the wider ocean.

Chimneys form because the solubility of the dissolved salts falls as the fluid cools. When the hot, acidic, metal-rich fluid meets cold seawater at the vent opening it is chilled almost instantly, and the metal sulfides and silica that dissolved in the hot water become insoluble and precipitate, along with calcium sulfate. The particles settle on the rim of the vent and on one another, and the deposit builds upwards and outwards into a chimney, usually with a narrow throat through which the fluid still flows.

Growth is fastest while the flow is strong and steady, and it can be rapid in the early life of a vent. If the flow shifts to a new opening, or the throat becomes blocked, the chimney stops growing and seawater begins to dissolve and erode it, so old chimneys are often dead, crumbling stacks.

• Vent fluid: under high pressure (so it does not boil), 300-400 °C, acidic, oxygen-poor, rich in sulfide, methane, silica, metals and nutrients.
• Plume: forms where the hot jet mixes with cold seawater, rises hundreds of metres, then spreads laterally with the current.
• Detection: temperature, salinity, turbidity, metal and nutrient anomalies can be traced kilometres from the vent.
• Chimney: salts precipitate because their solubility falls as the fluid cools, and the deposit grows around the vent throat.

Weathering: three types, and how it differs from erosion

Weathering is the breakdown of rock in place, by the action of the atmosphere, water and organisms. Nothing is carried away - the products stay where they formed. Erosion is the removal of that material and its transport by a moving agent such as water, ice, wind or gravity. The two work as a pair: weathering prepares the material and erosion takes it away, which is why a cliff can be weathered without being eroded if the sea is calm.

Physical weathering breaks rock into smaller pieces without changing its chemistry. The most important type on a coast is freeze-thaw: water gets into a crack, freezes, expands by about 9 per cent and prises the rock apart, and repeated cycles shatter it into angular fragments. Salt crystallisation is the marine version: seawater soaks into pores and cracks, the water evaporates, salt crystals grow, and the pressure of the growing crystals flakes the surface. Wetting and drying, heating and cooling, and the release of pressure as overlying rock is removed all act in the same way.

Chemical weathering changes the minerals themselves, usually by reaction with water and the gases dissolved in it. Carbonation dissolves limestone: rainwater with carbon dioxide dissolved in it forms weak carbonic acid, which reacts with calcium carbonate to make calcium bicarbonate, which is soluble and is carried away. Hydrolysis attacks silicate minerals such as feldspar and turns them into clay. Oxidation reacts with iron minerals to produce the rust-coloured iron oxides that crumble so easily. Solution simply dissolves soluble minerals such as halite. Chemical weathering is fastest where it is warm and wet.

Organic weathering is caused by living things. Plant roots widen cracks as they grow, burrowing animals break up the rock and bring fresh material to the surface, and on the shore piddocks and other bivalves bore into soft rock, sponges and worms bore into limestone and shells, and limpets rasp algae off the surface and take grains of rock with it. Algae and lichens also hold water against the rock and speed up chemical attack.

Erosion, transport and sedimentation

The syllabus names four agents of erosion: ice, water, wind and gravity.

Ice erodes by plucking and abrasion as a glacier slides over bedrock, and it carries the material far from its source before depositing it as till, or as outwash sorted by meltwater. Where a glacially deepened valley is flooded by the sea it becomes a fjord, and unsorted clay and boulders on a coast are a sign of ice rather than waves.

Water is the most important agent in the sea. Waves erode by hydraulic action, where air trapped in a crack is compressed by the wave and expands explosively as the water retreats; by abrasion, where sand and pebbles grind the rock; by attrition, where the moving particles wear one another down and become smaller and rounder; and by solution, where the rock is dissolved. Rivers carry the products of weathering to the coast, and tidal currents and longshore drift move them along it.

Wind erodes by deflation, lifting loose sand and silt, and by abrasion as sand is blasted against rock. On a coast it moves sand inland to build dunes and strips the finer material off the beach surface.

Gravity moves material downslope as mass movement - rockfalls, landslides, slumps and mudflows. It is what carries debris from the foot of a cliff to the shore, where the waves can then pick it up.

Sedimentation is the deposition of suspended particles. Whether a particle is removed, transported or deposited depends on its size and on the speed of the water. Fast water has the energy to lift and carry large particles; as the flow slows it loses the energy to hold them, and the largest and densest grains settle first, then the sand, then the silt, and finally the clay. The result is sorting: coarse sediment close to its source or high on the beach, finer sediment further out or in sheltered water.

Very fine clay can stay suspended in almost still water, but in seawater the particles clump together as the dissolved salts neutralise the charges on their surfaces, and the floccules settle far faster than single grains. That is why mud can accumulate in estuaries and sheltered bays where the current is never completely still.

The littoral zone and the five shore types

The littoral zone is the intertidal region of a shoreline, between the highest and lowest spring tide marks. It is the strip that is covered and uncovered twice a day, so its organisms have to survive drying out, sharp changes in temperature and salinity, and the direct force of the waves. The syllabus gives five examples to know: rocky shores, sandy shores, muddy shores, estuaries and deltas.

The morphology of every one of them comes from the same balance. Where erosion outpaces deposition, rock and sediment are removed and a steep, rocky shore develops. Where deposition outpaces erosion, sediment builds up and a sandy or muddy shore develops. Where the two are close, the shore shifts with the seasons and the weather, and the same beach can be cut back in winter and rebuilt in summer.

Rocky shores form where waves are strong and the land is hard rock. Wave action removes the weathering products as fast as weathering produces them, so there is very little loose sediment: the profile is a cliff, a wave-cut notch at its base and a wave-cut platform running out to sea, with boulders and rock pools on it. Weathering exploits joints and bedding planes, so the platform is irregular, and the steep gradient of exposure produces clear zonation of seaweeds and animals from the splash zone down to the sublittoral.

Sandy shores form where the energy is moderate and there is a supply of sand, from rivers, from eroding cliffs, or from shell and coral debris. Sand is deposited to build a gently sloping beach with a berm at the top, and the swash and backwash sort it, carrying the finest material offshore. Onshore winds build dunes behind the beach, and longshore drift moves sediment along the coast.

Muddy shores form in sheltered places where the water is calm enough for the finest particles to settle - behind a spit, in a bay, at the head of an estuary. Mud accumulates, the sediment is waterlogged and becomes oxygen-poor a few centimetres below the surface, and salt-tolerant plants colonise the upper shore to form saltmarsh. The fauna is dominated by burrowing worms and bivalves that can cope with the low oxygen.

Estuaries are where a river meets the sea and the water is brackish. The flow slows as it meets the tide, so fine sediment settles, and the salt makes the clay flocculate and settle faster still. The result is mudflats cut by channels, saltmarsh at the edges, and a turbidity maximum where the water is brown with suspended sediment. Deltas form where a river delivers sediment faster than the sea can remove it. The sediment builds out into the sea, the channel splits into distributaries, and the delta progrades seawards; the Nile and the Mississippi are the classic examples.

Tides: cause, range, spring and neap

Tides are the regular rise and fall of sea level, produced mainly by the gravitational attraction of the Moon, with a smaller contribution from the Sun, combined with the rotation of the Earth and Moon about their common centre. The Moon pulls the water on the side of the Earth nearest to it into a bulge, and the rotation of the pair throws water into a second bulge on the opposite side. The Earth turns through both bulges, so most coasts have two high and two low tides a day, about 12 hours 25 minutes apart.

The Sun's tide-raising effect is a little less than half the Moon's, because the Sun is far more massive but much further away. What decides the size of the tide is therefore not the size of either pull on its own, but whether the two pulls act together or at right angles.

Spring tides occur at new moon and at full moon, when the Earth, Moon and Sun are in a straight line, so the solar and lunar bulges coincide and the tidal range is at its greatest. Neap tides occur at the first and third quarters, when the Sun and Moon are at right angles as seen from Earth, so their pulls partly cancel, the bulges are smaller and the range is at its smallest. Spring and neap tides each occur about twice a month, and the range grows and shrinks smoothly between them.

The tidal range at any one place is not fixed by astronomy alone. Coastal geomorphology matters most: a funnel-shaped estuary or bay concentrates the incoming water and can amplify the range enormously - the Bay of Fundy has a range of more than 15 m - while a wide open coast has a small range, and a basin whose natural period of oscillation matches the tidal period resonates and amplifies it further. Wind piles water up against the coast when it blows onshore and holds it back when it blows offshore. Air pressure changes the level too: a fall of 1 hPa allows the sea surface to rise by about 1 cm. The size of the water body sets the scale, because a large ocean basin has far more water to move than a small enclosed sea such as the Mediterranean, where the range is only tens of centimetres.

A tide table gives the time and height of high and low water for a port, as heights above chart datum. The tidal range is high water height minus the following low water height. Reading a table or a graph, look for the pattern rather than the individual numbers: two highs and two lows a day, each about 50 minutes later than on the previous day, and a range that grows to a maximum at spring tides and shrinks to a minimum at neaps. A spring tide is identified by a large range and an unusually high high water, never by the time of year.

Ocean currents, upwelling and the global conveyor belt

Surface currents are driven mainly by wind. The wind drags on the surface of the sea and sets the water moving, and because the Earth is rotating, that moving water is deflected by the Coriolis effect - to the right, which is clockwise, in the northern hemisphere, and to the left, which is anticlockwise, in the southern hemisphere. The deflection is zero at the equator and greatest at the poles. Because each surface layer drags the layer below and each is deflected in turn, the net movement of the surface water is at a large angle to the wind, and the combination of wind and deflection produces the great circular gyres that fill each ocean basin.

Temperature and density drive the deeper flow. Cold water is denser than warm water, and salty water is denser than fresh, so cold, saline water sinks while warm, fresher water stays at the surface. These density differences set up a slow deep circulation that moves water between the ocean basins.

The shape of the seabed steers the currents. Water squeezed between a ridge and a continent speeds up, seamounts and islands deflect it and shed eddies, and the continental slope and the shelf edge guide it along the coast. Where a current meets the coast or another water mass it is turned or forced to mix.

Upwelling is the rise of deep water to the surface. It happens wherever surface water is moved away faster than it can be replaced: where winds blow along a coast and push the surface layer offshore, where two currents diverge, or where the flow is deflected away from the land. Deep water rises to fill the gap. Upwelled water is cold and rich in nutrients such as nitrate and phosphate, because it comes from depth where dead organisms have decomposed, so upwelling regions are among the most productive in the ocean; the Peru (Humboldt) upwelling supports one of the world's largest fisheries.

The global ocean conveyor belt is the thermohaline circulation, a single slow circuit driven by differences in temperature and salinity. Warm surface water flows polewards, gives up its heat to the atmosphere, and becomes colder and - because evaporation has raised its salinity - denser. In the North Atlantic it sinks to depth, flows south as North Atlantic Deep Water, circles Antarctica, and spreads into the Indian and Pacific Oceans, where it slowly mixes upwards and returns as a surface current. One complete circuit takes about a thousand years.

Its importance is that it mixes the water of all the oceans and moves heat, oxygen, nutrients and carbon around the planet. It carries heat northwards, which is why north-west Europe is milder than its latitude would suggest; it supplies oxygen to the deep sea, so that animals can live there; and it carries carbon into the deep ocean, where it is stored for centuries.

El Nino and La Nina: the ENSO cycle

The El Nino Southern Oscillation (ENSO) is an irregular cycle of coupled changes in the Pacific Ocean and its atmosphere, lasting about 2 to 7 years from one warm event to the next. It matters because the Pacific is the largest ocean, so a change in where its warm water sits alters rainfall and temperature around the world.

In a normal year the trade winds blow from east to west across the Pacific, piling warm surface water into the west near Indonesia and Australia and drawing cold, nutrient-rich water up along the coast of Peru and Ecuador. Convection and heavy rain sit over the warm western Pacific, the eastern Pacific is dry, and the Peruvian upwelling supports a huge anchoveta fishery.

During El Nino the trade winds weaken, and sometimes reverse. The warm water that was piled in the west spreads back eastwards, the thermocline deepens in the eastern Pacific, and upwelling there is suppressed because the wind no longer pulls surface water away from the coast. The results are a collapse in nutrient supply and plankton production, a failure of the anchoveta fishery, and a shift in the rain: heavy rainfall and flooding in Peru and Ecuador, drought and bushfires in Indonesia and eastern Australia, and coral bleaching where the water stays too warm. The very strong events of 1982-83 and 1997-98 are the standard examples.

During La Nina the pattern is reversed and intensified. The trade winds strengthen, more warm water is piled into the west, and the eastern Pacific becomes colder than normal, with stronger upwelling and higher productivity off Peru. Indonesia and eastern Australia become wetter and the eastern Pacific drier.

Because the atmosphere and the ocean drive each other, the change is self-reinforcing while it lasts and then decays, which is why the cycle is described as an oscillation rather than a one-off event. Its effects are felt well beyond the Pacific, through changed patterns of storm tracks, monsoon rainfall and hurricane activity, and it leaves a detectable mark on global mean temperature.