Earth's interior and plate tectonics

The first principle

The Earth is a heat engine. Heat left over from its formation and heat released by radioactive decay must escape from the interior, and the solid but slowly creeping mantle carries it upward by convection. The cold, rigid outer shell, the lithosphere, is broken into plates that ride on this moving system, so heat loss from within is what builds mountains, opens and closes oceans, and sets the places where earthquakes and volcanoes occur.

The core

How we know the interior: seismic waves

• No borehole has gone deeper than about 12 km, so the interior is read indirectly from earthquake waves, whose speed and path change with the density and state of the rock they cross. Primary (P) waves are compressional and travel through solids, liquids and gases, while secondary (S) waves are shear waves that cannot pass through a liquid, because a liquid has no rigidity to resist shear.
• Waves bend as density increases with depth, so rays curve back to the surface. Between about 105° and 145° from the epicentre no direct P waves arrive, because the core bends them sharply; beyond 105° no S waves arrive at all, and that S wave shadow is the proof that the outer core is liquid.
• Sudden changes in wave speed mark discontinuities: the Mohorovicic or Moho (crust and mantle, about 35 km under continents and 5 to 10 km under oceans), the Gutenberg (mantle and core, about 2,900 km) and the Lehmann (outer and inner core, about 5,150 km).

Layers by composition and by strength

• By composition the Earth has a light silica rich crust (continental crust granitic, oceanic crust basaltic), a dense iron and magnesium rich mantle, and an iron and nickel core whose outer part is liquid and whose inner part is solid under enormous pressure. Convection of the liquid outer core generates the Earth's magnetic field.
• By strength the outer 100 km or so, crust plus the topmost mantle, forms the rigid lithosphere. Below it the asthenosphere (to about 400 km) is close to its melting point, so it deforms plastically and lets the plates glide; it is also the main source of magma.

From continental drift to plate tectonics

• Alfred Wegener (1912) argued that all continents once formed one landmass, Pangaea, surrounded by one ocean, Panthalassa. His evidence was the jigsaw fit of South America and Africa, matching rocks and fossils (such as Mesosaurus and Glossopteris) across oceans, and tillite and coal found far from the climates that form them. It was rejected because his proposed forces, tidal and polar fleeing forces, were far too weak.
• Post war mapping found mid ocean ridges and trenches. Harry Hess (1962) proposed sea floor spreading: magma rises at ridges, forms new crust that moves sideways, and old crust sinks back into the mantle at trenches, which is why no ocean floor is older than about 200 million years.
• Palaeomagnetism clinched it. Iron minerals in cooling basalt lock in the direction of the Earth's field, which reverses every few hundred thousand years, so the ocean floor carries magnetic stripes that mirror each other on both sides of a ridge (Vine and Matthews, 1963). By 1968 these ideas were joined as plate tectonics: about seven major and many minor rigid plates moving over the asthenosphere.

Dimensions

Plate boundaries

• Divergent (constructive). Plates pull apart, the pressure on the hot mantle below falls, and it melts and rises to make new crust, as at the Mid Atlantic Ridge and, on land, the East African Rift.
• Convergent (destructive), of three kinds. Where ocean meets continent, the denser oceanic plate subducts, forming a trench and a volcanic mountain arc (Peru Chile Trench and the Andes). Where ocean meets ocean, one sinks under the other, forming a trench and an island arc (Mariana Trench, Japan). Where continent meets continent, neither is dense enough to sink far, so the crust crumples and thickens into fold mountains (Himalaya, Alps).
• Transform (conservative). Plates slide past each other, crust is neither made nor destroyed, and the locked fault releases energy in shallow earthquakes (San Andreas Fault).

What drives the plates

• Mantle convection. Hot mantle rises and cold mantle sinks, as Arthur Holmes suggested in the 1930s, and the plates are the cold top layer of this circulation.
• Slab pull. A subducting slab is colder and denser than the mantle around it, so its weight pulls the rest of the plate behind it; this is now thought to be the largest force, which explains why plates attached to long subducting edges, such as the Pacific plate, move fastest.
• Ridge push. New, hot lithosphere stands high at a ridge and slides down its flanks under gravity.

Mantle plumes and hotspots

• A mantle plume is a narrow column of unusually hot rock rising from deep in the mantle, perhaps from the core mantle boundary. Its mushroom shaped head melts as pressure falls near the surface and feeds a hotspot, a centre of volcanism that is not tied to a plate boundary.
• Because the plume stays roughly fixed while the plate moves over it, a hotspot leaves a chain of volcanoes that get older away from the active one. The Hawaiian Emperor chain records the Pacific plate's motion, and its bend about 47 million years ago records a change in that motion.
• Plumes play three roles in plate tectonics. They mark plate direction and speed, their huge first outbursts form flood basalts (the Deccan Traps erupted about 66 million years ago when India passed over the plume now under Reunion), and by heating and doming the lithosphere they can help break continents apart and start new oceans. Iceland is a plume sitting on the Mid Atlantic Ridge, which is why it stands above sea level.

In practice

How continents and ocean basins change shape and size

• Oceans open and close in a Wilson cycle. J Tuzo Wilson showed that a continent rifts, a narrow sea opens, a wide ocean grows, subduction begins at its edges, and the ocean finally closes in a collision. Today the East African Rift is the first stage, the Red Sea the second, the Atlantic the third, the Pacific the fourth and the Mediterranean the fifth.
• The Atlantic is widening and the Pacific shrinking. The Atlantic grows by a few centimetres a year at its ridge, while the Pacific, ringed by trenches of the Ring of Fire, loses more floor than it makes.
• India's journey. India broke from Gondwana, crossed the Tethys at up to about 15 cm a year, and collided with Asia about 50 million years ago. The Tethys sediments were squeezed into the Himalaya, which are still rising by a few millimetres a year because India keeps pushing north at about 5 cm a year.
• Continents grow and break. Continents grow as island arcs are welded to their edges and split when rifts open.

Learning about Earth from Jupiter: the Juno mission

• The Juno spacecraft of the National Aeronautics and Space Administration (NASA), launched in 2011, has orbited Jupiter since July 2016 in a polar orbit that passes close to the cloud tops. It measures Jupiter's gravity field (to infer the interior and core), its magnetic field, and microwave emission that reveals water and ammonia deep in the atmosphere.
• Its findings matter for Earth because Jupiter, the first and largest planet to form, shaped the whole solar system. Its gravity steered the asteroids and comets that likely delivered water and volatiles to the inner planets, so measuring Jupiter's water content tests theories of where Earth's water came from.
• Juno found that Jupiter's core is not a small compact ball but large and "dilute", mixed into the surrounding hydrogen, which forces scientists to revise models of how giant planets and their cores form. Its detailed map of an irregular, changing magnetic field helps us understand dynamos in general, including the one in Earth's outer core.
• This is comparative planetology: studying another planet as a natural experiment lets us test ideas about Earth's formation, interior and magnetic field that cannot be tested on Earth alone.

Issues and way forward

• Hazard on a collision boundary. The Himalaya store strain from India's continuing push, so large earthquakes are certain even if their timing is not. Strict building codes from the Bureau of Indian Standards, enforced in hill towns, and microzonation of cities, as the National Disaster Management Authority has recommended, are the remedy.
• Plate interiors are not safe. Earthquakes such as Latur (1993) occur inside plates along old weak zones, so preparedness cannot stop at plate boundaries.
• Gaps in observation. Denser networks of the National Centre for Seismology (Ministry of Earth Sciences), ocean bottom seismometers and continuous Global Positioning System (GPS) monitoring of plate motion are needed to refine hazard maps.

Common confusions

• Lithosphere and crust. The crust is a compositional layer; the lithosphere is a strength layer that includes the crust and the rigid top of the mantle.
• Continental drift and plate tectonics. Drift moved only continents, through the ocean floor, with no workable mechanism; plate tectonics moves whole plates of continent and ocean floor, driven by mantle forces.
• Hotspot and plate boundary volcanism. Hotspots are fed from deep plumes and can sit in the middle of a plate (Hawaii); boundary volcanism comes from ridges or subduction.
• The shadow zone. The P wave shadow lies between about 105° and 145°; the S wave shadow covers everything beyond 105°.

For the exam

• Keyword bank: P and S waves, shadow zone, Moho, asthenosphere, Pangaea, sea floor spreading, palaeomagnetism, subduction, slab pull, ridge push, mantle plume, hotspot, flood basalt, Wilson cycle, comparative planetology.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction (data): The ocean floor is nowhere older than about 200 million years, while continental rocks are up to four billion years old, which shows that ocean basins are constantly made and destroyed.
  • Introduction (scholar): J Tuzo Wilson showed that oceans open and close in cycles, so today's map of continents is only one frame in a moving picture.
  • Conclusion: Plate tectonics turns the Earth's internal heat into the shape of its surface, explaining mountains, oceans, earthquakes and volcanoes with one mechanism.
  • Conclusion: For India, still pushing into Asia, understanding plate motion is not academic but the basis of earthquake safety across the Himalaya.