Plate Tectonics & Earthquakes

The Earth is alive - and moving!

Junior level — plain language, no maths

The ground under your feet feels rock-solid and dead still. But speed up a film of Earth over millions of years and something astonishing appears: the continents drift across the surface like colossal jigsaw pieces adrift on a slow river of hot rock. That's plate tectonics, the single most important process shaping the face of our planet.

Earth's outer shell is cracked into about fifteen big slabs - the tectonic plates - creeping along at roughly the speed your fingernails grow, a few centimetres a year. Trivial by the day, staggering over deep time: 200 million years ago every continent was fused into one supercontinent, Pangaea. India sat down near Antarctica, the Atlantic Ocean didn't exist, and the Himalayas - the highest mountains on Earth - only began rising 50 million years ago, when India slammed into Asia.

When plates grind together, slide past, or pull apart, the energy that comes loose can be catastrophic. Earthquakes strike when two plates locked by friction suddenly break free, dumping centuries of stored strain in a matter of seconds. The 2011 Japan quake was so violent it shifted Earth's axis by 17 centimetres and shaved 1.8 microseconds off the length of a day. In the simulation below, move the plates and feel the consequences.

Plate boundaries, seismic waves, and the Wilson cycle

Student level — the core equations

Earth is layered like an onion: a solid iron inner core, a churning liquid outer core that spins up the magnetic field, a slowly convecting rocky mantle, and a thin brittle crust on top. The plates are the rigid outer ~100 km - crust plus the coldest mantle - riding on the softer asthenosphere beneath. What drives them isn't some conveyor belt shoving from below but mostly gravity: ridge push, as newborn crust slides off the mid-ocean rises, and - dominating - slab pull, where a cold, dense slab sinking at a trench drags the rest of the plate down behind it.

Plates meet in three ways. At divergent boundaries they pull apart and fresh crust wells up (mid-ocean ridges, the East African Rift); at convergent ones they collide, either subducting to build volcanic arcs and deep trenches or crumpling into fold mountains; at transform boundaries they grind sideways past each other (the San Andreas). Zoom out to hundreds of millions of years and these knit into the Wilson cycle - ocean basins rifting open, then subducting shut, again and again.

An earthquake breaks at a point called the hypocentre and floods the rock with seismic waves: fast P-waves that squeeze through both solid and liquid, and slower S-waves that shear, and so pass through solids only. Because P outruns S, the gap between their arrivals at a seismometer measures distance, and three stations pin the location. Size is reported as moment magnitude \(M_w = \tfrac{2}{3}\log_{10} M_0 - 10.7\), built from the seismic moment \(M_0 = \mu A d\) (rock stiffness × rupture area × slip). Each whole step up the scale is roughly \(32\times\) more energy - a magnitude 8 outstrips a magnitude 6 a thousandfold.

Mantle convection, geodynamics, and seismic tomography

Scholar level — full mathematical depth

01The mantle is a fluid - just a very slow one

Solid rock flows, if you wait. Over millions of years the mantle convects like a pot of syrup, and its vigour is set by the Rayleigh number \(Ra = \dfrac{\alpha g \Delta T d^3}{\kappa \nu}\), pitting buoyancy against viscosity and diffusion. Earth's mantle runs at \(Ra \sim 10^7\), thousands of times past the threshold where convection kicks in - so despite a viscosity of \(10^{21}\) Pa·s, the whole thing overturns, and plate tectonics is just the top of that convection breaking the surface into brittle pieces.

02How the planet sheds its heat

Convection is the planet's cooling system. The Nusselt number \(Nu \sim Ra^{1/3}\) says how much more heat convection carries than plain conduction would, and Earth bleeds about 47 TW in total - roughly 70% of it through the ceaseless manufacture of new oceanic crust at mid-ocean ridges. Tectonics isn't incidental to Earth's thermal history; it is how a rocky planet stays hot enough inside to keep going.

03A CT scan of the deep Earth

We can't dig to the core, but seismic waves can, and seismic tomography turns them into a 3-D image. Invert the arrival times of thousands of earthquakes and you recover where waves speed up or slow down: cold, stiff slabs run fast, hot buoyant plumes run slow. It's a planetary CT scan, resolving structure to a couple hundred kilometres globally and finer regionally - enough to watch subducted slabs plunge toward the core and plumes rise from it.

04The two blobs at the bottom of the world

Tomography's most startling find sits at the core-mantle boundary: two continent-sized regions of anomalously slow rock beneath Africa and the Pacific, the Large Low Shear Velocity Provinces. Hot, dense and ancient, they may be primordial piles left from Earth's early differentiation - or, in one bold recent idea, buried fragments of the Mars-sized body whose collision formed the Moon. Either way, they seem to steer where deep plumes and their surface volcanoes arise.

05The geodynamo

Earth's magnetic field is generated by convection in the liquid iron outer core, and its evolution obeys the induction equation \(\partial_t B = \nabla\times(u\times B) + \eta \nabla^2 B\): the first term stretches and amplifies the field, the second lets it diffuse away. Which wins is decided by the magnetic Reynolds number \(Rm = UL/\eta \sim 500\) - comfortably in favour of amplification - so the churning core sustains a field against ohmic decay, a self-exciting dynamo that has run for billions of years.

06When the compass flips

That dynamo is not steady. The magnetic record frozen into rocks shows the field has reversed polarity some 170 times in the last 100 million years, at wildly irregular intervals from 0.1 to 50 Myr apart, each flip taking a few thousand years. Why the field destabilises and reverses remains only partly understood - and with the north magnetic pole currently sprinting toward Siberia, it's not a purely academic question.