What if the continents stopped moving?

The ground under us is made of giant pieces, like a cracked eggshell. The pieces move very slowly.

What if they stopped moving? Let's imagine it.

Earth's hard outer layer is cracked into huge pieces called tectonic plates. The continents ride on them. The plates move only a few centimeters a year, too slowly to feel, but over millions of years they have carried continents thousands of kilometers.

What if the plates stopped? In this thought experiment, you can follow three paths: what the plates are doing today, what would happen to the mountains, and what might change for earthquakes and volcanoes.

Plate tectonics describes Earth's rigid outer shell, the lithosphere, broken into plates that move over a hotter, slowly flowing mantle. USGS explains that the plates are driven by forces scientists can neither precisely describe nor fully understand, linked to heat escaping from the interior.

This thought experiment imagines that motion stopping. Plate tectonics builds mountains, opens oceans and concentrates earthquakes and volcanoes along plate edges, so stopping it would change all three. The scenario is not a forecast: it is a way to see what plate motion does by imagining its absence.

The plates move a tiny bit each year, too slowly to feel.

Some pull apart. Some crash together and push up mountains. Some slide past each other.

Plates meet in three main ways. Where they pull apart, new rock forms, as along the Mid-Atlantic Ridge, which spreads about 2.5 centimeters a year. Where they push together, one plate may sink under another, or the crust may crumple into mountains. Where they slide past each other, as at California's San Andreas Fault, the ground grinds sideways.

The Himalayas formed when India crashed into Asia between 40 and 50 million years ago. USGS says they are still rising by more than 1 centimeter a year.

USGS describes divergent, convergent and transform boundaries, plus broad boundary zones. Spreading rates vary from under 2.5 cm/yr on the Arctic Ridge to over 15 cm/yr on the East Pacific Rise; the Mid-Atlantic Ridge averages about 2.5 cm/yr, 25 km per million years. Along the San Andreas, the Pacific Plate has moved past the North American Plate at about 5 cm/yr for 10 million years.

When continents collide, neither subducts, so the crust thickens and rises. India, moving north at about 9 m a century 80 million years ago, met Asia 40 to 50 million years ago; the Himalayas still rise by more than 1 cm a year.

Let's pretend the plates stopped. No more pushing.

Mountains might stop growing. Rain and ice might slowly wear them down. There might be fewer earthquakes. This is only pretend.

Now imagine every plate froze in place. The Himalayas might stop rising. But rain, rivers, ice and wind would keep wearing rock away, as they do now, so over millions of years the mountains might slowly get lower and rounder.

With plates no longer grinding past each other, earthquakes along plate edges might become rare, and many volcanoes fed by sinking plates might go quiet. These are ideas, worked out from what moving plates do today.

Halting plate motion would remove the main source of uplift while leaving erosion untouched. Over geological time, high ranges might be lowered and smoothed, as the Appalachians have been over 300 million years, and sediment might keep filling low ground.

Stress would no longer build along boundaries, so plate-boundary earthquakes might fade, along with arc volcanoes fed by subducting slabs and new crust at spreading ridges. Hotspots, which Wilson's classic theory treats as fixed sources of heat beneath the plates, might keep erupting, but in one place instead of building a line of islands (USGS adds that newer studies debate how deep and how fixed hotspots really are). All of this is reasoning, not a model run.

Deep inside, Earth is very hot. Slow, soft rock moves like thick soup heating on a stove.

That slow stirring helps move the plates on top.

Earth is hot inside. Below the hard plates, rock is so hot that it can flow very slowly, over millions of years, a bit like thick soup in a pot: hot material rises, spreads, cools and sinks again.

That heat comes from two main places: radioactive elements such as uranium slowly breaking down, and heat left over from when Earth formed 4.6 billion years ago. Scientists still do not fully understand exactly how this motion moves the plates.

Below the lithosphere, mantle rock under heat and pressure can flow slowly over long times. USGS compares this to convection in a pot of thick soup, while noting the huge differences in scale and speed. Heat comes mainly from radioactive decay of uranium, thorium and potassium and from residual heat of Earth's formation 4.6 billion years ago.

Many questions remain: how many convection cells exist, and how much plates are pushed at ridges or pulled by sinking slabs. That is one reason the scenario here is reasoning. Its central point is solid: uplift needs plate motion, while erosion needs only rain, ice, wind and gravity.

The Appalachian Mountains are very, very old.

Long ago, they were probably much taller. Rain and ice slowly wore them down.

The Appalachian Mountains in the eastern United States formed when continents collided between 500 and 300 million years ago. The National Park Service says that in their prime they probably had peaks as high as the Himalayas or the Alps today.

Over the past 300 million years, erosion has worn them down to more modest heights. They show what happens when the push from colliding plates stops but rain, rivers and ice keep working.

The Appalachian, Ouachita and Marathon mountains are one collisional range, built as continents collided 500 to 300 million years ago. NPS notes that at their peak they probably rivaled today's Himalaya-to-Alps collision zone, and that after 300 million years they are deeply eroded and partly buried by younger coastal-plain sediments.

Hawaii offers a second real example. The Pacific Plate carries volcanoes away from a hotspot; USGS notes that Kauai's oldest rocks are about 5.5 million years old and deeply eroded. Once a volcano moves off its heat source, erosion takes over, just as it might everywhere if plates stopped.