Why does the Earth have continents?

If you were to arrive in our solar system without having seen it before, you would be in awe of the variety. Gas giant planets with rings, moons ranging from tiny to huge, icy comets that rush from the edges, rocky planets all with varying amounts of atmospheres. It almost seems like there aren’t two planets/moons similarly trained, but one really stands out as an oddball.

It is the Earth. Our planet has liquid water (weird!) It has life (even stranger!) It has bubbling plate tectonics (weirdness continues!) It even has gigantic masses of rock that look like nothing The other in the solar system (totally weird!) These masses are the continents, made of rocks like granite, sandstone, gneiss, slate, andesite, rhyolite and more.

The other planets are almost entirely made of basalt or something close to it, but Earth. No, the earth hides most of its basalt surface beneath deep oceans, instead letting its bizarre flag fly with continental rocks showing off to all passers-by.

All these unique characteristics are linked. Tectonic plates can exist on Earth because we have liquid water on the surface. Life could be the product of abundant water and volcanism. The composition of Earth’s continents may be the product of life’s interactions with rock. It is the whole evolution over time of minerals, rocks and organisms that makes the Earth what it is.

What are the continents anyway?

A view of part of the Canadian Shield, part of the oldest continental crust on Earth, near Yellowknife, Northwest Territories. Credit: awmcphee/Wikimedia Commons.

There are still many unknowns about the formation of our continents. We are pretty sure that no other planet has the silica-rich landmasses that Earth has. Mars might have some of what geologists call “evolved” rocks (in other words, more silica than basalt). Venus might have some too. The moon has anorthosite highlands which are kind of like continents except they formed from lighter minerals floating in an ocean of primordial magma…that and those highlands are mostly the same thing.

No planet has the complex mixture of volcanic rocks, sediments, metamorphic rocks and cooled magma that are Earth’s continents. The current theory, based on the ages of tiny zircon crystals found in Australia, is that our continents may have begun to form more than 4 billion years ago. However, whether they all formed quickly close to their current size or have slowly increased over time is an open question.

What makes continents so special?

Well, they are less dense and much thicker than the other flavor of plate on Earth, the oceanic plates. Our ocean basins exist primarily because the underlying crust is made up of denser, thinner basalt plates, which means they sit lower on the Earth’s ductile mantle (note: the Earth’s mantle is not made of molten magma). Continents, on the other hand, sit high due to their lower density and thicker profile, much like a volleyball sits higher in a swimming pool than a tennis ball (a concept we let’s call isostasis).

This difference does more than just create the different shapes of the Earth’s surface. The continents are so buoyant that they cannot be pushed back into the Earth’s mantle like the denser continental crust. Thus are born elements such as mountain belts formed by continental collision and subduction zones (and their volcanoes) where oceanic crust dips beneath continental crust.

The continents also change. With plate tectonics comes the “Supercontinent Cycle” (also known as the Wilson bike) where continents collide to form massive supercontinents like Pangea, then pull apart over hundreds of millions of years. Today, the only thing we have near a supercontinent is the amalgamation of Europe, Asia and India.

The core of the continents

The oldest parts of our continents are called cratons (And if these rocks are exposed on the surface, they are called shields.) They represent the core of each major continent, usually much smaller than the continent as a whole. These areas have not seen many active tectonic processes such as collisions or fissures for hundreds of millions to billions of years.

In North America, the the craton expands from northern Canada and Greenland (where the oldest rocks date back 3-4 billion years) south to Texas, but only parts are exposed at the surface. Most continents are more than just cratons, so we know that not all continents formed at once in Earth’s early history. You can check out a world craton map below to get an idea of ​​the ancient cores of the continents.

Map of the geological provinces of the world. The areas in orange are the oldest cratons/shields. Credit: USGS.

One of the biggest questions might be What made the whole continent begin …and what keeps it going. This didn’t seem to happen on the other rocky planets in our solar system. This means that there are certain factors which are probably intrinsic to the Earth – our liquid water and our molten/solid core – which helped the continents to develop as fully as they did. However, as they say, that’s not all.

Next week, I’ll talk about new research that suggests the initial events that caused continents to form may have originated far out in space. At the same time, the processes that continue to make our continents such compositional oddballs in the solar system could be linked to life itself. Things get even weirder when it comes to Earth’s continents.