If you arrive in our solar system without having seen it before, you will be impressed by the diversity. Giant gas planets with rings, moons ranging from tiny to huge, icy comets hurtling from the edges, rocky planets, all with varying amounts of atmosphere. It seems that there are almost no two planets/moons formed in the same way, but one really sticks out like a freak.
This is Earth. Our planet has liquid water (weird!) It has life (even weirder!) It has rifting plate tectonics (the weirdness continues!) It even has giant masses of rock unlike anything else in the Solar System ( totally weird!) These masses are the continents made of rocks like granite, sandstone, gneiss, shale, andesite, rhyolite, etc.
The rest of the planets are almost entirely basaltic or something, but Earth. No, the earth hides most of its basaltic surface beneath deep oceans, instead letting its strange flag fly with continental rocks on display to any passerby.
All of these unique features are connected. Plate tectonics it can exist on Earth because we have liquid water on the surface. Life may have been a product of abundant water and volcanism. The composition of Earth’s continents may be the product of life’s interactions with rock. All of this is a deep evolution of minerals, rocks and organisms that make the Earth what it is.
What are continents anyway?
View of part of the Canadian Shield, one of Earth’s oldest continental crusts, near Yellowknife, Northwest Territories. Credit: awmcphee/Wikimedia Commons.
There are still many unknowns about the formation of our continents. We are almost certain that no other planet has the silica-rich continental masses that Earth has. Mars may have some of what geologists call “evolved” rocks (in other words, more silica than basalt). Venus may also have some. The moon has anorthosite mountains which are a bit like continents except that they formed from lighter minerals floating in a primordial magmatic ocean… this and those mountains are pretty much all the same thing.
Neither planet has the complex mix of volcanic rocks, sediments, metamorphic rocks, and cooled magma that are Earth’s continents. The current theory based on the age of small 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 to approach their current size or grow slowly over time is an open question.
What makes continents so special?
Well, they are less dense and much thicker than Earth’s other flavor of plates, the oceanic plates. Our ocean basins exist mainly because the crust beneath them is a denser and thinner basalt plate, which means they sit lower on 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 sitting higher in a pool than a tennis ball (a concept we call isostasy).
This difference not only creates 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. This is how features such as mountain belts formed by continental collision and subduction zones (and their volcanoes) where oceanic crust dives beneath continental crust.
Continents also change. With plate tectonics comes the “supercontinent cycle” (aka Wilson cycle), where continents collide to form massive supercontinents like Pangea and then break apart over hundreds of millions of years. Today, the only thing we have close to a supercontinent is the amalgam 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 form the core of any large continent, usually much smaller than the continent as a whole. These areas have not seen much in the way of active tectonic processes such as collisions or rifting for hundreds of millions to billions of years.
In North America, craton extends from northern Canada and Greenland (where the oldest rocks date back 3-4 billion years) south into Texas, but only parts of it are exposed at the surface. Most continents are more than their cratons, so we know that continents did not form suddenly in Earth’s early history. You can look at a map of the world’s cratons below to get an idea of the old cores of the continents.
Map of the world’s geological provinces. Areas in orange are the oldest cratons/shields. Credit: USGS.
One of the biggest questions can be what started the whole continent … and what sustains it. It doesn’t seem to have happened on the other rocky planets of our solar system. This means that there are some factors that are probably inherent to Earth – our liquid water and molten/solid core – that helped the continents develop as fully as they did. But as they say, that’s not all.
Next week I’ll talk about some new research that suggests that the initial events that caused the continents to start forming may have come from deep space. At the same time, the processes that continue to make our continents such compositional oddballs in the Solar System may be related to life itself. Things just get weirder when it comes to Earth’s continents.