Why Does the Earth Have Continents?

Should you have been to reach in our photo voltaic system by no means having seen it earlier than, you would be impressed with selection. Large gasoline planets with rings, moons spanning from minuscule to huge, icy comets that hurtle in from the sides, rocky planets all with various quantities of atmospheres. It nearly looks like no two planets/moons shaped the identical approach, however one actually stands proud as an oddball.

It is Earth. Our planet has liquid water (bizarre!) It has life (even weirder!) It has plate tectonics churning away (continued weirdness!) It even has gigantic lots of rocks not like anything within the photo voltaic system (completely bizarre!) These lots are the continents, made from rocks like granite, sandstone, gneiss, slate, andesite, rhyolite and extra.

The remainder of the planets are nearly completely basalt or one thing shut, however Earth. No, earth hides most of its basalt floor underneath deep oceans, as a substitute letting its freak flag fly with continental rocks displaying off to any passersby.

All of those distinctive options are related. Plate tectonics could exist on Earth as a result of we’ve got liquid water on the floor. Life may be a product of the ample water and volcanism. The composition of the Earth’s continents may be a product of life’s interactions with rock. It’s all deep time evolution of minerals, rocks and organism that make Earth what it’s.

What are continents anyway?

There may be nonetheless rather a lot unknown in regards to the formation of our continents. We’re fairly positive that no different planet has the silica-rich continental lots that Earth possesses. Mars may need just a little little bit of what geologists name “advanced” rocks (in different phrases, extra silica than basalt). Venus may have just a little bit as nicely. The Moon has anorthosite highlands which might be a bit like continents besides they shaped from lighter minerals floating in a primordial magma ocean … that and people highlands are largely all the identical stuff.

No planet has the advanced melange of volcanic rocks, sediment, metamorphic rocks and cooled magma which might be Earth’s continents. The present principle, based mostly on the ages of tiny zircon crystals found in Australia, is that our continents could have began forming over 4 billion years in the past. Nonetheless, whether or not they all formed quickly to shut to their present measurement or have been slowly rising over time is an open query.

What makes continents so particular?

Nicely, they’re much less dense and far thicker than the opposite flavour of plate on Earth, oceanic plates. Our ocean basins exist primarily as a result of the crust beneath them are denser and thinner basalt plates, that means they sit decrease on the Earth’s ductile mantle (be aware: the Earth’s mantle is not made of molten magma). The continents, then again, sit excessive due to their decrease density and thicker profile, very like a volleyball sits increased in a pool than a tennis ball (an idea we name isostasy).

This distinction does extra than simply create the completely different shapes of Earth’s floor. Continents are so buoyant that they cannot get shoved again into Earth’s mantle just like the denser continental crust. Thus is born options like mountain belts shaped from continental collision and subduction zones (and their volcanoes) the place oceanic crust dives beneath continental crust.

The continents change as nicely. With plate tectonics comes the “supercontinent cycle” (also referred to as the Wilson Cycle) the place continents collide to type huge supercontinents like Pangaea after which cut up aside over a whole lot of thousands and thousands of years. At this time, the one factor we’ve got near a supercontinent is the amalgam of Europe, Asia and India.

The core of continents

The oldest elements of our continents are referred to as cratons (and if these rocks are uncovered on the floor, they’re referred to as shields.) They characterize the nucleus of every main continent, often a lot smaller than the continent as a complete. These areas have not seen a lot within the methods of energetic tectonic processes like collisions or rifts for a whole lot of thousands and thousands to billions of years.

In North America, the craton stretches from northern Canada and Greenland (the place the oldest rocks going again 3-4 billion years) to the south into Texas, however solely elements of it are uncovered on the floor. Most continents are extra than simply their cratons, so we all know that the continents did not type suddenly within the early historical past of the Earth. You’ll be able to take a look at a map of the world’s cratons beneath to get a way of the outdated cores of continents.

One of many largest questions may be what got the whole continent thing started … and what retains it going. It did not appear to occur on the different rocky planets of our photo voltaic system. Which means there are some elements which might be possible intrinsic to Earth — our liquid water and molten/strong core — that helped continents develop as absolutely as they’ve. Nonetheless, as they are saying, that is not all.

Subsequent week, I am going to discuss some new analysis that means that the preliminary occasions that acquired continents to begin forming may be from far out in house. On the similar time, the processes that preserve making our continents such compositional oddballs within the photo voltaic system could possibly be linked to life itself. Issues simply get weirder in terms of Earth’s continents.