There may be nonetheless so much unknown in regards to the formation of our continents. We’re fairly positive that no different planet has the silica-rich continental plenty that Earth possesses. Mars may need a little bit little bit of what geologists name “developed” rocks (in different phrases, extra silica than basalt). Venus may have a little bit 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 principally 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 idea, 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 (word: 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 similar to 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 can not 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” (often known as the Wilson Cycle) the place continents collide to type large supercontinents like Pangaea after which cut up aside over a whole lot of tens of millions of years. At the moment, the one factor we’ve got near a supercontinent is the amalgam of Europe, Asia and India.
The core of continents
The oldest components of our continents are known as cratons (and if these rocks are uncovered on the floor, they’re known as shields.) They signify the nucleus of every main continent, normally 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 tens of millions 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 components 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 all of sudden within the early historical past of the Earth. You possibly can try a map of the world’s cratons beneath to get a way of the previous cores of continents.
