I’ve said this before on this space, but Earth is weird. Consider this:
Altitude distribution across the many planets and moons in our solar system. Credit: Vita-Finzi and Fortes, 2013.
These diagrams show the distribution of surface heights over a range of large and small bodies in our solar system. Something has to jump off really fast: they all have one peak in altitude except for Earth and Mars. It’s weird! Why does any planet have two different peaks of altitude distribution?
For Earth, the explanation is relatively straightforward: we have Two types of crust. The oceanic crust that lies beneath the oceans is thin, dense, and low in silica (silicon oxide) while the continental crust is thick, buoyant, and rich in silica. This creates a planet where much of the Earth’s surface is below sea level (in this case, 0 km elevation), creating ocean basins. The rest of the planet sticks out up to 8 km above sea level, forming the continents.
Our planet’s surface is made up of tectonic plates, crustal plates, and the upper mantle (the next layer down). Depending on who you talk to, there are probably dozens of major, small, and small motherboards on the floor. They all interact in different ways to create or destroy a shell and rearrange themselves.
Map of tectonic plates on Earth. Credit: Eric Gaba, Wikimedia Commons.
What we think of as continents is not directly equivalent to the tectonic plates of the same name. North America (the plate) contains both the land mass we think of as North America (the continent) as well as the oceanic crust that extends from the east coast to the mid-Atlantic mountain range. This means that many plates are a mixture of oceanic and continental crust.
Well, why does that make for weird Earth? No other planet* has such types of specific binary crust that create the twin peaks of the terrain. Something different is happening on Earth and it has happened in different ways for more than 4 billion years (*we’ll be back to Mars soon).
Young Earth began continents
Early Earth was a completely different place. Counting 4 billion years ago the Earth’s surface was mostly basalt – the material of the oceanic crust. After the planet’s formation (and the subsequent formation of the Moon with great impact), the planet likely had an ocean of magma that solidified into dark basalts.
The Earth was much hotter at that time, too. This heat was coming from remnants of planet formation and all of the influences that occurred during the early history of the solar system. There was also more heat generated from the radioactive elements that had died down since then.
In fact, it was hot enough inside the Earth that as the basalt cooled and began to sink, it began to melt. When you melt just a little bit of rock, you don’t get magma that is the same composition as the rock you’re melting – a process called partial melting Because only some metals melt. This means that the melting of basalts in early Earth was creating magmas that were more silica-enriched, just like our modern continents.
Perhaps this was the birth of the continents as we know them on Earth. These areas of silica-rich magma have been cooled into rocks such as Tone And granodiorite (Close to granite) Suddenly the Earth’s surface wasn’t just basalt. Over hundreds of millions of years, this movement created massive bodies of new crust that were rich in silica and buoyant.
This process did not last forever. As the Earth cooled, the dominant process of formation of new crust changed to plate tectonics. There is evidence of this 60-70% of the Earth’s continental crust was formed about 3 billion years ago. It continues to grow today, but at more than 3 times slower rates. Most of this growth occurs where plates collide to form subduction zones where oceanic plates are pushed back into the mantle as the Cascades or Andes.
Mars is the only planet with two peaks in height, but the reason for the division of Mars is controversial. It may be tectonic, but there are theories Massive influences In the southern or northern hemisphere, it could have created elevation differences between the two hemispheres. It could just be a product of our calculation of the elevations of the surface of Mars, a world without oceans today.
Of course, the big “wait a second” question here is why our sister planet, Venus, hasn’t really developed distinct continents. There are some suggestions that a high altitude area (Ishtar Terra) could be something similar to early “continental” Earth material, but without exploring the surface of Venus, it’s just speculation. In any case, Venus is by no means Earth’s twin.
Earth may just have had the right balance of heat, water, and formation history (like the collision forming the Moon) to start the ball rolling toward the continents. It is difficult to imagine our planet without the heights of the continents and the bottoms of the oceans. A world that lacks such features is out there and any life that originated on it might have been very different.