4. Plate Tectonics & Erosion

Think about a recently accreted planet. It has a lot of energy near the center. That energy is enough to keep the mantle, most of the planet, in a semi-solid semi-liquid molten phase. The outermost layers of the mantle lose energy to the atmosphere and cool to solid rock. You can think of the outer layer as a scab around the ball of molten rock. We call this scab the crust.

The crust is incredibly thin relative to the size of the planet, similar to the skin on an apple. This rocky scab has been the home of every organism that has ever existed (that we know) along with every place you have ever stood.

The rocks of the crust are split into two categories: less dense material called felsic rock and more dense material called mafic rock. Felsic rock makes up a portion of crust that I like to think of as “fluffy.” Scientists call it continental crust and it makes up the majority of what you are standing on.

Mafic rock forms a thinner, denser layer of crust. Gravity causes the water of our planet to rush to the lowest point. The water pooled on top of the mafic crust, creating the oceans. We call this layer oceanic crust and it makes up most of the surface of our planet.

Early Earth crust
Early Earth crust

Although the crust is made up of solid rock, the mantle beneath is constantly churning. The high energy mantle, in its churning, will build up pressure as gaseous particles try to break out through the crust. In order to release that pressure, a crack will form in the crust so that those gases can escape. The cracks will then spread as more gases pour out from different parts of the planet’s surface. The cracks will follow the point of greatest weakness (usually along the thinner oceanic crust) and eventually meet up with other cracks that have formed elsewhere. We call these cracks fault lines. The solid portions between the cracks are called tectonic plates.

The crust now has openings and the mantle can pump out energetic molten rock through a crack. That rock, though liquid-like in the mantle, will solidify quickly when it reaches the surface. The newly formed solid will then be pushed out of the way to make way for more molten rock to rise up from the crack. The rising and solidifying rock, as well as the churning mantle, pushes the tectonic plates apart. We call this a divergent plate boundary and there is a very active one currently expanding the Atlantic Ocean.

Internal view of mid-Atlantic Ridge
Internal view of mid-Atlantic Ridge

If two plates are pushed apart, they will eventually push into other plates. Two plates coming together is called a convergent plate boundary and there are a few different possible scenarios here.

  • Scenario 1: only oceanic crust converges.
    The older crust, the one that formed first, has had more time to absorb the water sitting on top of it and is therefore more dense. The older, denser crust will sink underneath the newer crust in a process called subduction. The old crust will then be melted as it enters the mantle to be recycled and eventually create more crust somewhere else. The lighter crust will slide atop the older crust pushing it down further.
  • Scenario 2: oceanic and continental crust converges. 
    The oceanic crust is much denser than the “fluffy” continental crust. The thinner mafic crust will subduct underneath to be recycled by the mantle. These boundaries are often coupled with mountain ranges or volcanoes along the coast.
  • Scenario 3: only continental crust converges.
    Neither of these plates are dense enough to be pushed into the mantle. Instead, the crust bends and wrinkles upward to form what we call mountains.

Plates don’t always move directly away or toward each other, sometimes they move parallel to each other. These are called transverse plate boundaries and they will only move when enough pressure builds up to shift the plates. The movement of plates, of any kind, will send shockwaves through the solid crust; greater force of movement creates greater shockwaves. We call these waves earthquakes.

Meanwhile, the rock of the crust is exposed to the water and air that sits on top of it. We call moving air wind and it, along with water, can knock off small bits of rock and carry it away. The process of wind or water chipping away at rock is called erosion and it is a huge reason for the shape of the rocks we see.
Some rock formations are found in strange shapes. The reason is because less dense pieces were chipped away by erosion leaving only the denser parts of rocks still standing. Smaller rocks, like sand, have been eroded more than larger rocks.


When rocks are put under high heat and pressure, like being buried under a forming mountain, they break down and resolidify in a denser form. We call these types of rocks metamorphic because of the change they underwent.

Okay! This section went through a lot of information so let’s review. The crust is the thin layer of our planet that solidified on the outside. The pressure of escaping molecules from the mantle caused the crust to crack into pieces called plates. Those plates move around, collide, and recycle when pulled back to the mantle. Meanwhile, air and water are constantly flowing above and sometimes crash into the rocks, knocking off small chunks.

Crust is formed from and recycled back into the mantle, mountains rise up and are then whittled down by wind. Everything is recycled, everything is conserved; your planet is spectacular!

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