Earth Has Layers

Earth is hot and melty in the middle and cold and crispy at the top.

About 2000 miles below us is a molten iron core which sank to the center of the molten primordial Earth. It's super hot and its spin creates the magnetic field that deflects the charged particles of solar wind that otherwise would long since have stripped away our atmosphere. 

"Floating" on the iron core is Earth's mantle. It's made of oxides of silicon, iron, and magnesium and constitutes the great bulk of our planet. In the few places where this alien material is found at the surface it is green and weird -- olivine and pyroxene rocks called "peridotite." 

Even though it's solid and incredibly dense, the mantle is hot enough that it behaves as a super thick fluid over geologic time. Think of it like a lava lamp. Mantle rock that's in contact with the outer core gets hot enough that buoyant plumes of it rise thousands of miles. When these plumes approach the surface they spread out, dragging the stiffer outer layers sideways. The sideways-moving slabs cool and eventually sink toward the core as blobs of denser mantle rock. This churning convection over many millions of years is called "plate tectonics."

Earth's crust is a paper-thin veneer of more familiar rock 5 to 30 miles thick derived from the mantle by partial melting, extrusion, and refreezing. Ocean crust covers about 70% of the planet. It's made when about 10% of mantle material melts and floats to the top, then "refreezes" as basalt. Basalt is a fine-grained black volcanic rock that makes up the entire ocean crust as well as the land of Hawaii and Iceland. Land crust is based on granite, which is made by melting about 10% of basalt, letting it float to the top and then refreezing that.

As mantle convection pushes plates of ocean and land crust around, they bump into one another. Continental crust made of granite is less dense than ocean crust made of basalt. Where ocean and land plates collide, the denser ocean basalt sinks underneath and gets remelted into the underlying mantle. Some of the magma burbles up and becomes volcanic mountain ranges like the Cascades or the Andes. Two ocean plates colliding produce arcs of volcanic islands like Japan or Indonesia. Where two granite continental plates collide, neither of them can sink into the mantle so they make gigantic crash zones like the Himalayas and the Alps.

Where hot blobs of mantle rock spread apart, the overlying crust r-r-R-R-I-I-P-P-s apart! When these rips happen in ocean crust the underlying mantle peridotite partly melts and basaltic lava flows upward to fill the void with brand-new ocean crust called mid-ocean ridges. When it happens on land, gigantic valleys form with earthquakes and volcanos in them. Prominent example is the Great African Rift Valley. Rift valleys that keep spreading actually open up to become new ocean basins.

The oceans and atmosphere are an ethereal blanket of volatile elements that float between the planet's impossibly hot/dense rocky interior and the eternal vacuum of space.  When the solar system formed, materials that make air and water and life must've boiled away in the heat of the young Sun and the cosmic impacts that built the inner planets. These volatile elements were then re-introduced later when icy chunks of CHON from "beyond the snowline" (think Jupiter and Saturn) fell into the warmth near the central fire.