# Geology (land science) explained

The earth itself is composed of several types of rocks, categorized by how they form:

- Igneous rocks form when lava from volcanoes cools down.
- Sedimentary rocks form through pressure applied over time to particles from water or air.
- Metamorphic rocks are changed from other rocks through heat, pressure, or reactive fluids.

Inside the earth, there are multiple layers from all the pressure (which generates heat):

1. The crust only goes down for 5-12 kilometers, and is the only thing we've ever drilled into.
2. The mantle is about 3,000 kilometers thick and made of semi-solid rock. The pressure makes it circulate slowly, and there are a few dozen sections of crust that extend into the mantle as "plates" that move around.
3. The outer core is about 3,000-5,000 kilometers thick and made of churning liquid rock, which probably churns from radioactive decay of uranium and thorium and somehow generates the earth's magnetic field.
4. The inner core is 1,220 kilometers thick and might be made of solid iron. There might be an innerer core as well.

Underneath the earth, there are many plates made of that shift. This can create earthquakes, but also can move continents.

The highest naturally occurring element is uranium, and most natural radioactive elements are deep underground. The further down, the hotter it gets. Digging past the crust will quickly yield the mantle, then the core, which is so hot that everything is a liquid.

Obvious questions that remain unanswered:

- How could we put carbon back in the earth?
- Why do rocks sail across the desert floor?