# What are the four layers of the crust?

The outermost layer that we live on is the crust. It is very thin, only $4$ miles or so under the ocean and $25$ miles under the land masses or continents. The crust is made up mostly of solid rock, with silicon and oxygen making up almost 75% of the crust in silicon dioxide, out of which much of sand, rock and glass is made. The rest are metals of various sorts: aluminium and iron being dominant.
Most of the volume of the Earth is the mantle, which is largely magma, molten rock inside the Earth. When magma erupts out of the crust it becomes lava. By weight, magma contains around 40% oxygen, 20% magnesium and 20% silicon, with smaller amounts of iron, calcium and aluminium. Most of the heat on and in the Earth doesn't come from the sun, but inside the mantle, where radioactive decay of elements like thorium and uranium have given off masses of energy for millions of years. The mantle is around $1800$ miles thick.
Surrounded by the mantle is the outer core, a viscous liquid of molten nickel and iron compounds, known as NiFe. The temperature of the outer core ranges from around $4300 K$ to $6000 K$. Since nickel and iron are conductive, it is thought that the Earth's magnetic fields are moving because of currents in the NiFe, and geophysicists predict that the Earth's magnetic poles could completely flip before long. (Before long in geology means within a few million years or thereabouts.) The outer core is $1400$ miles thick on average.
On the very inside of the Earth is the inner core. This is the hottest part of the Earth, though it is not actually a liquid, unlike the mantle and outer core. This is because, even though the heat is so high, and ordinarily it should be a liquid, the pressure from the rest of the Earth bearing down on it compresses it into a solid ball of iron and nickel. It is around $6000 K$ and $750$ miles thick.