Earth's Interior

In survival games, you can dig straight down to bedrock in a few minutes. In real life, the deepest hole humans ever drilled, the Kola Superdeep Borehole in Russia, reached about 12 km (7.5 miles). Sounds deep, right? That's only about 0.2% of the way to Earth's center. They stopped because the rock was so hot it behaved like putty.

So how do we know what's down there? Clever detective work.

When an earthquake hits, it sends seismic waves through the whole planet. Scientists record them with instruments called seismographs all over the world.

• P waves (primary): the fastest. They squeeze and stretch rock, like a Slinky pushed from one end. They travel through solids AND liquids.
• S waves (secondary): slower. They shake rock side to side, like flicking a rope. They travel ONLY through solids.

Here's the big clue: S waves never make it through the center of the Earth. They disappear on the far side of the planet in an area called the shadow zone. Since S waves can't pass through liquid, that tells us part of the core must be liquid. Waves also bend and change speed when they hit different materials, which shows us where the layers are.

Other clues: meteorites (leftover pieces from when planets formed), volcanoes that bring up deep rock, and Earth's magnetic field.

Earth is built like a peach: thin skin, thick fleshy middle, hard pit in the center.

1. Crust: the thin rocky outer shell we live on. Oceanic crust is thin (about 7 km), dense, and made of basalt. Continental crust is thicker (30 to 70 km), lighter, and mostly granite. If Earth were an apple, the crust would be thinner than the peel.
2. Mantle: the thickest layer, about 2,900 km deep. It's solid rock, but so hot that it slowly flows over millions of years, like super-thick silly putty. Makes up about 84% of Earth's volume.
3. Outer core: liquid iron and nickel, about 4,000 to 5,000 °C.
4. Inner core: solid iron and nickel ball, about as hot as the surface of the Sun (around 5,200 °C). It stays solid because the pressure is so crushing that the atoms can't spread out to melt.

Scientists also divide the outer layers by how they behave:

• Lithosphere: the rigid outer layer, made of the crust plus the very top of the mantle. It's broken into giant pieces called plates.
• Asthenosphere: the soft, slowly flowing layer of mantle right below. The plates float and slide on top of it, like crackers on warm pudding.

Why does the mantle move? Convection. Hot material rises because it's less dense. As it nears the top, it cools, gets denser, and sinks again. This creates huge circular currents, like a pot of boiling soup or a lava lamp.

These convection currents in the mantle are a major force that drags the plates around on the surface. They're the engine behind earthquakes, volcanoes, and mountain building. You'll see a lot more of that in the next chapter.

As Earth spins, the liquid iron in the outer core swirls and creates a giant magnetic field. It's why compasses point north. It also protects us by deflecting deadly charged particles from the Sun. When those particles hit the magnetic field near the poles, they make the northern lights (aurora). Every so often, northern Idaho gets a glimpse.

Fun fact: Earth's magnetic poles have flipped hundreds of times in its history. North becomes south. That record is frozen in rocks, and it turned out to be key evidence for plate tectonics.