The Earth's inner forces

Ah... the Earth! Our home planet. So solid and stable. A steady place to plant your feet on, right?

Hey! What was that? How could the Earth shake if it's solid? The Earth might look solid... ... but actually consists of layers that aren't so solid and stable at all.

Let's open the Earth up, and have a look inside. At its center, is the inner core... ... which is made of heavy metals, like iron and nickel. In the deepest part of the core, it's really hot. About 6,000 degrees Celsius.

Outside the hot metal core, you'll find the layer that makes up most of the mass of our planet. This is the mantle. The mantle doesn't have as much metal in it as the core, and is not as hot. Not hot enough to be in liquid state... ... but neither is it fully solid.

It's slow flowing -- viscous. A gigantic slowly moving viscous sludge of solid and molten rock. The reason that the mantle is moving has to do with heat. The deepest part of the mantle is heated by the core. The heated, slow moving rock starts moving up, toward the outermost part of the mantle.

Here, closer to the Earth's surface, it cools down, is pushed sideways, and sinks downward again. When heat causes this kind of movement, we call it: convection. Hot rock from deep down is transported by convection from the Earth's core, and eventually reaches the outermost layer, the crust. The crust is the layer that we walk around on, and build our houses on. Under your feet, the crust is some thirty to fifty kilometers thick.

But the crust is only this thick under the continents. Compare it to the crust you'll find at the bottom of the sea... Down there, at the ocean bed, the crust is much thinner. The oceanic crust can be as thin as five kilometers. Both on land and under the ocean, the crust is in solid form, rock hard and rigid.

But staying put, it certainly isn't... Because the crust sits on the mantle... ... and the mantle moves and shifts, so the crust follows along. Little by little, these movements shift the ground under your feet, pushing and shoving... The crust moves about as slowly as...

your fingernails grow. A slow movement, but with a huge effect. Because, if you just wait a hundred million years or so, these tiny shifts, added up together, create tall mountains and deep valleys. But sometimes, the different sections of the crust get stuck to each other, and can't move. The movement in the mantle keeps pulling on the crust, until eventually, it comes loose, and then the crust moves a lot, all at once.

We call this... ... an earthquake. When an earthquake takes place under the water, it sometimes generates an enormous wave, a tsunami. And when a tsunami strikes land, it can crush buildings and boats along an entire coast. In some places, rock in the form of molten-magma from the earth's mantle finds a pathway through the crust.

Then we have a volcano. Fascinating. And deadly. All of these things are caused by forces from within the earth -- the Earth's endogenous forces. Endogenous - comes from within.

So the Earth may feel solid, but its endogenous forces constantly change its shape and properties. Sometimes slowly... ... and sometimes very suddenly.