I kept the graphic for your amusement. Now, let's get to know the layers better.
Lithosphere
Let's start with a technical definition. The word LITHO means "rock" and a sphere is a round 3D object. By this definition, a lithosphere is a big ball of rock.
It's a good definition. The outer crust of the earth is composed of rock and light minerals. Everything which we can see is part of the crust of the earth. The truth is, all we have ever seen is the thin crust of the earth.
How thin is the lithosphere? Good question. At the mid-ocean ridge it's probably just over a kilometer and a half thick. For you Americans, that's about a mile. Under the mountains, the crust (uh, lithosphere) may be around 300 km thick (ok, around 185 miles) but it is unknown exactly how thick the lithosphere can get.
That seems pretty thick, but when you think that the earth is
over 6,300 km deep (over 3,900 miles deep) from the outside to the center, the crust is thin indeed.
The crust is solid and it floats upon dense liquid. Like an iceberg, there is more crust under the surface of the ground than above it. This principle is called isostasy.
The crust isn't one single chunk of rock, but it is broken into sections called PLATES. Scientists study the motions of these plates in a science called PLATE TECHTONICS. It is the motion of these plates responsible for earthquakes.
Asthenosphere
Asthenosphere means "weak sphere". This is a funny name considering the asthenosphere supports the lithosphere. Probably
it got its name because it is made of partially molten rock, making this layer vulnerable to stress. This semi-liquid slips around just under the surface, carrying the crust on top of it.
Have you ever eaten a bowl of soup that has been left to cool a while? It gets a thin skin on the surface. The skin will stay in place even when the liquid underneath is jarred. In a similar way, the lithosphere seems more stable than the asthenosphere.
Scientists didn't suspect there was an asthenosphere until the 1920s. This belief was confirmed in the 1960s by analysis of earthquake waves. Scientists knew that earthquake waves passed quickly through solids, but went slower through softer, more liquid rock. By studying earthquake data at various points on the surface, they were able to determine the existence of the weak little asthenosphere.
By the way, you may read that the asthenosphere is a plastic layer. This doesn't mean it's made from plastic, but that it is moveable and bendable. The asthenosphere is also a thin layer, and it marks the transition from the lithosphere to the mantle.
Mantle
Use your imagination for a moment. Picture a big, giant saw cutting the earth in half. Picture the two halves in your mind. Does it look like a boiled egg, or do you see liquid there? Is it hot or cool in your imagination?
If we could really take the earth apart, we would find that about 84% of the volume of the earth is found in the mantle. The mantle is very deep. About 2900 km deep (1800 mi). That's a lot of rock!
A mantle was originally the name of a cloak or coat worn in the winter to keep warm. It's a good name for this thick layer of the earth. The mantle is made of molten rock under pressure, and keeps the earth warm, keeps the plates moving and makes new crust.
No one has ever seen the mantle of the earth. We know it's there because of the behavior of the rocks above it. We know its composition (roughly) by studying volcanoes. We have been able to tell its thickness by studying earthquakes. That's a lot to know without ever seeing it.
Outer Core
The transition from mantle to outer core is a gradual one, but the biggest difference is the composition. The densest materials will be toward the center of the earth. It is believed that the mantle is made of silicon and magnesium, mostly, and the outer core has heavier elements like iron and nickel.
By studying earthquake waves, scientists have determined that the outer core is liquid. This liquid core layer is about 2,200 km (1,380 mi) thick. It is liquid because of the intense heat found there.
Of course, no one has ever seen the core. It would be impossible to survive a rotating sea of molten nickel and iron. Again, we owe much of what we've learned to earthquake studies. We know about this layer because of the earth's magnetic field as well. Together, the outer core and the inner core generate this field. Continue reading to find out more.
Inner Core
The earth's inner-most core is solid nickel and iron. Although it is blistering hot at the center of the earth, there is so much pressure generated by gravity that the inner core remains a solid mass. We also know it's solid because liquids can't have magnetic fields, and the earth indeed has a giant magnet as its core. We also know that it's iron and nickel because these are the only two elements we know are capable of holding magnetic fields.
Scientists believe that the earth is an electro-magnet. When electrical charges rotate around an iron or nickel core, a magnetic field is produced. You can make your own magnet by wrapping wire a bunch of times around a nail and connecting the ends of the wire to a battery. The nail becomes a magnet. In a similar way, the tidal forces cause the outer core to rotate around the inner core. This produces a magnetic effect which is easily detectable if you've ever used a compass.
The inner core is a mere 12,600 kilometers (780 miles) deep. It's not very big compared to the mantle, but it has a big job. That solid core is the center for all the goings-on around it.
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