The different layers have different densities and so float one
upon another. Some of the molten layers have different temperatures and different viscosities, which affect how they mix or do not mix together.
A few lakes at the Earth's surface have a similar effect, resulting in two distinct layers of water that never mix, with the warm water staying at the surface, and the much colder water staying at the bottom.

The earth has different layers for a reason similar to chocolate milk. When you put chocolate syrup in milk, where does it immediately go? To the bottom. We don't exactly know how the earth itself formed, but the way it developed its layers is by the heaviest elements sinking to the bottom. The earth's core is made of iron and nickel, with maybe a little sulfur - that's the heaviest part. The mantle is made of heavier minerals in the silicate family, and within the mantle there are certain regions where there is a "phase transition" from one type of crystal structure to another, where the weight of the overlying layers is heavy enough to squish the atoms closer together. The lightest material floats like a crust on top - we call it the crust of the earth, even.

There are many ideas about why the Earth has many different layers, and no one really knows for sure. Most scientists believe that the existence of layers is because of the materials that make up rocks and minerals. Under different pressures and temperatures, rocks and minerals change "state", in other words, they can melt or crystallize to form different rocks. For example, do you know about the gemstone called Garnet? It's a shiny red rock that people polish and cut and use in rings and necklaces. Anyway, this rock, when buried about 1,500 feet (about the same as the height of the tallest skyscraper in the U.S.) below the earth's surface, will change. It becomes heavier because the crystals that make up the garnet actually become packed closer together. (You can recreate this by seeing how many marshmallows you can fit into your open hand. Now make a fist and squish them in. You can fit a lot more in, right?) So, rocks at different depths under the earth's surface will look and feel differently, forming the layers that you learned about.


Has the size of the earth changed since it was created? Well, scientists don't know the answer to this question for sure either because no one can go back in time. We think that since the earth formed all of its layers, that the size hasn't changed too much, but the shape of it has. It has always looked like a ball, but if you look really closely, it's actually a bit flattened at the north and south poles, and it bulges at the equator. This flattening and bulging is caused by forces caused by the spinning of the earth (rotation). The shape has changed slightly over time. The location of the continents and oceans has changed too, as you learned in class.

The earth is a fascinating thing to learn about - keep up the enthusiasm!

The layered structure of the Earth seems to be evidence that at one time the Earth was hot enough to be molten (about 3000 degrees F or higher). A molten Earth would have allowed the more dense materials (metals) to settle in the core while the lighter materials (silicates, for example) would have stayed more towards the surface. As the Earth cooled below the melting point, the layered structure would be preserved.

The earth has different layers because as it formed, the lighter parts (like continental crust) floated to the surface, and the really heavy parts (like iron and nickel in the core) sank to the middle. It is just like when you mix oil and water: the oil will float to the surface because it is lighter (or less dense) than the water.
As far as I know, the earth size has not changed drastically since it was formed.

Basically the Earth is layered because of gravity. The earth formed in the molten state. The liquid iron which is very dense fell to the center. The rest of the earth is made up of silicate materials with SiO2 being the main component. The Earth's mantle is made up of low silica compositions (around 40% silica) and that is _denser_ than the high silica stuff that makes up the continental crust. You can think of it in terms of shaking up a bottle filled with some fluid and suspended minerals of different density. If you shake it up (mix) and then put it on table and let gravity act, you will note that the denser minerals sink faster and hence accumulate on the bottom. This is a simple analogy and many details of reality are left out, but it does give the essence of the mechanics.

The radius of the Earth has not substantially changed since it accreted 4.56 billion years ago...once it had cooled to the point that it was mainly solid as it is today. Of course the outer core is still molten...