Which layer of Earth acts as a very effective "insulator," because it's made of rock?

The earth is made up of layers (crust, mantle, outer core, and inner core). Each one has a unique make-up to it.

Crust

This is the outer layer of the Earth about 10 miles or so thick. Mostly made up of rock and loose material. The crust found underneath continents is 3 times thicker than the crust underneath the oceans.

Mantle

The next layer beneath the crust is called the mantle. It extends to a depth of approximately 1,800 miles and is made of a thick solid rocky substance that represents about 85% of the total weight and mass of the Earth.

The first 50 miles of the mantle are believed to consist of very hard rigid rock. The next 150 miles or so is believed to be super-heated solid rock. Below that for the next several hundred miles, the Earth mantle is once again made up of very solid and sturdy rock materials.

Outer Core

Traveling deeper within the Earth, we next would encounter the Earth’s outer core, which extends to a depth of around 3000 miles beneath the surface. It is believed that this outer core is made up of super-heated liquid molten lava. This lava is believed to be mostly iron, and nickel.

Inner Core

Finally, we would reach the Earth’s inner core. The inner core extends another 900 miles inward towards the center of the Earth. It is believed that this inner core is a solid ball of mostly iron, and nickel.

The mantle is made of much denser, thicker material, because of this the plates "float" on it like oil floats on water. Many geologists believe that the mantle "flows" because of convection currents.