Geophysical studies have revealed that the Earth has several distinct layers. Each of these layers has its own properties. The outermost layer of the Earth is the crust. This comprises the continents and ocean basins. It is distinguished from the underlying mantle rocks by its composition, lower density, and the lower velocity at which it conducts seismic energy. Most of the crust can be classified either as continental or oceanic. These two types of crust differ in their average age, composition, thickness and mode of origin. The crust has a variable thickness, being 35-70 km thick in the continents and 5-10 km thick in the ocean basins. The crust is composed mainly of alumino-silicates. Silicates are insoluble metal salt with silicon and oxygen in its anion, and with the addition of quartz describe the chemical group of minerals which make up most of the Earth's crust. The next layer is the mantle, which is composed mainly of ferro-magnesium silicates. The mantle is the layer in the Earth's interior between the crust and the core. The Mantle makes up most of the earths mass and is composed of iron, magnesium, aluminum, silicon, and oxygen silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a highly viscous manner, similar to hot tar. It is about 2900 km thick, and is separated into the upper and lower mantle. This is where most of the internal heat of the Earth is located. Large convective cells in the mantle circulate heat and may drive plate tectonic processes. The last layer is the core, which is separated into the liquid outer core and the solid inner core. The outer core is 2300 km thick and the inner core is 1200 km thick. The outer core is composed mainly of a nickel-iron alloy, while the inner core is almost entirely composed of iron. Earth's magnetic field is believed to be controlled by the liquid outer core. The Earth is separated into layers based on mechanical properties in addition to composition.