The Mantle Sitting on top of the outer core, we find the mantle. This layer is by far the thickest layer of Earth, about 1,800 miles thick! It also makes up about 85% of Earth's volume. Like the core, the mantle contains mostly iron, but in the form of silicate rocks. You might be surprised to learn that this rock actually moves like a fluid, similar to how silly putty moves. If you poke silly putty hard, it acts like a solid, but if you slowly pull it apart, it acts like a liquid. We call this ability of rock to move without breaking plasticity. The mantle can also be divided into two portions, the upper mantle and the lower mantle. The lower mantle is completely solid because, like the solid inner core of Earth, the pressure is just too great for it to melt and flow. The upper mantle is also known as the asthenosphere, which flows as convection currents. Convection occurs in all fluids and is the rising of warm particles and sinking of cool particles. So, as the material in the upper mantle warms, it rises straight up, and as it rises, it cools and then sinks back down.
There are two different types of crust: thin oceanic crust that underlies the ocean basins and thicker continental crust that underlies the continents. These two different types of crust are made up of different types of rock. The thin oceanic crust is composed of primarily of basalt and the thicker continental crust is composed primarily of granite. The low density of the thick continental crust allows it to "float" in high relief on the much higher density mantle below.
What is Plate Techtonics?
Plate tectonics is the theory that Earth's outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core. The plates act like a hard and rigid shell compared to Earth's mantle. This strong outer layer is called the lithosphere.he driving force behind plate tectonics is convection in the mantle. Hot material near the Earth's core rises, and colder mantle rock sinks
There are three main types of plate boundaries:.
1. Convergent boundaries: where two plates are colliding. Subduction zones occur when one or both of the tectonic plates are composed of oceanic crust. The denser plate is subducted underneath the less dense plate. The plate being forced under is eventually melted and destroyed.:
i. Where oceanic crust meets ocean crust Island arcs and oceanic trenches occur when both of the plates are made of oceanic crust. Zones of active seafloor spreading can also occur behind the island arc, known as back-arc basins. These are often associated with submarine volcanoes.
ii. Where oceanic crust meets continental crust The denser oceanic plate is subducted, often forming a mountain range on the continent. The Andes is an example of this type of collision.
iii. Where continental crust meets continental crust Both continental crusts are too light to subduct so a continent-continent collision occurs, creating especially large mountain ranges. The most spectacular example of this is the Himalayas.
2. Divergent boundaries – where two plates are moving apart. The space created can also fill with new crustal material sourced from molten magma that forms below. Divergent boundaries can form within continents but will eventually open up and become ocean basins.
On land Divergent boundaries within continents initially produce rifts, which produce rift valleys.
ii. Under the sea
The most active divergent plate boundaries are between oceanic plates and are often called mid-oceanic ridges.
3. Transform boundaries – where plates slide passed each other.