Rocks and Earth

As you might expect - since most of the earth's surface is covered by water - molten material from inside the earth often breaks through the floor of the ocean and flows from fissures where it is cooled by the water resulting in the formation of igneous rocks. Some low grade metamorphism often occurs during and after the formation of the rock due to the intrusion of the material by the sea water. As the molten material flows from the fissure, it begins forming ridges adjacent to it.

If we examine the rock cycle in terms of plate tectonics, as depicted in the figure above, we see that igneous rocks form on the sea floor as spreading ridges. As the rocks cool, and more magma is introduced from below, the plate is forced away from the spreading ridge, and acquires a sediment cover. As shown in the figure, in this case, the oceanic plate eventually "dives" under the adjacent continental plate. As the oceanic plate travels deeper, high temperature conditions cause partial melting of the crustal slab. When that occurs, the surrounding "country rock" (existing adjacent rock) is metamorphosed at high temperature conditions by the contact. The molten material is either driven to the surface as volcanic eruptions, or crystallizes to form plutonic igneous rocks.

an apple can be used to represent the structure of the earth. The peel represents the crust, the white portion of the apple, the mantle. The core of the apple represents the core of the earth. The thickness of each "layer" of the apple approximates the relative thickness of the earth's structural features. Almost all of our minerals, oil and gas come from just a thin outer portion of the crust at maximum depths of about three miles.