The Earth is Made of Rocks
Thousands of different types of rocks and minerals have been found on Earth. Most rocks at the Earth's surface are formed from only eight elements: oxygen, silicon, aluminum, iron, magnesium, calcium, potassium, and sodium. These elements are combined in a number of ways to make rocks that are very different. Rocks are divided into three basic types, igneous, sedimentary and metamorphic, depending upon how they were formed. Igneous rocks form when melted rock from deep within the Earth cools and becomes solid. The chemical composition of the magma and cooling rate determine the type of igneous rock it becomes. Sedimentary rocks are formed from pre-existing rocks. Weathering breaks rocks into smaller bits and pieces. Wind, water, and ice carry these pieces and deposit them and they accumulate in layers on the Earth’s surface. Metamorphic rocks form when rocks are subjected to high heat and pressure. The heat and pressure substantially changes them from their original form.
Picture
Picture
The Rock Cycle
The Earth’s crust is made of huge chunks of land called plates. There are seven major plates and lots of little ones. They move around very slowly. When plates shift, pull apart, or push together, mountains are formed. Of course this takes a very long period of time. The movement of these plates is called plate tectonics. The plate tectonics theory provides an explanation for how rocks are recycled from igneous to sedimentary to metamorphic and back to igneous again.
The movement of these plates results in intense heat and pressure. Some rocks get pushed deep into the Earth and melt. The melted rock is called magma. When magma cools and hardens, it forms igneous rock. If the magma cools and hardens slowly underground, large crystals are formed and the rock is called intrusive igneous rock. If the magma cools quickly, perhaps after erupting from a volcano, small crystals are formed and the rock is called extrusive igneous rock.
On the surface, weathering breaks rocks into bits and pieces and erosion carries them away. These bits of rock are deposited on the Earth’s surface, often under water. Over time, many layers accumulate, creating pressure and heat. Salts that are present in the sediment crystallize as water is squeezed out of the layers. This helps to cement the particles of sediment together making sedimentary rock.