Geologic processes in the rock cycle determine the formation of the many rock types.
The rock cycle describes the continuous creation, destruction and recycling of Earth’s crust. Not only does this cycle define the formation of igneous, sedimentary and metamorphic rocks, it describes the relationship between the rocks and the geologic processes that form them. Therefore, the key components of the rock cycle are not only the rocks themselves, but the mechanisms by which they were formed.
Cooling and Solidification – Formation of Igneous Rocks
Mountainous regions are composed of igneous rocks.
All rocks start out as magma, the molten material beneath Earth’s crust, which is composed of minerals in a liquid form. Magma rises to the surface, whether through volcanism, internal pressures, uplift of mountainous or other regions, or other modes, and begins to cool and solidify as it reaches the cooler upper crust and atmosphere. As the magma cools, mineral crystals are able to grow, which causes rock to form. Magma that has cooled and solidified becomes igneous rocks, of which there are several types, including granite and basalt.
Weathering, Transport, Deposition – Formation of Sediment
Sediment on the slope of a weathering mountainside.
Mountain chains and other areas of uplift that have created igneous rock formations, are subject to the processes on Earth’s surface. Weathering and erosion by wind and water break down rocks, freeing bits and pieces of minerals and creating sediment. These bits are transported downslope by water, wind or gravity until it is deposited in the lowest place possible. Sediments will accumulate in basins or other low-lying flat areas, where it lies in horizontal layers.
Compaction and Cementation – Formation of Sedimentary Rocks
It has taken millions of years to form the beds of sedimentary rock at Bryce Canyon in Utah.
The horizontal layers of sediment accumulate and settle on top of each other, adding weight, which begins to press the sediment together, much like packing sand into a bucket at the beach. This packing of minerals tighter and tighter is called compaction, which pushes individual grains together and removes the open pore space in between. The sediments are then “glued” together by mineral-rich waters. Natural glues, like silica, calcite or clay, infiltrate the open space between the sediment grains and bonds them together.
Deformation Pressures and Heat – Formation of Metamorphic Rocks
Mineral grains are aligned in this piece of marble, a metamorphic rock.
Rocks that are deeply buried in the Earth, such as under a mountain range or deposited sediments that buries older rocks, are subjected to intense heat and pressures. These two factors cause physical changes in the rocks and minerals. Combined, heat and pressure can actually pull and push the rock formations, creating faults, folds, and changing the appearance of the minerals, and thus, the rocks themselves. Any rock that has been altered by heat and pressure is a metamorphic rock and how it looks is determined by its parent rock, or the original formation. For instance, limestone is the parent rock of marble. When heat and pressure are applied to limestone, the mineral grains within it align themselves and create new patterns, which is classified as an entirely different rock — marble.
If rocks are exposed to enough heat, the minerals will eventually remelt into magma, thus recycling them back into the mantle to go through the cycle again.