The Rock Cycle

The Rock Cycle & Classification of Rocks

The rock cycle is the formation, breakdown and reformation of a rock as a result of sedimentary, igneous, and metamorphic processes. The rock cycle is an illustration that is used to describe how the three rock types are related and how Earth processes change a rock from one type to another over time. Each group contains a collection of rock types that differ from each other on the basis of the size, shape and arrangement of mineral grains. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks are forced to change as they encounter new environments as they do not remain in equilibrium.

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Weathering:
At the Earth’s surface, rocks are broken down; this is caused by rainwater, extremes of temperature, and biological activity. Weathering should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind and gravity. There are three types of weathering: physical, chemical and biological. Physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, water ice and pressure. Chemical weathering involves rain water combining with mineral grains in rocks to form new minerals. Living organisms such as trees, bacteria, and animals contribute to biological weathering.

Erosion
Erosion is the removal of soil and rock particles. It usually occurs due to transport by wind, water or ice.

Deposition of sediment
Deposition is the laying down of sediment carried by wind, water, or ice. Deposition occurs when the forces responsible for sediment transportation are no longer sufficient to overcome the forces of particle weight and friction which resist motion.
Burial and compaction:
Layers of sediments are piled one upon another, so the weight of the layers compacts the sediment grains.

Sedimentary rock
Sedimentary rocks are those formed by other pieces existing rocks, bones or other formerly living things. Rocks are weathered and eroded to smaller units, then transported and deposited along with other pieces of rock called sediments.
With the weight and pressure of thousands of feet of sediments above them, sediments are cemented together and compacted and hardened over time. Sedimentary rocks are important as they contain information about the history of Earth. The composition sediments have clues of the original rock. Unlike igneous and metamorphic rocks, sedimentary rocks can contain fossils as they form at pressures which do not destroy fossil remains. Roughly three-quarters of the Earth’s bedrock on the continents is sedimentary.
Sedimentary rocks are formed in three main ways: by deposition of weathered remains of other rocks – this is known as clastic sedimentary rocks, by the disposition of the results of biogenic activity and by precipitation from solution.

Clastic
Clastic sedimentary rocks are made from fragments of materials of other minerals. Rocks such as sandstone were formed from rocks which were broken down into clasts by weathering, which were then transported elsewhere. The classification of clastic sedimentary rocks involve many variables such as particle size, composition of particles, the cement and the matrix must be taken into consideration. All rocks disintegrate when bared to chemical and mechanical weathering at the Earth’s surface. The breakdown of rock into particles without producing changes in the chemical composition of the minerals in the rock is known as mechanical weathering. The process of chemical weathering is the breakdown of rock by chemical reaction where the minerals within the rock are altered into smaller particles which are easier to be carried away.

Biogenic
Biogenic sedimentary rocks are the process where living organism extract ions dissolved in water to make such things as shells and bones. Biogenic sedimentary rocks include corals and foraminifera which cover the ocean floor with layers of calcite which then form limestone.

Precipitate
When mineral solutions such as sea water, evaporate precipitate sedimentary rocks form. Halite and gypsum are such rocks.

Deformation and metamorphism
Deformation describes the processes by which rocks are folded and faulted. Rocks become deformed when the Earth’s crust is stretched or compressed. Tension occurs when tectonic plates are pulled apart and the crust becomes thinner. Compression take places as tectonic plates are pushed together and the crust becomes shorter and thicker.
Metamorphic rocks have their appearance altered due to extreme pressure and heat. Metamorphic rocks can be formed either, beneath mountain ranges where rocks are buried and squashed by movements of the Earth’s crust, or at shallower depths if heated up y nearby intrusions of hot magma.

Metamorphic rock
Metamorphic rocks are the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism. The protolith may be a sedimentary rock, igneous rock or another older metamorphic rock. Metamorphic rocks are classified by texture and chemical and mineral assembly, and also make up a large part of the Earth’s crust. These rocks are formed by the intrusion of magma, into solid rock and form at the place of contact between magma and solid rock where temperature is high. They are also formed deep beneath the Earth’s surface. Sedimentary rocks can become metamorphic rocks if the thousands of feet of sediments above them can apply enough heat and pressure. Metamorphic minerals form at high temperatures and pressures only, associated with the process of metamorphism. Examples of such minerals are sillimanite and kyanite. The change of the size of rock particle during metamorphism is called recrystallisation.

Foliated and non-foliated
There are two basic types of metamorphic rocks: foliated and non-foliated. Foliated rocks are those such as schist and slate that have a layered or banded appearance that is formed by direct pressure and heat. Non-foliated rocks are those such as marble which do not have a layered appearance.

Crystallization of magma:
To form magma from the melting of a solid rock is controlled by: temperature, pressure and composition, these factors control the viscosity of the magma.

Igneous Rock
All igneous rocks start out as melted rock, then crystallize or freeze. Igneous rocks are formed either above ground or underground, and originated from cooling and solidification of molten rock. Igneous rocks form in two different environments, which are intrusive and extrusive.

Intrusive
When underground, then molten rock is called magma. The magma becomes intrusive igneous rocks when the magma cools underground. Examples of such rocks are gabbro and granite.

Extrusive
Extrusive rocks cool quickly on the Earth’s surface. Igneous rocks can form when volcanoes erupt, which causes the magma to rise above the Earth’s surface. Examples of such rocks are basalt and dacite.
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