Earth’s Dynamic Geology: Rock Cycle and Formation Processes
Geology: Earth’s Materials, History, and Phenomena
Geology is the science of Planet Earth as a whole. It describes the materials that form it, helps us learn its history and evolution, and seeks to understand the cause of endogenous and exogenous phenomena.
The Rock Cycle: Earth’s Dynamic Processes
The rock cycle describes how magma becomes igneous rocks, and how sediments can be generated, forming sedimentary or metamorphic rocks. Igneous and sedimentary rocks, in turn, can give rise to metamorphic rocks and new magma.
Magma: Molten Rock Beneath Earth’s Surface
Magma is a fluid composed mainly of incandescent, molten silicate minerals and oxides.
Crystallization: Mineral Formation from Magma
During the cooling of magma, tiny solid minerals form. These minerals tend to create crystalline bodies, characterized by their regular spatial forms and chemically homogeneous material.
Igneous Rocks: Formation and Classification
On Earth, igneous rocks form in two main geographical environments: oceanic and continental. In oceanic environments, these rocks are typically rich in ferromagnesian minerals and are termed basic or ultrabasic rocks. In continental environments, they are abundantly rich in silica and aluminum minerals and are called acid rocks. These classifications are based on the chemical composition of the rocks.
According to their depth of formation, igneous rocks can be:
- Plutonic rocks: Formed when magma cools inside the Earth’s crust.
- Volcanic rocks: Formed when magma cools on the Earth’s surface.
Weathering, Erosion, and Transport: Shaping Earth’s Surface
Sediments are formed through the processes of weathering, erosion, and transport of materials that make up the Earth’s crust.
Weathering (also known as denudation), as a precondition to erosion and transportation, is the alteration of rocky material exposed to air, moisture, or the effect of organic matter. There are two main types of weathering:
- Mechanical weathering: Refers to the physical disintegration of materials.
- Chemical weathering: Refers to their chemical decomposition.
Other forms of alteration, not strictly weathering, include tectonic and hydrothermal alteration, which are particularly important in the Andean environment.
Products of weathering include: gravel, sand, silt, clay, and siliceous, carbonated, and iron solutions, among others.
Erosion is the process of detachment of altered rock units through agents such as ice, water, and wind. Gravity is also a key agent.
Sediment: Accumulated Earth Materials
Sediments are rock materials, dead organic matter, chemicals, and other substances that accumulate as a result of weathering and rock alteration. They can also form through the precipitation of dissolved elements in the hydrosphere or the accumulation of organic matter in continental or marine environments.
Diagenesis and Lithification: Forming Sedimentary Rocks
When sediments are buried, various chemical and physical processes occur that can lead to significant changes in the original material.
Diagenesis encompasses all those changes occurring at relatively low temperatures and pressures in areas not too deep below the Earth’s surface. Key diagenetic processes include cementation, consolidation, drying, and crystallization.
Perhaps the most obvious effect of diagenesis is the transformation of loose, unbound particles into a compact and hard sedimentary rock. This specific aspect is called lithification, which is the conversion of materials like sand into sandstone, clay into claystone (or argillite), and peat into coal.
Sedimentary Rocks: Common Types and Occurrence
The most important sedimentary rocks, due to their abundance, include: shale, sandstone, and limestone.
Although sedimentary rocks constitute a very small proportion of the Earth’s crust volume, there is a high chance of finding them on the surface, where three-quarters of all sedimentary rocks are exposed. For example, the Colombian Eastern Cordillera is mainly sedimentary in nature.
Metamorphism: Transforming Existing Rocks
Metamorphism is the change of one coherent class of rock into another. This process occurs below the zone of sedimentation and weathering but above the fusion zone or magma production.
There are three primary agents of metamorphism, and at least two of them are always present: temperature, pressure, and chemically active fluids. The new rock undergoes mechanical, chemical, or chemo-mechanical processing, resulting in a change in its structure or mineral composition without altering its overall chemistry.