The Three Major Rock Types: Formation and Classification

Major Rock Types and Their Formation Processes

Igneous Rocks

Igneous rocks are formed by the solidification of molten material (magma) or lava.

Classification of Igneous Rocks

• Plutonic Rocks (Intrusive): Igneous rocks that have been formed from slow cooling at depth in large bodies of magma.
• Volcanic Rocks (Extrusive): Those that were formed by the rapid cooling of lava on Earth's surface or beneath the sea.
• Hypabyssal Rocks (Intermediate): These rocks are formed similarly to volcanic and plutonic rocks, but the material fills grooves and seeps into large rocky veins or fissures.

Sedimentary Rocks

Sedimentary rocks are formed by the accumulation of sediments, subjected to physical and chemical processes (known as diagenesis), leading to more or less consolidated material of a certain consistency.

Classification of Sedimentary Rocks

• Detrital or Clastic Sedimentary Rocks: Formed by the accumulation of debris resulting from erosion and deposited primarily by gravity.
• Chemical Sedimentary Rocks: Formed by the precipitation of dissolved products of erosion. These rocks are mainly classified according to their chemical composition.
• Organogenic Sedimentary Rocks: They consist mainly of accumulations of remains of living organisms. This category includes sedimentary fuels such as carbonaceous deposits (like coal) and kerogen deposits.

Metamorphic Rocks

Metamorphic rocks are those formed from other rocks by a process called metamorphism. Metamorphism never involves the fusion of the rock and occurs equally in igneous, sedimentary, or other metamorphic rocks when they are subjected to high pressures, high temperatures, or fluid interaction.

Classification of Metamorphic Rocks (By Grade)

• Low-Grade Metamorphic Rocks: Formed at temperatures of about 200 degrees Celsius. They contain minerals such as chlorite, muscovite, and biotite.
• Intermediate-Grade Metamorphic Rocks: Contain minerals such as garnet and staurolite.
• High-Grade Metamorphic Rocks: Formed at temperatures of about 800 degrees Celsius. In this environment, the rocks are about to melt. Under this intense heat, a mineral called sillimanite forms, which is a common mineral in the rock gneiss.