Classification and Formation of Earth's Rocks

What Are Rocks?

Rocks are solid aggregates of minerals. They are more common than individual minerals.

The Three Types of Rocks

There are three different types of rocks:

• Igneous
• Sedimentary
• Metamorphic

Igneous Rocks

Igneous rocks are formed from magma that has cooled and solidified.

Plutonic Rocks

These rocks are formed deep underground, and the cooling process is very slow. The slower the cooling process, the larger the crystals formed (e.g., granite).

Volcanic Rocks

These rocks are formed on the surface, often related to volcanoes. The cooling process is very quick, preventing the formation of visible crystals (e.g., obsidian, pumice).

Sedimentary Rocks

Sedimentary rocks are made of fragments of other rocks (sediments). They often contain fossils.

Formation by Grain Size

• Conglomerate: Big grains.
• Sandstone: Small grains, but still visible.
• Clay: Very small grains, which are not visible.

Formation by Chemical Precipitation

Sedimentary rocks can also be formed by chemical precipitation. These types can also contain fossils (e.g., limestone).

Fossils and Fossilization

Fossils are the remains or traces of an organism from the past that have been preserved.

The Fossilization Process

1. An organism dies and is quickly covered by sediment.
2. The soft parts of the body decompose, while the hard parts remain and make a print of their outer surface in the sediment.
3. Sometimes, the hard parts become mineralized and remain preserved.
4. More frequently, the hard parts dissolve, and the space left fills with groundwater. Fossils formed this way are the external mold of the organism.

Metamorphic Rocks

Metamorphic rocks are created by the transformation of existing rocks due to high pressure and/or high temperature. Some exhibit foliation, a feature unique to metamorphic rocks (e.g., slate).

Foliation

Foliation is the layered disposition of minerals within the rock.

Practical Applications of Rocks

Aggregates:

Used in construction (gravel, sand, crushed stone).

Building Stones:

Used in construction due to strength and resistance (they do not easily breach).

Ornamental Rocks:

Used to decorate exteriors and in sculptures.

Cementing Materials:

Materials that hold building materials together (cement, lime).

Other Uses:

• Brick/Tiles
• Glass

Fossil Fuels:

• Coals (peat, lignite, black coal, anthracite)
• Petroleum (petrol, butane, tar, oil)

The Rock Cycle

Any kind of rock can be transformed through the following processes:

• Melting: Transforms into magma, leading to the formation of igneous rocks.
• Heat and Pressure: Leads to the formation of metamorphic rocks.
• Weathering, Erosion, and Deposition: Breaks the rock into pieces, leading to the formation of sedimentary rocks.