Understanding the Three Major Rock Types and Their Formation

The Composition and Classification of Rocks

Rock is any natural, inorganic material made up of one or more minerals. Rock forms the solid part of the Earth's crust.

Rocks formed of only one type of mineral are called mono-mineralic rocks, but rocks are usually formed of more than one mineral.

Exceptions to the Definition

There are two notable exceptions often referred to as 'rocks':

• Oil: Considered a liquid 'rock'.
• Coal: Formed from the remains of plants (organic material).

The Three Main Types of Rocks

Rocks are classified into three primary types based on their formation process:

1. Igneous Rocks: Rocks formed when magma cools and solidifies. (Magma is molten rock.)
2. Sedimentary Rocks: Rocks formed by the accumulation and cementation of fragments of other rocks or organic matter.
3. Metamorphic Rocks: Rocks formed by the transformation of existing rocks due to an increase in temperature, pressure, or both simultaneously.

Sedimentary Rocks

Sedimentary rocks are formed by fragments of other rocks or animal remains. Depending on the origin of the fragments, they are divided into three main groups:

1. Detrital Sedimentary Rocks

   These rocks are formed from fragments of other rocks. They are classified based on the size of the fragments:

   • Big fragments (≥ 2mm):
     • Conglomerate: Rounded fragments.
     • Breccia: Angular fragments.
   • Medium fragments (Smaller than 2mm, but larger than 1/256mm).
   • Small fragments (≤ 1/256mm).

2. Chemical Sedimentary Rocks

   These rocks are formed from precipitation when water evaporates, leaving dissolved minerals behind.

3. Organic Sedimentary Rocks

   These rocks originate from the transformation of the remains of living beings buried millions of years ago.

Igneous Rocks and Volcanic Processes

Igneous rocks are rocks formed from the cooling and solidification of magma.

Classification by Cooling Rate

• Plutonic Rocks (Intrusive): Formed if magma cools and solidifies slowly under the ground.
  • Characteristics: They have mineral grains visible to the naked eye (e.g., Granite).
• Volcanic Rocks (Extrusive): Formed if lava cools and solidifies quickly on the surface or under the sea.
  • Characteristics: Due to the rapid cooling, they either lack visible crystals or contain vesicles (bubbles) (e.g., Basalt).

Magma vs. Lava

Magma

Located inside the Earth's crust. Magma is a mixture of molten rock and gases.

Lava

Located outside the Earth's crust or under the sea. Lava is essentially magma that has lost its dissolved gases.

Metamorphic Rocks and Metamorphism

Metamorphic rocks are rocks that have undergone a process called metamorphism.

Metamorphism is a change in the conditions of an existing rock caused by:

• Increasing pressure.
• Increasing temperature.
• A combination of both.

As a result of metamorphism, the preexisting rock changes into a new one, known as a metamorphic rock.