What is a Rock?

• According to National Geographic, rock is a Natural substance composed of solid crystals of different minerals that have been fused together into a solid lump.

• According to Brittanica Encylopedia, it is naturally occurring and a coherent aggregate of one or more minerals.

Rocks are classified into three types:

1. Igneous Rocks

2. Sedimentary Rocks

3. Metamorphic Rocks

1. IGNEOUS ROCKS

   It comprises 95% of the Earth’s crust.

How do igneous rocks form?

Magma - Solidification of Lava - Igneous Rocks

There are two types of Igneous Rocks

1. Intrusive Igneous Rocks (PLUTONIC)

   • Magma cools down and solidifies inside the earth’s surface.

   • Slow cooling = Large crystals

     Examples of Intrusive Igneous Rocks:

     Diorite: It is a mixture of feldspar, pyroxene, hornblende, and sometimes quartz.

     Gabbro: It contains feldspar, pyroxene, and sometimes olivine.

     Granite: it contains mainly quartz, feldspar, and mica minerals

2. Extrusive Igneous Rocks (VOLCANIC)

   • Magma cools down and solidifies above the Earth’s surface

   • Fast cooling = small crystals

     Examples of Extrusive Igneous Rocks:

     Andesite: It is composed mainly of plagioclase with other minerals such as hornblende, pyroxene, and biotite.

     Basalt: it is composed mainly of plagioclase and pyroxene.

     Obsidian: forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form.

2. SEDIMENTARY ROCKS

   Comprised of Sediments

   Lithification: it is a complex process whereby freshly deposited loose grains of sediment are converted into rock.

How do Sedimentary rocks form?

Lithification processes:

• Compaction: Reduces pore space due to the weight of sediments above.

• Cementation: Dissolved substances precipitate out and act as glue.

• Recrystallization: Form new crystalline mineral grains from the old ones.

• Diagenesis: the cycle repeats.

3 CLASSIFICATIONS OF SEDIMENTARY ROCKS:

• CLASTIC ROCKS: formed when individual sediment made by fragments of rocks, minerals, or shells are cemented together.

• CRYSTALLINE ROCKS: Chemical sedimentary rocks. It is formed by the crystallization of chemical precipitates.

• BIOCLASTIC ROCKS: organic sedimentary rocks. Formed mostly by the remains of organisms.

3. METAMORPHIC ROCKS

   When igneous rocks and sedimentary rocks are exposed to high temperature and pressure.

   Metamorphism: change from one rock formation to another.

• FOLIATED METAMORPHIC ROCKS: Have layered or branded appearance. It is produced by exposure to heat and directed pressure.

• NON-FOLIATED METAMORPHIC ROCKS: Doesn’t have a layered or branded appearance. It formed around igneous intrusions where temperatures are high but the pressures are relatively slow.