Igneous Rocks Classification and Formation

Classification

• Igneous rocks are classified based on their origin, texture, and mineral composition.

Formation

• Formed from magma (beneath Earth’s surface) or lava (on the surface).

Extrusive Rocks

• Formed from lava that has erupted onto the Earth's surface.

Intrusive Rocks

• Formed from magma that hardens beneath the Earth’s surface.

Basalt

• Most common extrusive rock, significantly constitutes the oceanic crust and features in shield volcanoes and lava plateaus.

Granite

• Most abundant intrusive rock in continental crust; coarser grained and often forms batholiths in mountain ranges.

Rock Origin

• Classified as either extrusive or intrusive; classification based on formation process.

Texture

• Depends on size and shape of mineral crystals; similar mineral compositions can yield different textures.

Texture Variations

• Different textures include fine-grained, coarse-grained, glassy, or porphyritic.

Cooling Processes

• Rapid cooling of lava results in fine-grained rocks with small crystals.
• Slow cooling of magma results in coarse-grained rocks with larger crystals.

Intrusive vs. Extrusive Crystals

• Intrusive rocks usually have larger crystals compared to extrusive rocks, which typically have smaller crystals.

Porphyritic Texture

• A texture resembling a gelatin dessert with fruit; characterized by large crystals scattered throughout a finer background.

Formation of Porphyritic Rocks

• Occurs in two stages: initial slow cooling forms large crystals, followed by rapid cooling leading to smaller crystals, often as magma rises toward the surface.

Extrusive Rock Texture

• Typically fine-grained or glassy; basalt's crystals are generally too small to be seen without a microscope.

Silica Content

• Affects the fluidity of magma or lava; lower silica typically leads to darker rocks like basalt, while higher silica results in lighter rocks like granite.

Durability

• Many igneous rocks are hard, dense, and durable, making them suitable for tools and construction materials throughout history.

Historical Use of Granite

• Used by ancient Egyptians for statues over 3,500 years ago, and by Incas in Peru for fortress construction; granite was popular for bridges and public buildings in the 1800s and early 1900s.

Use of Basalt

• Commonly crushed for gravel in construction.

Pumice

• Features a rough surface, making it effective as an abrasive for cleaning and polishing tasks.

Perlite

• Formed from heated obsidian; often mixed with soil for starting vegetable seeds.