Igneous Rocks Classification and Formation

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Questions and Answers

How are igneous rocks classified?

Igneous rocks are classified according to their origin, texture, and mineral composition.

How are igneous rocks formed?

Igneous rock is formed from magma or lava.

What are extrusive rocks?

Extrusive rocks are igneous rocks formed from lava that erupted onto the Earth's surface.

What are intrusive rocks?

Intrusive rocks are igneous rocks that formed when magma hardened beneath the Earth's surface. Signup and view all the answers

What is basalt?

Basalt forms much of the Earth's crust, including the oceanic crust and shield volcanoes; it is the most common extrusive rock. Signup and view all the answers

What is granite?

Granite is the most abundant intrusive rock in the continental crust and is coarser grained. Signup and view all the answers

What does the term 'origin' refer to in igneous rock classification?

Origin includes extrusive and intrusive igneous rocks. Signup and view all the answers

What affects the texture of an igneous rock?

The texture of an igneous rock depends on the size and shape of its mineral crystals. Signup and view all the answers

What are the different textures of igneous rocks?

The textures can be fine-grained, coarse-grained, glassy, or porphyritic. Signup and view all the answers

What type of rock forms from rapid cooling lava?

Rapid cooling lava forms fine-grained igneous rocks with small crystals. Signup and view all the answers

What type of rock forms from slow cooling magma?

Slow cooling magma forms coarse-grained rock with large crystals. Signup and view all the answers

What is true about intrusive rocks?

Intrusive rocks usually have larger crystals. Signup and view all the answers

What is true about extrusive rocks?

Extrusive rocks usually have smaller crystals. Signup and view all the answers

What does a gelatin dessert with chopped fruit mixed in resemble?

A porphyritic texture. Signup and view all the answers

What is porphyritic texture?

A porphyritic texture is a rock with large crystals scattered on a background of much smaller crystals. Signup and view all the answers

When do porphyritic rocks form?

Porphyritic rocks form when intrusive rocks cool in two stages. Signup and view all the answers

Extrusive rocks have a fine-grained or glassy texture.

True Signup and view all the answers

How does the silica content of magma and lava affect its flow?

The silica content affects how easily the magma or lava will flow. Signup and view all the answers

What type of rocks usually forms from lava that is low in silica?

Usually forms dark-colored rocks such as basalt. Signup and view all the answers

What will magma that is high in silica usually form?

Usually forms light-colored rocks such as granite. Signup and view all the answers

What are many igneous rocks known for?

Many igneous rocks are hard, dense, and durable. Signup and view all the answers

What is notable about granite in construction?

Granite is one of the most abundant igneous rocks and has a long history as a building material. Signup and view all the answers

What is basalt crushed for?

Basalt is crushed to make gravel used in construction. Signup and view all the answers

What is pumice good for?

Pumice has a rough surface which makes it a good abrasive for cleaning and polishing. Signup and view all the answers

What is perlite formed from?

Perlite is formed from the heating of obsidian. Signup and view all the answers

Study Notes

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.

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Description

Explore the fascinating world of igneous rocks in this quiz, which covers their classification based on origin, texture, and mineral composition. Learn about extrusive and intrusive rocks, their formation processes, and key examples like basalt and granite.