Definitions

1. Extrusive rocks are usually distinguished from intrusive rock s on the basis of their texture and mineral composition.
2. Extrusive rocks are igneous rocks that have formed (and cooled more quickly) at the surface of the Earth (like obsidian).
3. Extrusive rocks are formed when magma reaches the surface of the Earth, then cools and hardens.
4. Extrusive rocks are magma ejected from volcanos and cooling rapidly on the Earth's surface.
5. Extrusive rocks are fine-grained in texture and crystallise quickly from lava on or near the earth's surface.

Cool

1. Because extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals.

Volcanoes

1. Extrusive rocks are those which have erupted from volcanoes.

Texture

1. Most extrusive rocks which cool quickly have this texture.

Intrusive

1. They may be classified as intrusive or extrusive rocks.

Small Holes

1. Some extrusive rocks are riddled with vesicles, small holes in the rock left by gas bubbles during cooling.

Crystal Growth

1. Aphanitic rocks are extrusive rocks that cooled so quickly that crystal growth was inhibited.

Crystals

1. Most extrusive rocks exhibit an aphanitic texture, meaning that their crystals are not visible to the unaided eye.

Basalt

1. In the case of extrusive rocks, the answer is obvious: we can see basalt (for example) forming when lava flows cool; there is no difficulty there.

Small Crystals

1. The quick cooling doesn't allow the formation of large crystals so most extrusive rocks have small crystals or none at all.

Volcanic Rocks

1. Extrusive rocks are also called volcanic rocks.

Fine-Grained

1. However, extrusive rocks are generally too fine-grained to identify their minerals by eye.

Rocks

1. These rocks are called volcanic or extrusive rocks.

Pumice

1. In some extrusive rocks, like pumice and scoria, air and other gases are trapped in the lava as it cools.

Obsidian

1. Some extrusive rocks, such as obsidian and pumice, cool so rapidly that they completely lack crystal structure and are considered a volcanic glass.
2. Extrusive rocks defined mainly by texture are discussed under rock textures: obsidian, scoria, pumice, tuff, and volcanic breccia.
3. Basalt and obsidian are two kinds of extrusive rocks that are quite solid.

Intrusive Rocks

1. Since magma that forms intrusive rocks solidifies slowly, most intrusive rocks have larger crystals than extrusive rocks.
2. Those which began their lives below the surface are called intrusive rocks, while those which cooled on the surface are referred to as extrusive rocks.

Mineral Composition

1. Extrusive rocks are usually distinguished from intrusive rocks on the basis of their texture and mineral composition.

Aphanitic

1. In extrusive rocks, where cooling is much more rapid, the individual mineral crystals are usually not visible and these rocks are termed aphanitic.
2. Small-grained rocks generally form from volcanic eruptions and are often referred to as extrusive rocks; they are also called aphanitic.

Extrusive Rocks

1. Igneous rocks which form by the crystallization of magma at the surface of the Earth are called extrusive rocks.
2. Rocks formed at the surface of the earth are called extrusive rocks.
3. Extrusive rocks are formed on the surface of the Earth from volcanic activity.

Categories

1. Intrusive Rocks
2. Aphanitic
3. Mineral Composition
4. Matter > Materials > Rocks > Volcanic Rocks
5. Materials > Rocks > Igneous Rocks > Obsidian
6. Books about "Extrusive Rocks" in Amazon.com
   

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