What does Obsidian glassy texture indicate?

What does Obsidian glassy texture indicate?

If a rock looks like a block of (colored) glass, with no visible mineral crystals, it has a glassy texture. Superficially, a glassy texture suggests cooling that was so extremely fast that no crystals could form. However, composition is also vitally important.

What rock texture is glassy?

Obsidian
Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass. Obsidian is usually black. Now let us briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks.

When the groundmass is glassy in a porphyritic texture it is called?

Porphyritic – This texture describes a rock that has well-formed crystals visible to the naked eye, called phenocrysts, set in a very fine grained or glassy matrix, called the groundmass.

How is Obsidian’s glassy texture formed?

obsidian, igneous rock occurring as a natural glass formed by the rapid cooling of viscous lava from volcanoes. Obsidian is extremely rich in silica (about 65 to 80 percent), is low in water, and has a chemical composition similar to rhyolite. Obsidian has a glassy lustre and is slightly harder than window glass.

What causes some igneous rocks to have a glassy texture?

Intrusive or plutonic rocks crystallize from magma beneath the earth’s surface. Extrusive or volcanic rocks crystallize from lava at the earth’s surface. Very quickly cooled lavas, typically those quenched in water, will have a glassy texture. They cool too quickly to form crystals.

What determines a rock’s texture?

The texture of a rock is the size, shape, and arrangement of the grains (for sedimentary rocks) or crystals (for igneous and metamorphic rocks). Also of importance are the rock’s extent of homogeneity (i.e., uniformity of composition throughout) and the degree of isotropy.

Is glassy intrusive or extrusive?

Igneous Textures

1. Textural Terms Based On Crystal Size
Intrusive Textures Extrusive Textures
Pegmatitic: Slow cooling plus high water content Glassy: Fast cooling plus high silica content

Is glassy texture intrusive or extrusive?

Are glassy rocks intrusive or extrusive?

Because extrusive rocks cool quickly, they only have time to form very small crystals such as basalt or none at all. On the other hand, intrusive rocks grow larger crystals because they take longer to cool. Extrusive rocks are usually fine-grained or glassy while intrusive rocks are coarse-grained.

What does Obsidian glassy texture indicate? If a rock looks like a block of (colored) glass, with no visible mineral crystals, it has a glassy texture. Superficially, a glassy texture suggests cooling that was so extremely fast that no crystals could form. However, composition is also vitally important. What rock texture is glassy? Obsidian Obsidian…