Table of Contents
What causes an igneous rock to have a glassy texture?
Very quickly cooled lavas, typically those quenched in water, will have a glassy texture. They cool too quickly to form crystals. Glasses do not have an orderly arrangement of atoms and there are therefore no minerals, in the strict sense, in them.
What is the composition of a glassy rock?
volcanic glass, any glassy rock formed from lava or magma that has a chemical composition close to that of granite (quartz plus alkali feldspar). Such molten material may reach very low temperatures without crystallizing, but its viscosity may become very high.
What does glassy mean in rocks?
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.
Which igneous rock is made mostly of quartz and has a vesicular glassy texture?
2.1 Felsic Extrusive Igneous Rocks. Rhyolite is extrusive equivalent of granite magma. It is composed predominantly of quartz, K–feldspar and biotite. It may have any texture from glassy, aphanitic, porphyritic, and by the orientation of small crystals reflecting the lava flow.
How do light colored igneous rocks differ from dark colored rocks?
Light-colored igneous rocks are more granitic (felsic) in composition and contain more feldspar and silica. Dark-colored igneous rocks are more basaltic (mafic) in composition and contain more iron and magnesium. Texture is the size, shape, and distribution of the particles that collectively constitute a rock.
What is the texture of intrusive igneous rocks?
If magma cools slowly, deep within the crust, the resulting rock is called intrusive or plutonic. The slow cooling process allows crystals to grow large, giving the intrusive igneous rock a coarse-grained or phaneritic texture. The individual crystals in phaneritic texture are readily visible to the unaided eye.
What does composition of a rock mean?
Composition refers to both the types of minerals within a rock and the overall chemical makeup of the rock (the two are obviously related). Texture refers to the size and arrangement of the minerals or grains that make up a rock. Igneous Rock Textures.
What mineral is glassy?
Vitreous
Of the nonmetallic lusters, glassy is the most common and means the surface of the mineral reflects light like glass. Pearly luster is important in identifying the feldspars, which are the most common type of mineral….Luster.
| Table 1. Six types of non-metallic luster. | |
|---|---|
| Luster | Appearance |
| Vitreous | Glassy |
Are glassy rocks intrusive or extrusive?
Crystal Size and Texture Extrusive rocks are usually fine-grained or glassy while intrusive rocks are coarse-grained. Extrusive rocks may contain trapped bubbles of gas called vesicles.
What is the name of the texture if extremely small crystals form Where do rocks like this form?
APHANITIC TEXTURE
Textural terms include: APHANITIC TEXTURE – Igneous rocks that form on the earth’s surface have very fine-grained texture because the crystals are too small to see without magnification.
Which igneous rock has a vesicular texture and a mafic composition *?
Classification of Igneous Rocks
| TEXTURE | Felsic | Mafic |
|---|---|---|
| Phaneritic | Granite | Gabbro |
| Aphanitic | Rhyolite | Basalt |
| Vesicular | Pumice | Scoria |
| Glassy | Obsidian |