How does glassy texture form?
Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include obsidian.
How are glassy igneous rocks formed?
These fine-grained rocks are known as aphanitic—from a Greek word meaning “invisible.” They are given this name because the crystals that form within them are so small that they can be seen only with a microscope. If lava cools almost instantly, the rocks that form are glassy with no individual crystals, like obsidian.
What type of rock has a glassy texture?
extrusive igneous rocks 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.
Why is the texture of the igneous rock obsidian glassy?
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. Volcanic glass is called obsidian.
What does 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.
What makes a rock glassy?
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 are glassy igneous rocks?
Description. A glassy igneous rock consists of dense volcanic glass. The very porous, foamy variety is called Pumice. The most important rock type belonging to this group is: obsidian.
What is obsidian’s texture?
Obsidian
| Type | Igneous Rock |
|---|---|
| Texture | Glassy |
| Origin | Extrusive/Volcanic |
| Chemical Composition | Felsic |
| Color | Reddish Brown mottled with black |
What does a glassy texture tell us about the history of an igneous rock?
occurrence in igneous rocks Aphanitic and glassy textures represent relatively rapid cooling of magma and, hence, are found mainly among the volcanic rocks.
Is glassy texture 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 |
How is it formed basalt?
Basalts are formed by the rapid cooling of basaltic lava, equivalent to gabbro-norite magma, from interior of the crust and exposed at or very close to the surface of Earth. These basalt flows are quite thick and extensive, in which gas cavities are almost absent.
How was obsidian formed?
Obsidian is an "extrusive” rock, which means it is made from magma that erupted out of a volcano. If it was an igneous rock that formed from magma underground and did not erupt, it would have been called an "intrusive" rock.
What characteristics indicate a glassy texture?
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.
Is glassy a texture?
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.
What is basalt texture?
Basalt has several textural varieties such as glassy, massive, porphyritic, vesicular, scoriaceous.
When was basalt formed?
Basalt forms when lava reaches the Earth's surface at a volcano or mid ocean ridge. The lava is between 1100 to 1250° C when it gets to the surface. It cools quickly, within a few days or a couple weeks, forming solid rock. Very thick lava flows may take many years to become completely solid.
What is the texture of obsidian?
Obsidian
| Type | Igneous Rock |
|---|---|
| Texture | Glassy |
| Origin | Extrusive/Volcanic |
| Chemical Composition | Felsic |
| Color | Reddish Brown mottled with black |
Why does the obsidian rock have a black glassy appearance?
That's because obsidian is volcanic glass. It forms when volcanic lava cools so quickly, there's no time for crystals to grow. This gives the rock a black, smooth, glossy shine.
How basalt is formed?
Basalts are formed by the rapid cooling of basaltic lava, equivalent to gabbro-norite magma, from interior of the crust and exposed at or very close to the surface of Earth. These basalt flows are quite thick and extensive, in which gas cavities are almost absent.
What are basalt characteristics?
Basalt is rich in iron and magnesium and is mainly composed of olivine, pyroxene, and plagioclase. Most specimens are compact, fine-grained, and glassy. They can also be porphyritic, with phenocrysts of olivine, augite, or plagioclase. Holes left by gas bubbles can give basalt a coarsely porous texture.
How obsidian is formed?
Obsidian is an "extrusive” rock, which means it is made from magma that erupted out of a volcano. If it was an igneous rock that formed from magma underground and did not erupt, it would have been called an "intrusive" rock.
What rock looks like glass?
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.
How obsidian rock is formed?
Obsidian is an "extrusive” rock, which means it is made from magma that erupted out of a volcano. If it was an igneous rock that formed from magma underground and did not erupt, it would have been called an "intrusive" rock.
Where are basalt rocks formed?
Basalt is the most common composition of lava rocks that cool from magma, liquid rock that rises from the deep Earth at volcanoes. Today basalt is forming at many active rifts, including Iceland, the East African Rift Valley, the Red Sea and the Rio Grande Valley of New Mexico and Colorado.
What is the texture of a basalt?
Basalts have almost always aphanitic or fine-grained mineral texture resulting from rapid cooling of volcanic magma.
Why do some rocks look like glass?
There are a lot of rocks that look like glass, but they are not. Rocks that look like glass tend to be made up of quartz and feldspar. They can also contain pieces of mica or olivine in them. Rocks can have the heft and feel of stone while composed entirely of either quartz or feldspar.