Why are extrusive igneous rocks glassy?
Extrusive Igneous Rock Exposed to the relatively cool temperatures of the atmosphere, the lava cools quickly meaning that mineral crystals don’t have much time to grow. This results in rocks with a very fine-grained or even glassy texture. Hot gasses are often trapped in the quenched lava, forming bubbles (vesicles).
Are glassy igneous rocks extrusive?
Extrusive rocks are formed on the surface of the Earth from lava, which is magma that has emerged from underground. If lava cools almost instantly, the rocks that form are glassy with no individual crystals, like obsidian. There are many other kinds of extrusive igneous rocks.
What igneous rock is hard and glassy?
Pumice is formed when gas-rich magma froths up to form a sort of “mousse”. The lava is glassy-looking and is so full of bubbles that it floats on water!
What kind of texture do extrusive igneous rocks have?
fine-grained crystals
The texture of extrusive rocks is characterized by fine-grained crystals indistinguishable to the human eye, described as aphantic. Crystals in aphantic rocks are small in size due to their rapid formation during eruption.
What does obsidian’s 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. Here are three different types of a glassy volcanic rock called obsidian.
Are glassy igneous rocks felsic or mafic?
Classification of Igneous Rocks
| TEXTURE | Felsic | Mafic |
|---|---|---|
| Phaneritic | Granite | Gabbro |
| Aphanitic | Rhyolite | Basalt |
| Vesicular | Pumice | Scoria |
| Glassy | Obsidian |
How do igneous rocks with glassy textures 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. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye.
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. However, composition is also vitally important.
What is the most common extrusive igneous rock?
basalt
The most common extrusive igneous rock is basalt, a rock that is especially common below the oceans (Figure 4.6). Figure 4.5: Extrusive or volcanic igneous rocks form after lava cools above the surface.
Is glassy intrusive or extrusive?
Both lava flows and pyroclastic debris (fragmented volcanic material) are extrusive; they are commonly glassy (obsidian) or finely crystalline (basalts and felsites). Many extrusive rocks also contain intrusive components; this mixture of fine- and coarse-grained textures is described as porphyritic.
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.