What Are the Rocks Formed From Lava & Have Cooled Quickly Called?

Two Primary Classification of Igneous Rock

• Igneous rocks are broken down into two main classes, based on where they are formed. Volcanic, or extrusive, igneous rocks are formed when lava cools on the surface of the Earth's crust. Plutonic, or intrusive, igneous rocks are formed when magma solidifies underneath the Earth's crust.

Distinctions In Composition

• Igneous rocks are further broken down by the amount of silica they contain. Igneous rocks containing 45 percent to 55 percent silica are called basalt, if extrusive. They are called gabbro, if intrusive. Igneous rocks that contain 55 percent to 65 percent silica are called andesite, if extrusive. They are termed diorite, if intrusive. Igneous rocks that contain 65 percent to 75 percent silica are called rhyolite, if extrusive. They are referred to as granite, if intrusive.

Effects Of Silica

• Silica affects the eruption temperature and viscosity of lava. As the silica content rises, lava's eruption temperature decreases and its viscosity increases (thickens). This not only affects the distance the lava will travel before solidifying, it effects how rapidly the lava will cool. Basalt lava flows have a higher temperature and flow greater distances. Rhyolite lava flows have a lower temperature and tend to pile-up, cooling much quicker.

Rapidly Cooled Lava

• The speed at which lava cools is a matter of relativity. All extrusive, igneous rocks cool much more rapidly than intrusive rocks and therefore can all be considered to form from rapid cooling. However, in terms of igneous rocks formed on the surface from lava flows, rapid surface cooling produces very small crystals and results in the formation of either an aphanitic texture or volcanic glass, called obsidian. The glass-like texture of obsidian is formed in a process that is very similar to the man-made process of creating glass, where rocks that are high in silica content are melted and then rapidly cooled in water.

Factors Contributing to Obsidian Formation

• While all extrusive lava flows (basalt, andesite and rhyolite) are capable of producing obsidian, several factors increase the likelihood it will be formed. The higher the silica content, the more likely obsidian will be formed, meaning rhyolite flows are the best candidate. However, interaction with water greatly increases the cooling rate, producing obsidian rocks as well. This is more likely to occur in locations where low-viscosity flows of basalt lava reach water sources, such as oceans. The type of volcanic eruption also contributes to the likelihood of obsidian formation. Explosive eruptions are the result of high-levels of tapped gases in the magma, which is a characteristic of rhyolite. With rhyolite's lower temperature and higher viscosity, its presence increases the chance of obsidian formation.