(RM) Igneous Rocks Lesson

Igneous Rocks

Any rock formed from magma or lava is an igneous rock. The word "igneous" comes from the Latin word ignis, meaning "fire". Igneous rocks are formed both above ground and below ground and are the most commonly found rock on Earth. When they are formed below ground they are called intrusive rocks. Granite is the most common intrusive, igneous rock. Underground, they are formed when the melted rock, called magma, deep within the earth becomes trapped in small pockets. As these pockets of magma cool slowly underground, the magma becomes igneous rocks.

When they are formed above ground they are called extrusive rocks. Basalt is the most common extrusive, igneous rock. Extrusive igneous rocks are formed when volcanoes erupt, causing the magma to rise above the earth's surface.

Intrusive rocks tend to have a course grain while extrusive rocks tend to have a fine grain. Igneous rocks can be light in color or dark in color.

•Andesitic igneous rocks have mineral compositions between those of basaltic and granitic rocks.

Igneous rocks are classified according to:

  • origin
  • texture
  • mineral composition

Mineral crystals make up most igneous rocks. The exception to this is volcanic glass; there are different kinds of volcanic glass.

Volcanic glass most commonly, refers to obsidian, a rhyolitic glass with high silica content.

Other types of volcanic glass include:

  • Pumice, which is considered a glass because it has no crystal structure.Lipari Obsidienne Image
  • Apache tears, a kind of nodular obsidian.
  • Tachylite (also spelled tachylyte), a basaltic glass with relatively low silica content.
  • Sideromelane, a less common form tachylyte.
  • Palagonite, a basaltic glass with relatively low silica content.
  • Hyaloclastite, a hydrated tuff-like breccia of sideromelane and palagonite.
  • Pele's hair, threads or fibers of volcanic glass, usually basaltic.
  • Pele's tears, tear-like drops of volcanic glass, usually basaltic.
  • Limu o Pele (Pele's seaweed), thin sheets and flakes of brownish-green to near-clear volcanic glass, usually basaltic.

The texture of an igenous rock will depend upon the size and shape of its mineral crystals. Sometimes an igneous rock may have more than one texture. A rock that has two textures are a result of two different stages of cooling.

Mineral Composition

Magma and lava have various levels of silica content. The amount of silica present will affect the type of rock that is formed. Lava that has a low silica content will usually form dark colored rocks, basalt for example. Magma with a high silica content level usually forms light-colored rocks, like granite.

Putting Igneous Rocks to Use

Igenous rocks are typically hard, dense and durable. These rocks are good to use in construction and for making tools. Basalt is crushed to make gravel, pumice is often used as an abrasive for cleaning and polishing and perlite can be mixed with soil to promote garden growth.

The first use of obsidian by people probably occurred when a sharp piece of obsdian was used as a cutting tool. People then discovered how to skillfully break the obsidian to produce cutting tools in a variety of shapes. Obsidian was used to make knives, arrow heads, spear points, scrapers and many other weapons and tools.

Obsidian in Modern Surgery

Although using a rock as a cutting tool might sound like "stone age equipment", obsidian continues to play an important role in modern surgery. Obsidian can be used to produce a cutting edge that is thinner and sharper than the best surgical steel. Today, thin blades of obsidian are placed in surgical scalpels used for some of the most precise surgery. In controlled studies, the performance of obsidian blades was equal to or superior to the performance of surgical steel.

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