Extrusive Igneous Rock Help

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Extrusive Igneous Rock

Once magma exits the crust (ejected during an eruption) and cools rapidly, it creates a finely textured or glassy rock with small crystals. Basalt is an example of igneous rock that is quickly cooled from magma and extruded at the surface from lava. Extrusive igneous rock is on the rocket ship of movement to the surface. These rocks are commonly called volcanic rock , since they blast to the surface as either lava or rock fragments from your local neighborhood volcano. When the grains in igneous rock are not easily seen, even with a magnifying glass, the rocks are called aphanites .

Volcanic glass is a natural glass formed by the quick cooling of molten lava that hasn’t had time for crystals to form.

Depending on the cooling rate and amount of different gases trapped in cooling lava, volcanic glass can be smooth or full of holes. Most volcanic glass is in one of the three forms:

(a) pumice , a light-weight rock with lots of holes from escaped gases,

(b) obsidian , a glassy-smooth, dense solid, or

(c) porphyry , a mixed texture rock with large crystals suspended in a fine crystalline matrix. (This type is neither aphanite nor phanerite, but uniquely textured.)

The different textures of the three volcanic glass types are illustrated in Fig. 6-2.

Igneous Rock


Fig. 6-2. Volcanic glass can be very different in appearance.

Pumice rock is a favorite among movie set designers and practical jokers. It looks pretty much like regular rock, but it is very light. People can pretend superhuman strength when lifting a large pumice boulder. Pumice is sometimes called a glassy froth, like a molten milk shake. Since pumice is full of closed air pockets, it’s really light. Pumice rock can even float! Impossible, right? No. There is enough trapped air to keep the pumice afloat.

When lava blasts out of a volcanic vent or fissure with terrific force and heat, the surrounding air is trapped with the exploding volcanic particles. After quick cooling, a lot of air is sealed within holes in the rock. Some pumice contains almost more air than it does rock. It has a rough uneven texture. Commonly, pumice stones are used as an abrasive tool in the beauty industry to smooth rough heals and calluses.

Obsidian is a black or dark-colored glassy volcanic rock, much like granite in chemical makeup, but formed by super-fast cooling when shot to the surface at low pressure during an eruption. It is the most “glassy” in appearance of the three types of volcanic glass types. It is shiny and smooth to the touch, but sharp at the edges.

Since it fractures fairly easily with light pressure along curved planes within the rock, obsidian was a favorite stone for early flint knappers. Its razor-sharp fracture edges made obsidian perfect for early knife blades, scrapers, spear, and arrowheads. In abandoned Native American settlements in the western United States and elsewhere in the world where early peoples lived, obsidian can be found lying on the ground in chips and fragments.

Obsidian is also excellent for dating ancient artifacts such as tools. By using a technique called obsidian hydration-rim dating , scientists can date tools from periods like the Aztec age in Mexico or preceramic Japanese era dating from 23,000 BC. The way it works is by testing for the presence of a perlite rim, formed when water molecules on the outside of the sample move inward (hydrating the sample) through cut obsidian edges. When this happens, the obsidian at the edges change to perlite. Most perlites have more water molecules than obsidian. The thickness of the perlite rim allows scientists to figure out how obsidian was shaped by human hands long ago.

Porphyry rock has some specific minerals most commonly associated with its igneous origin, but in general, is thought of as a smooth igneous rock with large crystals thrown into the rock like marshmallows in hot cocoa. These large crystals, called phenocrysts , formed while magma is still below the Earth’s surface. Like phanerites, they are shot to the surface during a volcanic eruption. Phenocrysts have well-formed crystals since they were created within fluid magma and didn’t compete with other crystals growing and crowding them into warped shapes with irregular grain boundaries. We will look at these volcanic glass types in more detail.

Basalt , a fine-grained, aphanitic, extrusive igneous rock is found everywhere under the sediments of the oceans’ floors. It’s like the wood or concrete floor of a house that lies under the carpet. The main minerals found in basalt are olivines, pyroxenes, and plagioclase feldspar. Basalt is the most abundant extrusive igneous rock on Earth.

Practice problems of this concept can be found at: Igneous Rock Practice Test

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