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.)
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 isfull 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 theEarth’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.
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.
Chemical/Mineral Composition
Igneous rocks are divided into two types depending on composition: felsic and mafic. Felsic rock is affected by heat, either from magma coming to the surface from extreme depths in the Earth or by the friction between continental plates. Although igneous rock contains some combined rock like the deep continental plate rock melted by moving magma, overall igneous rocks are either felsic or mafic.
Felsic
The first igneous rock type, felsic, is made up of light-colored igneous rocks that have high levels of silica-containing minerals like quartz and feldspar. Plagioclase feldspar that is higher in calcium crystallizes at higher temperatures than plagioclase having higher levels of sodium. When a rock is formed by different minerals, it tends to melt at a temperature below that of any one contributing mineral. This happens because different elements have different chemical properties that have an effect on their melting points.
Granite and granodiorites are the best known and most frequently seen intrusive igneous rocks containing about 70% silica. These mostly lightcolored quartz and orthoclase feldspar minerals are found only in the continental crust. When different minerals like quartz and feldspar mix with granite, it takes on its well-known gray or pink color.
Felsic rock has high levels of silica in its composition. The name felsic comes from a combination of the words feldspar and silica.
Rhyolite has the same composition as granite, but it is an extrusive igneous rock. It has a pale brown to gray color and is finely grained. The majority of rhyolites are made up of volcanic glass with no obvious crystals. They are much less common and found in much smaller pockets of extrusive igneous rock than their intrusive cousins.
Intermediate Igneous Rock
The volcanic igneous rocks in the intermediate class, that are a lot like granodiorite, are dacite and andesite.
Granodiorite has a lot of quartz like granite, but not a lot of silica. Diorite, the phaneritic cousin of andesite, contains feldspar and a lot of other mafic minerals like biotite and pyroxene that give it a darker color. Diorite is in between the granites and gabbro in composition and properties. A transition rock type between the rhyolitic and basalitic magmas is an intermediate igneous rock like andesite. This type doesn’t contain particularly high or low levels of silica, but is pretty average. Andesite is named after the volcanic Andes Mountains of South America and is made up of plagioclase and amphibole. Andesites and diorites are both equally common.
Mafic Rock
Mafic rock contains high levels of magnesium and ferric (iron-containing) minerals. The word mafic comes from a combination of these two mineral names. Rocks which are low in silica, but high in magnesium and iron, form darkcolored mafic rocks like pyroxenes and olivines. Their main minerals are calcium-rich feldspar and calcium- and magnesium-rich pyroxenes. The Earth’s ocean floor is made up of basalt. Mafic rock contains only about 50% silica by weight. It is commonly dark gray, but can be green, brown, or black.
When geologists looked closer at these two rock types, they found rocks that looked very different, but had the same composition. Some geologists thought that texture differences must be related to the way magma crystallized and then reached the surface, either by slow boat or by rocket ship.
