Felsic and mafic rocks

Felsic and mafic rocks are two geological classifications of igneous rocks, a type of rock formed from cooled magma. The rocks are classified according to their silica content. Silica is a mineral compound formed by the combination of silicon and oxygen. It comes in several forms, such as quartz and sand. Silica is a relatively light material, meaning rocks high in silica content tend to be lighter and less dense. Felsic rocks have higher levels of feldspar and silica. The name felsic comes from a combination of the words feldspar and silica. Mafic rocks have less silica content, making them denser and heavier than felsic rocks. Mafic rocks are high in magnesium and iron. The name mafic is derived from magnesium and ferric, the Latin word for iron.

Background

The interior of Earth is divided into four layers. The inner core is a solid ball of iron about 1,500 miles (2,414 kilometers) across. This core is very hot, but is under such intense pressure that the iron remains in solid form and does not melt. Surrounding the inner core is the outer core, a 1,400-mile (2,253-kilometer) thick layer of liquid iron. Above the outer core is the 1,800-mile (2,897-kilometer) wide mantle. The mantle is a solid layer of rock under such extreme heat and pressure. It moves and flows in currents. The rocky, outermost layer of Earth is the crust. The crust varies in thickness from 5 to 25 miles (8 to 40 kilometers).

The heat and pressure found within Earth can turn solid rock into a molten or semi-molten mixture called magma. Magma ranges in temperature from about 1300 to 2400 degrees Fahrenheit (704 to 1315 degrees Celsius). It typically consists of a hot liquid rock base, crystallized minerals, some solid rock, and dissolved gases. It originates in the lower part of the crust or the upper mantle. Depending on its temperature, pressure, composition, and location, magma can move and form in various ways.

Overview

When magma cools, it forms a type of rock known as an igneous rock, a term derived from ignis, the Latin word for fire. Igneous rocks are divided into two types based on where they form. Intrusive igneous rock forms when magma remains trapped below the planet’s surface. This magma can take thousands or millions of years to completely solidify. The slow-cooling process allows mineral grains to form within the rock, giving it a coarse, grainy texture. Extrusive igneous rock forms when magma erupts from volcanoes or fissures near the surface. This magma cools very rapidly, giving mineral grains very little time to form. Extrusive rock has a fine-grained or glass-like texture. At times, trapped gas bubbles can create a pockmarked appearance in the rock.

The severity of volcanic eruptions depends on the amount of gas trapped within the magma and the magma’s viscosity. Viscosity is the thickness, or gooeyness, of the molten rock. Typically, cooler magma with more trapped gases and higher viscosity will produce more explosive eruptions. Hotter, lower-viscosity magma can give the gases time to escape, resulting in less explosive eruptions.

The terms felsic and mafic are another way geologists use to categorize igneous rocks. Felsic rocks are high in silica content and low in iron content. To be considered felsic, a rock must contain more than 65 percent silica. The elements that make up felsic rocks are typically lighter, such as silicon, oxygen, aluminum, and potassium. Felsic rocks can usually be recognized by their lighter colors, such as white, pink, tan, light brown, or light gray. This rule does have exceptions, as obsidian—a glass-like felsic rock—is black in color. Granite is the most common type of felsic rock. It is an intrusive igneous rock that contains about 70 percent silica. Other examples are quartz, muscovite mica, and different types of feldspar. Rhyolite is an example of an extrusive felsic rock.

Felsic rocks usually form in more explosive volcanic eruptions from magma containing more gases and higher viscosity. Magma that produces felsic rocks typically spends a longer period of time trapped below the surface. When the magma does reach the surface, it usually does so in a spot where two sections, or plates, of Earth’s crust are colliding. This type of geological activity occurs in some of the most mountainous areas on Earth, such as the Himalayas in Asia or the Andes in South America.

Mafic rocks also contain silica, but in lower quantities. Geologists classify mafic rocks as those containing between 45 and 55 percent silica. Mafic rocks contain larger amounts of heavier elements, such as iron, magnesium, calcium, and sodium. The presence of iron in the rocks gives them a darker color that ranges from black to dark brown, gray, or green. Common mafic rocks include basalt, dolerite, and gabbro. Basalt can also be classified as extrusive igneous rock, while gabbro is intrusive.

Mafic rocks are usually produced by magma with less trapped gases and lower viscosity. Eruptions that produce mafic rocks tend to be less violent and may create flowing rivers of lava—magma that has reached Earth’s surface. They generally form in areas where Earth’s plates are moving away from each other, such as in mid-ocean ridges, or places where magma is welling up through the crust in a “hot spot.” The volcanic activity of the Hawaiian Islands is an example of a hot spot.

In addition to felsic and mafic, some geologists also classify igneous rocks as ultramafic and intermediate. Ultramafic rocks have a very low silica content and are extremely viscous. They are primarily found in the upper mantle and rarely reach the surface. As the name suggests, intermediate rocks have a silica content that falls between the ranges of felsic and mafic.

Bibliography

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