Formation and Composition of Basalt

Formation

Basalt formation begins deep within the Earth's mantle. The mantle's partial melting generates basaltic magma, which is low in silica and high in iron and magnesium. This magma ascends through the Earth's crust due to its buoyancy.

Extrusive and Intrusive Mechanisms

When basaltic magma reaches the surface, it results in volcanic activity, creating volcanic rocks. This extrusive igneous rock solidifies rapidly, forming fine-grained textures. In contrast, if the magma cools beneath the surface, it forms intrusive equivalents, which are coarse-grained.

Flood Basalts

One of the most spectacular formations of basalt is through flood basalt events. These involve massive volcanic eruptions that create extensive lava plateaus. The Columbia River Basalt Group is a prime example, covering large areas of Washington, Oregon, and Idaho.

Pillow Lavas

Underwater volcanic activities give rise to pillow lavas. These formations occur when basaltic lava erupts underwater, leading to distinctive pillow-shaped structures due to rapid cooling by water.

Composition

Basalt is primarily composed of plagioclase feldspar and pyroxene. It may also contain olivine and magnetite. The rock is characteristically low in silica but rich in iron and magnesium, classifying it as a mafic rock.

Tholeiitic Basalt

One of the most common types of basalt is tholeiitic basalt. This rock type is prevalent both on land and in oceanic settings. It is distinguished by its low sodium content and is often found at mid-ocean ridges and hotspot volcanoes.

Chemical Indicators

The chemical composition of basalt reveals significant information about its source. Elements like iron, magnesium, and calcium are abundant, while silica content is relatively low. These indicators help geologists understand the conditions in the mantle where the basaltic magma originated.

Basalt

Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. Basalt has a composition similar to gabbro, with the primary difference being that basalt is fine-grained while gabbro is coarse-grained.

Formation and Composition

Basaltic magma is commonly produced by direct melting of the Earth's mantle, the region below the outer crust. On continents, the mantle begins at depths of 30 to 50 kilometers. The chemistry of basalts provides clues to processes deep in Earth's interior. Basaltic lavas are frequently spongy or pumiceous; the steam cavities become filled with secondary minerals such as calcite, chlorite, and zeolites.

Types of Basalt

There are two main chemical subtypes of basalt: tholeiites and alkali basalts. Tholeiites are silica-saturated to oversaturated and dominate the upper layers of oceanic crust and oceanic islands. Alkali basalts are silica-undersaturated and are common on oceanic islands and in continental magmatism.

Geological Significance

Basalt underlies more of Earth's surface than any other rock type. Most areas within Earth's ocean basins are underlain by basalt. Although basalt is much less common on continents, lava flows and flood basalts underlie several percent of Earth's land surface. Basalt is a very important rock in the study of geology and volcanology.

Notable Formations

Columbia River Basalts: These flood basalts erupted 15 to 17 million years ago and cover most of southeastern Washington and regions of adjacent Oregon and Idaho.
Olympus Mons: This shield volcano on Mars is composed of basalt and is the largest known volcano in our solar system.

Extraterrestrial Basalt

Basalt is also an important rock type on other planetary bodies in the Solar System. For example, the bulk of the plains of Venus, which cover approximately 80% of the surface, are basaltic. The lunar maria are plains of flood-basaltic lava flows, and basalt is a common rock on the surface of Mars.

Uses of Basalt

Basalt is used in a variety of applications due to its durability and abundance. It is commonly used as an aggregate in construction projects, as a decorative stone, and in the production of basalt fiber, which is used in various industrial applications.