Basalt is classified as a mafic igneous rock, a category used for rocks formed from the cooling and solidification of molten material. Igneous rocks are primarily classified based on their chemical composition, with silica content acting as the main differentiator. Basalt is the extrusive, or volcanic, counterpart to the intrusive rock gabbro, and both share the same chemical makeup. Basalt’s specific mineral content, rich in iron and magnesium, dictates its physical appearance and behavior when molten.
The Classification System: Mafic vs. Felsic
The terms mafic and felsic describe the chemical makeup of silicate minerals, magmas, and the igneous rocks they form. This classification is based on the relative amount of silica (\(\text{SiO}_2\)) within the rock’s composition. Rocks with a lower silica content, typically between 45% and 55% by weight, are described as mafic.
The word “mafic” is derived from “magnesium” and “ferric” (iron). These rocks are rich in heavier elements like iron and magnesium, resulting in a characteristically dark color, often black or dark gray, and a higher density. Mafic minerals commonly include pyroxene, olivine, and calcium-rich plagioclase feldspar.
Conversely, felsic rocks have a higher silica content, generally exceeding 65% by weight. The term “felsic” combines “feldspar” and “silica,” indicating a composition enriched in lighter elements like silicon, aluminum, sodium, and potassium. Felsic rocks and minerals, such as quartz and potassium feldspar, are typically light-colored (white, pink, or light gray) and possess a lower density.
Basalt’s Defining Composition and Properties
Basalt’s silica content typically falls within the 45% to 52% range. This lower silica content means the rock is rich in magnesium oxide (\(\text{MgO}\)) and calcium oxide (\(\text{CaO}\)), alongside iron oxide (\(\text{FeO}\)).
The primary minerals in basalt include calcic plagioclase feldspar and pyroxene, often accompanied by olivine. The abundance of iron and magnesium within these dark silicate minerals gives basalt its characteristic dark gray to black color and contributes to its high density.
When molten, basaltic lava has a low viscosity due to its low silica content, allowing it to flow quickly across vast distances. This fluid nature is responsible for the gentle, effusive eruptions associated with basaltic volcanoes. Basalt forms from lava that cools rapidly on or near the Earth’s surface. This quick cooling prevents large crystals from forming, giving basalt a fine-grained, or aphanitic, texture where individual mineral grains are too small to be seen without magnification.
Geological Settings Where Basalt Forms
Basalt is the most abundant volcanic rock on Earth, making up more than 90% of all volcanic rock. The process that generates basaltic magma is the partial melting of the Earth’s mantle, which occurs at depths of 50 to 100 kilometers.
A majority of the Earth’s oceanic crust is composed of basalt, continuously created at mid-ocean ridges. At these divergent plate boundaries, decompression melting occurs as hot mantle material rises and pressure decreases, producing basaltic magma that solidifies into new ocean floor. When erupted underwater, this lava cools almost instantly, forming distinctive rounded structures known as pillow basalts.
Basalt also forms in continental regions and at oceanic hotspots, independent of plate boundaries. Iconic examples include the shield volcanoes of the Hawaiian Islands, built almost entirely from successive flows of low-viscosity basaltic lava. Additionally, massive outpourings known as flood basalts can cover hundreds of thousands of square kilometers on continents, such as the Columbia River Basalt Group in the United States.