Basalt is a common igneous rock that forms from the rapid cooling of molten lava at or near the Earth’s surface. It is typically dark in color and has a fine-grained texture. Basalt is primarily composed of minerals like pyroxene, plagioclase feldspar, and sometimes olivine. This volcanic rock has low viscosity when molten, allowing it to flow easily during eruptions.
Basalt’s Widespread Presence
Basalt is one of the most abundant rock types on Earth, constituting a significant portion of both oceanic and continental crust. More than 90% of all volcanic rock found on our planet is basalt. It is globally distributed, forming vast geological features on land and beneath the ocean floor.
Oceanic Basalt Formations
The vast majority of Earth’s basalt is located within oceanic environments, primarily forming the oceanic crust. Mid-ocean ridges, which are underwater mountain ranges, are where new oceanic crust is generated as tectonic plates pull apart. Basaltic lava continuously erupts and solidifies there.
Submarine eruptions at mid-ocean ridges and other underwater volcanic settings often produce distinctive pillow lavas. These formations get their name from their characteristic pillow-shaped structures, which result from hot lava rapidly chilling upon contact with cold seawater. As new lava emerges, it breaks through the solidified outer crust, creating a series of interconnected, rounded shapes.
Oceanic islands, such as the Hawaiian Islands, Iceland, and the Azores, are also prominently composed of basalt. These islands form from volcanic activity associated with hotspots, where magma upwells from deep within the Earth’s mantle. The basaltic lava erupts onto the seafloor, accumulating over time to build these volcanic landmasses.
Continental Basalt Formations
Basalt is also found in significant formations on continents, often associated with specific types of volcanic activity. Shield volcanoes, characterized by their broad, gently sloping profiles, are built from numerous effusive basaltic lava flows. A notable example is Iceland.
Large Igneous Provinces (LIPs), often referred to as flood basalts, represent enormous accumulations of basaltic lava that cover vast continental areas. These provinces can span hundreds of thousands of square kilometers and reach thicknesses of several kilometers. Prominent examples include the Deccan Traps in India, the Columbia River Basalt Group in the northwestern United States, and the Siberian Traps in Russia.
Continental rift zones, where the Earth’s crust is being pulled apart and thinned, also host significant basaltic volcanism. Examples include the East African Rift System and the Rio Grande Rift in the United States. As the crust stretches, deep fractures allow basaltic magma to rise to the surface, resulting in lava flows and volcanic features within these elongated depressions.
Why Basalt is Found Where It Is
The presence of basalt in these locations is driven by geological processes that facilitate the melting of the Earth’s mantle. Decompression melting is a primary mechanism, occurring where hot mantle rock rises and pressure decreases, even without a temperature increase. This process is prevalent at mid-ocean ridges, where tectonic plates diverge, and at hotspots, where mantle plumes bring material closer to the surface.
Mantle plumes, which are upwellings of hot material from deep within the Earth’s mantle, are thought to be responsible for the formation of hotspots and many continental flood basalt provinces. As these plumes rise, they cause extensive melting, producing large volumes of basaltic magma. This explains the massive outpourings of lava seen in regions like the Deccan Traps.
Continental rifting also leads to basalt formation through decompression melting, as the stretching and thinning of the continental lithosphere reduce the pressure on the underlying mantle. While subduction zones are known for volcanic activity, the initial magma generated there is often basaltic. However, as this magma rises through the thicker continental crust, it can evolve into more silica-rich compositions, though basalt still forms in these settings.