Basalt Rocks stand as a fundamental component of our planet’s crust, representing a common type of extrusive igneous rock, also known as volcanic rock. Characterized by their dark color and fine-grained texture, these rocks are notably low in silica content yet rich in iron and magnesium. This unique composition and formation process give basalt rocks distinctive properties and a significant role in geological landscapes worldwide.
Composition and Texture of Basalt
Basalt rocks exhibit a fascinating range of textures. While some varieties are glassy, known as tachylytes, and others are remarkably fine-grained and compact, it’s more typical to find basalts with a porphyritic structure. This means they contain larger crystals, called phenocrysts, embedded within a finer crystalline matrix or groundmass. Common phenocrysts in basalt include olivine, augite, and feldspar, with olivine and augite being particularly prevalent. Plagioclase feldspars are also frequently observed in porphyritic basalts.
The formation of basalt often involves volcanic activity where basaltic lavas cool and solidify rapidly. During this process, steam cavities can form, resulting in a spongy or pumiceous texture. Over time, these cavities can become filled with secondary minerals such as calcite, chlorite, and zeolites, further diversifying the mineral composition of basalt rocks.
Classifying Basalt: Tholeiitic and Alkali Basalts
Basalt rocks are broadly categorized into two primary groups based on their chemical and petrographic characteristics: tholeiitic basalts and alkali basalts.
Tholeiitic basaltic lavas are distinguished by their mineral assemblage, typically featuring calcic plagioclase alongside augite, pigeonite or hypersthene, and sometimes olivine as the dominant mafic minerals. Notably, some tholeiitic basalts are olivine-free. With a silica content ranging from 45 to 63 percent, tholeiitic basalts are iron-rich and encompass tholeiites, which are basalts containing calcium-poor pyroxene. These basalts are prevalent in mountain belts and are known to create vast plateaus, such as the Columbia River Plateau in the northwestern United States, the Deccan Traps in India, and the Paraná Traps in South America. Active volcanoes like Mauna Loa and Kilauea in Hawaii are renowned for erupting tholeiitic lavas.
Normal alkali basalt, in contrast, typically contains olivine and commonly features a diopsidic or titaniferous augite. Alkali basalts are the dominant type found in ocean basins and are also common among mafic lavas in the forelands and backlands of mountain ranges. Regions like the Brito-Icelandic province, including the Inner Hebrides, Antrim, and the Faroe Islands, showcase extensive successions of both tholeiitic and alkali basalts from the Paleogene and Neogene periods.
Basalt Varieties and the Presence of Feldspathoids
A diverse range of basaltic rocks within the alkali group incorporates minerals from the feldspathoid group. Nepheline, analcime, and leucite are the most commonly encountered feldspathoids in these basalts, with haüynite appearing less frequently. When nepheline completely replaces feldspar, the rock is classified as nepheline-basalt. Partial replacement leads to the term nepheline-basanite. Similar classifications exist for analcime and leucite, resulting in analcime-basalts, leucite-basalts, and leucite-basanites.
Nepheline-basalts are generally fine-grained, very dark rocks, with a significant portion dating back to the early Cenozoic Era. They are relatively common in areas of Germany and also occur in regions of the United States like New Mexico, as well as in Libya, Turkey, and other locations. Leucite-basalts are primarily found in Italy, Germany, eastern Africa, Australia, and within the United States in Montana, Wyoming, and Arizona.
In conclusion, basalt rocks are a diverse and widespread group of volcanic rocks, crucial to understanding Earth’s geological processes and history. Their varied compositions and textures, classified primarily into tholeiitic and alkali types, reflect different magmatic origins and geological settings across the globe.
