Notable Formations of Basalt

Giant's Causeway

The Giant's Causeway is one of the most famous basalt formations in the world, located in Northern Ireland. It is celebrated for its roughly 40,000 interlocking basalt columns, which are the result of an ancient volcanic fissure eruption. The columns have a predominantly hexagonal shape, but some have four, five, seven, or eight sides. This unique geological formation has given rise to numerous legends and myths, including stories of giants striding across the sea to Scotland.

Devils Postpile National Monument

The Devils Postpile National Monument in California is another exemplary site of basalt column formation. This U.S. National Monument is home to the Devils Postpile, an impressive array of columnar basalt that formed about 100,000 years ago as a result of volcanic activity. The columns, which can reach up to 60 feet in height, are remarkably symmetrical and are often compared to those of the Giant's Causeway. The formation process involved the cooling and contraction of lava, leading to the creation of these striking hexagonal joints.

Columnar Jointing

Columnar jointing is a geological feature that occurs in many types of volcanic rocks, including basalt. This process involves the formation of a series of regular, closely spaced fractures that divide the rock into polygonal columns, typically hexagonal in cross-section. Columnar jointing can be observed in various locations around the world, such as the Devils Tower National Monument and Gilbert Hill.

Other Notable Formations

Cape Stolbchaty

Cape Stolbchaty, located on the island of Kunashir, is known for its columnar basalt formations, which bear a striking resemblance to the Giant's Causeway. This site showcases a similar natural phenomenon where the cooling of basaltic lava led to the creation of vertical columns.

Stuðlagil

Stuðlagil is located in the Eastern Region of Iceland and is renowned for its large, well-preserved columnar basalt formations. These columns line the cliffs of the Jökuldalur valley, creating an awe-inspiring natural spectacle.

Daepo Jusangjeolli Cliff

The Daepo Jusangjeolli Cliff on Jeju Island in South Korea features spectacular examples of columnar jointing. Formed by the cooling and contraction of lava from the island's volcano, Hallasan, these prismatic columns create a dramatic coastal landscape.

Basalt Fan Structure

A basalt fan structure is a type of formation where columnar basalt columns have slumped into a fan shape. These are often found in regions with significant volcanic activity and can provide valuable insights into the geological history of an area.

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.