THE WORD basalt is said to have an Ethiopian origin, meaning “black stone.” Basalt is a dark gray to black, dense to finely grained igneous rock that is the result of lava eruptions. Basalts are not only the most abundant lavas, but they are also the most voluminous. Basalt, when exposed to the air, becomes covered with a brown crust, consisting largely of oxides of iron. The continental masses are mainly built up of granite material, but at some places they are injected with or penetrated by material of greater density from the layers below. These lower layers may be the source of the great basaltic extrusions on the Earth’s surface. Basalt flows are noneruptive, voluminous, and characterized by relatively low viscosity. They generally advance less than one mile or kilometer per hour on gentle slopes and may reach more than 30 mi (50 km) from their source or erupting vent.
A lava flow is defined as the product of a single eruption that may be divisible into one or more flow units, which represents a single pulse or surge of lava that flowed away from the eruptive vent and then cooled separately. The length of the lava flow is determined largely by the magma effusion rate. Basalt lava flows that originate from fissures spread for distances that are roughly proportional to the third power of their thickness.
These lava flows are called flood basalts because a large volume of lava erupted in a short period of time. Each flood of lava has its own unique chemical composition. The dark area that forms part of the moon’s face is a flood basalt eruption. Flood basalt regions exist on every continent. In plains basalt provinces such as Snake River and ICELAND flows are much less than 1 cubic mi or km. Basaltic lava flows in HAWAII extend for more than 21 mi (35 km) with an average thickness of 16 ft (5 m). Andesite flows have higher viscosity and few extend more than 9 mi (15 km). One andesite flow of Pleistocene age in the Cascade range is 50 mi (80 km) long. One Icelandic basaltic flow reached 93 mi (150 km).
In the past, fissure eruptions have taken place on a gigantic scale. The basaltic lava of the Deccan Trap in INDIA, and the lava flows of the Snake River Plains in the UNITED STATES, appears to have been poured out from fissures. The basalt flows of the northeast of IRELAND and in Skyne, Mull, and other islands of the Hebrides are of a similar type, and are merely the remains of an enormous lava field that probably extended as far as GREENLAND. The flows apparently came from fissures. In western OREGON and western IDAHO, the lava spilled out of giant fissures in the Earth’s crust that stretched for more than a 100 mi or km during a period from 15 to 17 million years ago covering an area of 20,000 square mi (52,000 square km). Hundreds of such flows were erupted in this region. The basalt oozed in large quantities from fissures. Some examples include the following.
The Deccan Trap, an extensive area in western and central India, has been covered with basaltic lavas. The horizontality of the lavas has resulted in the formation of flat-topped plateaus at different elevations with either steep walls or terraces on the lower flows. These have a steplike arrangement. In the area east of the Western Ghats, the scenery is like that of a country of mesa and buttes.
The Giant’s Causeway, a World Heritage Site in Ireland, is an area of 40,000 tightly packed basalt columns resulting from a volcanic eruption 60 million years ago. Vertical joints form hexagonal columns and give the impression of having been artificially constructed. The columns were formed as a natural consequence of lava cooling. The tallest are about 40 ft (13 m) high. The tops of the columns form stepping stones that lead from the cliff foot and then disappear in the sea.
Columbia River basalt, where the flood basalt flows beneath the Columbia basin, accumulated to a thickness of more than 5,000 ft (1,724 m). Their many layers are clearly visible along the paths of the Snake. The Columbia River plateau was created by a series of basalt flows. It covers more than 62,000 square mi (164,000 square km) of area of the Pacific Northwest. Over 17 million years ago cracks in the Earth’s surface began spewing molten basaltic lava and the basalt floods continued in one form or other, until about 6 million years ago. Much of this lava spread to cover large parts of Oregon and WASHINGTON. Columbia River basalt consists of 270 individual lava flows within an average volume of 135 cubic mi (561 cubic km) per flow. Out of them, 21 poured through the Columbia River gorge. The uniqueness and beauty of the Columbia River gorge is attributed to basalt flows. As the flows flooded the region’s lowest areas, they filled canyons and permanently altered the river’s path on several occasions. Basalt flows exposed in the walls of the gorge feature a jointing arrangement. These arrangements were created as the lava flow solidified.
On the Snake River plain, the lava flows of the Snake River are complex, in terms both of structure and emplacement processes. Basalt flows consist of long tortuous bodies with many side lobes. Most flows accumulate as small, low shields, fissure flows, or large tube-fed flows.
Flood basalt flows from even earlier times and their impact on the Earth’s environment are now being looked at as a possible cause for the extinction of the dinosaurs and other forms of life. The eruption of the Deccan flood basalts in India, ten times larger than the Columbia River basalt, occurred at the time the dinosaurs died out 60 million years ago. These tremendous lava eruptions could dramatically alter the climate, giving rise to the conditions that could have caused mass extinction, common through the history of the Earth.