Flood basalts

In some of the world's continental regions, layer after layer of basalt lava erupted at various times in the earth's history to form extensive accumulations of thick lava flows. These features are called flood basalts (or plateau basalts). In many cases, flood basalts were generated as the continents were torn apart to form ocean basins during the processes of continental drift and seafloor spreading.

Volcanic Rocks

Basalt is a dark-colored, relatively iron-rich rock that commonly occurs as lava flows, such as those on the islands of Hawaii. It is produced by the partial melting of dense, iron-and magnesium-rich rocks in the earth's mantle called peridotites. Volumetrically, basalt is the most important igneous rock type (rocks crystallized from molten magma) on the planet in that it constitutes the floors of all the earth's oceans, makes up most of the world's oceanic islands, and has poured out on the continents as well. The most impressive of these continental outpourings are the flood basalts, also known as plateau basalts. The last of the great flood-basalt areas (the Columbia River Plateau) ceased erupting lava some 10 million years ago, with activity lasting about 11 million years. Since then, no area on the earth has experienced such tremendous outpourings of basalt lava over such a short period of geologic time.

Volcanic rocks are responsible for several types of surface features. The most familiar are volcanoes, generally cone-shaped edifices that spew out lava from a central crater or vent. Wherever basaltic lavas form volcanoes, they are always low-profile forms that extend laterally over immense areas. Called shield volcanoes because they resemble giant shields lying on the ground, they occur on the Hawaiian Islands and on many other islands and continents. Other kinds of lava form relatively high, pointed mountains that do not spread out so much at their bases. Fujiyama in Japan is an example of this kind of volcano (called a stratovolcano), as are many of the Cascade volcanoes of the Pacific Northwest in the United States.

Chemical Composition of Lava

To understand why some volcanoes are low and spread out while others are high and cone-shaped requires a knowledge of the chemical composition of the lavas that make up the respective volcanoes. Basalt lava has a low abundance of the important component silica, while the lava of which stratovolcanoes (mostly andesite) are composed is relatively enriched in silica. Silica forms molecular chains in lavas called polymers that tend to become entangled with one another in silica-rich magmas, making them sticky and resistant to flow. Basalt, in contrast, flows quite readily over the land surface because its silica polymer chains are lubricated, so to speak, by other chemical constituents in the magma, such as iron, magnesium, calcium, and even water. For this reason, when basalt flows out on the land, it tends to spread over wide areas much as water or thin oil would, which produces the low-profile shield volcanoes typical of basaltic terrains. Stratovolcanoes are composed of materials that do not flow readily, so the lava tends to congregate near the central vent, eventually building up a high, cone-like structure.

The tendency for basalt lava to flow readily leads to a second type of volcanic eruption called a fissure flow. A fissure flow is simply a flow of lava that emanates from an elongated fissure (in many cases, a fault or other fracture) and flows away quickly to either side. Instead of a volcanic cone, this process produces horizontal sheets of basaltic lava that eventually harden to form a layer of black basalt rock. Fissure flows seldom occur alone. After the first flow cools and hardens, another flow will pour out to cover it, followed by others over some period of time. A single flow may cover tens of thousands of square kilometers and measure up to 100 meters (328 feet) thick or more. Total accumulations of basalt in fissure-flow areas may reach several kilometers in thickness and cover an area of many thousands of square kilometers. These areas are termed flood or plateau basalts, and nearly every continent has at least one example of these impressive geologic features.

Rifting

Nearly all the major flood-basalt regions in the world cover vast areas, and most occur on or near the margins of continents rather than in the interiors. The exceptions are the Lake Superior and Siberian regions, which occur far from any ocean basins. The occurrence of most flood basalts near continental margins is no mere coincidence. Geologists who study flood basalts are convinced that they arise during rifting episodes in which continental landmasses are literally pulled apart to make seas and, eventually, ocean basins. Tension produced during rifting causes the formation of deep fractures (faults) that penetrate through the continental crust to the mantle. Dense rocks in the mantle called peridotite are very hot. In some areas, these rocks are hot enough to make silicate liquid, a liquid that is compositionally basalt.

When faults penetrate into these partially molten rocks, pressure is released, causing the rocks to melt even more and the resulting magma to squeeze up through the faults. Eventually, this magma is joined by similar liquids produced near the deep faults, and much of it makes its way to the surface to be extruded as basalt lava flows. Some basalt magma, however, becomes trapped below the surface to crystallize as coarse-grained rocks called gabbro. Deeply eroded flood-basalt areas display excellent exposures of these “intrusive” rock bodies that, without doubt, accompany all flood basalts at depth.

During rifting events, hardened basalt lava flows produced during earlier eruptions are later split apart by subsequent rifting and covered over by younger lava flows. This process is repeated over and over again as the rift progressively tears the continent in two. Eventually, the continent splits into two parts with an ocean basin in between, an important part of the processes of continental drift and seafloor spreading. In fact, an ocean basin is composed of the basalt lava flows that pour out from the rift area between two continents. The ocean basin expands by “spreading” apart at an underwater rift, with new ocean floor issuing forth as basalt lava. Oceans form deep basins that hold water because basalt is a relatively dense material compared to continents (composed on average of low-density granitic rocks) and, thus, the basalt slowly sinks into the mantle below to form a basin depression.

Flood basalts occur on continental margins because they are some of the earliest basalt flows to be erupted during rifting. Once the ocean basins are formed, these flood basalts are left high and dry on opposite sides of the rift on the continents that have slowly drifted apart over millions of years. The rift area itself, complete with deep faults and basalt eruptions, sinks below the watery depths to become what geologists call mid-oceanic ridges.

The Lake Superior and Siberian flood basalts erupted over a very short period of geologic time in rift regions that seem to have been aborted at an early stage, before a sea or ocean had time to develop. For example, the Lake Superior basalts were erupted 1.1 billion years ago in a rift that has been traced below the surface from the Lake Superior area southwest to the middle of Kansas. The basalt itself is exposed on the north and south shores of Lake Superior, but the complete exposed system also includes a giant intrusion of gabbroic rocks called the Duluth complex. Inexplicably, this rift system ran out of energy before it succeeded in cutting the North American continent in two, an event that would have had a profound effect on present-day geography.

Flood Basalt Areas

Flood basalt areas that are explicitly associated with the formation of present-day ocean basins include the Paraná area in Brazil, the Deccan basalts in India, the Brito-Arctic area (Northern Ireland, Scotland, and the east coast of Greenland), and the Karoo area in South Africa. The Paraná has basalt flows of equivalent age in southwest Africa, extruded when Africa and South America were joined together about 100 million years ago. The Paraná basalts were extruded prior to the separation of Africa from South America, resulting in the formation of the South Atlantic Ocean.

The Deccan Plateau in India was made by outpourings of basalt during the separation of southern India from eastern Africa about 65 million years ago that eventually helped to make the Indian Ocean. At about the same time that the Deccan basalts were erupted, rifting occurred in the present vicinity of Northern Ireland and Scotland, which were attached to Greenland at that time. Basalts and gabbroic intrusions proliferated in the British Isles, including the small Inner Hebrides islands of Rhum, Skye, Mull, and others that attracted considerable attention from British geologists in the early twentieth century and later. The Karoo basalts resulted from the separation of Antarctica from southeastern Africa about 150 million years ago to make the southern Indian Ocean.

The best-known flood-basalt area in North America is the Columbia River Plateau of Washington and Oregon in the northwestern United States. Within a period of somewhat less than 2 million years (short in geological terms), between 17 and 15 million years ago, basalt flows were extruded that covered more than 220,000 square kilometers (84,942 square miles) of land to depths up to 10 kilometers (6 miles), with an average thickness of 1 kilometer (a little over 1/2 mile). The total area covered by basalt flows in eastern Washington, northern Oregon, and west-central Idaho is larger than the entire state of Washington. The name of this region comes from the Columbia River, which carves a rugged, scenic gorge through the basalt flows as it winds its way south across central Washington, then turns west to form the boundary between Oregon and Washington. As it cuts down through the many lava flows, the river reveals the layer-cake aspect of this thick volcanic pile that resembles the layered sedimentary rocks of the Grand Canyon.

The origin of the Columbia River Plateau is not as straightforward as the other occurrences already described. It is known that until about 400 million years ago, the western margin of the North American continent was located near the present western border of Idaho. Crustal blocks currently to the west of this boundary are “exotic,” having been moved into their present positions as microcontinents that formed in earlier times to the south. In some boundary areas between these microcontinents and the ancient continental margin, oceanic crust (basaltic) is known to lie not far below the surface. It is probably no coincidence that the main eruption areas of Columbia River basalt flows are located near one of these thin-crust plate junctures where mantle-derived basaltic magma could be tapped relatively easily.

Subduction Zones

What caused basalt lava to pour out in those eruption areas? Around 24 million years ago (during the early Miocene epoch), the North American continent, moving inexorably to the northwest, overrode the mid-oceanic ridge that lay just west of the continent in the present-day Pacific Ocean. Before that happened, the North American lithospheric plate was overriding the Pacific Ocean plate that was driving under the continent to produce a subduction zone. In subduction zones of this type, basaltic oceanic material dives deeper and deeper under the continent until it is heated up to melting. Resulting magmas rise up on the continental margin to make volcanoes similar to those in the Cascade Mountains of the northwest United States. Volcanoes similar to the Cascades were erupting in the area of the present Sierra Nevada range in California and Nevada prior to the mid-oceanic ridge being overridden by the North American plate, but they ceased to erupt sometime after this event because subduction was replaced by horizontal movement manifested by the present San Andreas fault. Shortly after, however, the basalts of the Columbia River Plateau began pouring out, lasting from 17 million to 6 million years ago (95 percent of the basalt erupted in the first 3.5 million years).

An oceanic ridge is an especially hot, active area in which basalt pours out from fractures caused by intense tensional (tearing) forces. Many geologists believe that this very hot material sliding under the continent melted to produce the Columbia River basalt flows. It is also important that this area was being stretched and pulled apart by the rising Cascade Mountains to the west and by the rise to the east of granitic rocks in the Idaho batholith. This stretching effect between two rising crustal masses was probably enough to release pressure on uprising magmas, allowing them to rush to the surface to produce numerous lava flows in the Columbia River Plateau area.

Noncontinental Flood Basalts

Flood basalts can occur in places other than continental areas. If the basalt fissure flows produced at mid-oceanic ridges can be considered flood basalts, then the earth's ocean basins are covered with flood basalts from shore to shore. Geologists, however, restrict the term “flood basalt” to those that occur in continental areas only. The dark areas on the Moon (lunar maria) are considered flood basalts, having erupted in huge impact basins more than 3 billion years ago. Similar areas on the planets Mercury, Venus, and Mars are also flood basalts. In fact, it turns out that the grand majority of all basalt in the solar system beyond Earth occurs as flood-basalt flows. The much rarer shield volcanoes, such as those in the Hawaiian Islands and Olympus Mons and its kin on Mars, are caused by the unusual situation in which magma is concentrated along only certain areas of the access fractures. In most cases, however, magma pours out all along the fracture, producing fissure flows, which accumulate over time to make what are termed flood basalts.

Principal Terms

basalt: dark, fine-grained, silicate igneous rock crystallized from lava flows

continental drift: the horizontal movement of continental masses relative to one another, caused by plate movement involving both the crust and the upper mantle

lithospheric plate: a segment of the rigid crust and upper mantle that moves horizontally, sliding past, away from, or under other plates, resulting in mountains, volcanoes, rift valleys, and earthquakes

magma: molten silicate liquid plus any crystals, rock fragments, and gases trapped within that liquid

mid-oceanic ridges: linear rift areas that bisect all the world's ocean basins; oceans are floored by basalt that pours out of these ridges and spreads out to either side

peridotite: the most common rock type in the upper mantle, composed of dense, iron-and magnesium-rich silicate minerals

rift: an area where tensional forces tear the crust and upper mantle apart, producing valleys or basins, generally with abundant volcanic activity

seafloor spreading: the expansion of the ocean floor with the creation of new material by extrusion of basalt lava at mid-oceanic ridges

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