Most of Earth's basalt is produced at divergent plate boundaries on the mid-ocean ridge system see map. Here convection currents deliver hot rock from deep in the mantle. This hot rock melts as the divergent boundary pulls apart, and the molten rock erupts onto the sea floor. These submarine fissure eruptions often produce pillow basalts as shown in the image on this page. The active mid-ocean ridges host repeated fissure eruptions. Most of this activity is unnoticed because these boundaries are under great depths of water.
At these deep locations, any steam, ash, or gas produced is absorbed by the water column and does not reach the surface. Earthquake activity is the only signal to humans that many of these deep ocean ridge eruptions provide. However, Iceland is a location where a mid-ocean ridge has been lifted above sea level. There, people can directly observe this volcanic activity. Thermal image of a hot basalt flow on the flank of Hawaii's Kilauea volcano.
Hot lava at the front of the flow is revealed in yellow, orange and red colors. The channel that it flowed through on the previous day appears as a purple and blue track. United States Geological Survey image. Another location where significant amounts of basalt are produced is above oceanic hotspots. These are locations see map above where a small plume of hot rock rises up through the mantle from a hotspot on Earth's core.
The Hawaiian Islands are an example of where basaltic volcanoes have been built above an oceanic hotspot. Basalt production at these locations begins with an eruption on the ocean floor. If the hotspot is sustained, repeated eruptions can build the volcanic cone larger and larger until it becomes high enough to become an island.
All of the islands in the Hawaiian Island chain were built up from basalt eruptions on the sea floor. The island that we know today as "Hawaii" is thought to be between , and , years old.
It began as an eruption on the floor of the Pacific Ocean. The volcanic cone grew as recurrent eruptions built up layer after layer of basalt flows. About , years ago it is thought to have grown tall enough to emerge from the ocean as an island. Today it consists of five overlapping volcanoes.
Kilauea is the most active of these volcanoes. It has been in amost continuous eruption since January, Basalt flows from Kilauea have extruded over one cubic mile of lava, which currently covers about 48 square miles of land. These flows have travelled over seven miles to reach the ocean, covering highways, homes and entire subdivisions that were in their path.
Columbia River Flood Basalts: The Columbia River Flood Basalts are an extensive sequence of stacked lava flows that reach a cumulative thickness of up to feet. The outcrops in the foreground and in the distance of this photo are all made up of layered basalt flows. Although basalt is typically a dark black rock, it often weathers to a yellow-brown color similar to the rocks shown here.
Public domain image by Williamborg. Basalts are often porphyritic and can contain mantle xenoliths. The coarse and medium-grained equivalents of basalt are gabbro and dolerite respectively. Basalt is distinguished from pyroxene andesite by its more calcic plagioclase.
There are two main chemical subtypes of basalt: tholeiites which are silica saturated to oversaturated and alkali basalts that are silica undersaturated. Tholeiites dominate the upper layers of oceanic crust and oceanic islands, alkali basalts are common on oceanic islands and in continental magmatism. Basalts can occur as both shallow hypabyssal intrusions or as lava flows.
Picrites are basalts containing abundant olivine. Basalts with alkali feldspar in addition to plagioclase are known as the trachybasalts. Basalts are erupted in a wide variety of tectonic environments on Earth e.
Basalts also occur on other terrestrial planets and the Moon and constitute an important class of meteorites basaltic achondrites. Basalt magmas are parental to most of the more evolved magma types involved in continental and oceanic igneous activity, that develop through fractional crystallization in the crust. In view of their parental role, and also because of their abundance and relatively simple mineralogy, basalts provide the most logical starting point for a systematic study of igneous rocks.
Many basalts are too fine - grained to permit confident microscopic identification of every mineral present, and may indeed have a glassy matrix from which one or more latent minerals have failed to crystallize. On this basis, most basalts consist predominantly of the normative minerals - Olivine, Clinopyroxene, Plagioclase, and Quartz or Nepheline. The basalt tetrahedron can be divided into three compositional volumes, separated by planes: - The plane Cpx-Plag-Opx is the critical plane of silica saturation.
Compositions that contain Qtz in their norms plot in the volume Cpx-Plag- Opx-Qtz, and would be considered silica oversaturated. Basalts that plot in this volume are called Quartz Tholeiites.
Normative compositions in the volume between the critical planes of silica undersaturation and silica saturation are silica saturated compositions the volume Ol - Plag - Cpx - Opx. Silica saturated basalts are called Olivine Tholeiites. Alkali Basalts, Basanites, Nephelinites, and other silica undersaturated compositions lie in the silica undersaturated volume.
The critical plane of silica undersaturation appears to be a thermal divide at low pressure. This means that compositions on either side of the plane cannot produce liquids on the other side of the plane by crystal fractionation. To see this, look at the front two faces of the basalt tetrahedron. The critical plane of silica undersaturation is a thermal divide at low pressures less thanabout 10 kb and is not a thermal divide at higher pressures.
The Oceanic Ridges are probably the largest producers of magma on Earth. Magma is both erupted and intruded near the central depressions that form the oceanic ridges.
Thus, both basalts and gabbros are produced. The main melting mechanism is likely decompression melting as rising convection cells move upward through the mantle beneath the ridges.
In modern literature "normal" actually refers to the very common, incompatible-element depleted variety, whereas "enriched" refers to the much less frequent, but nevertheless not uncommon, incompatible element enriched basalts. But some E-MORB, and this includes many small seamounts, cannot be attributed to input from any obvious plumes.
These types of lava flows are called "flood basalt" because they literally flood the land with lava. The western United States has quite a bit of both types. Much of the Klamath Mountains of northern California and southern Oregon is broken-up pieces of to million year old seafloor crust the million year old Josephine Ophiolite is a good example , while most of eastern Washington and Oregon the Columbia River Basalt , and northeastern California the Modoc Plateau Basalt are underlain by flood basalt.
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