Multiple locations where hot lava streams into the ocean can be seen in this image along with a red-hot lava flow traversing the lava field. This photo shows the enormous extent of the flows.
They extend from the shoreline up to the horizon. USGS image. 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. Basalt is by far the most common volcanic rock type.
Basaltic magma is formed by partial melting of material from the upper mantle, and and is therefore typical for volcanism at hot-spots and at rift-zones. In these areas, upwelling of the mantle either caused by a rising mantle plume underneath hot-spots, or by a divergent plate boundary at mid-ocean rift zones decreases the pressure of the hot rock and therefore causes partial melting.
Oceanic crust and submarine volcanoes consist largely of basalt, because most of them are formed at rift-zones all ocean floor or hot-spots. Among subaerial volcanoes, basaltic lava is primarily found at shield volcanoes. These areas of basalt are often a piece of the seafloor which has been folded and pushed up onto the land and is now called an "ophiolite".
Other areas of basalt are related to what is called "continental rifting," where the land is being pulled apart and basaltic magma is allowed to escape to the surface. These types of lava flows are called "flood basalt" because they literally flood the land with lava.
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