How Do Volcanoes Erupt With Basaltic Lava & Tephra?
Tephra and Basaltic Lava: Two Different Types of Eruptions, From Different Types of Volcanoes
Volcanoes come in a variety of shapes, sizes and flavors. Some are broad and flat, some are tall and cone shaped, some are underwater, and some only erupt every several hundred thousand years. But in terms of eruption characteristics, all volcanoes fall into one of two categories: felsic or mafic.
Felsic Eruptions: Explosions of Tephra
In geology, "felsic" refers to relatively light, low-density rock with a high silica content. The most common felsic rock is granite. Other examples include rhyolite and pumice. When it melts--because of the low-density silica--felsic rock produces slow-moving, viscous magma/lava as opposed to the runny, fast-flowing magma produced by mafic rock (discussed in the next section).
In felsic volcanoes, the sticky, silica-rich magma accumulates and forms a plug over the vent (crater), sealing in heat, pressure and volatile gases. Over time, the pressure builds inside the volcano. Finally, when there's enough pressure to breach the magma plug, a catastrophic explosion occurs.
In such explosions, like that of Mount St. Helens in 1980, huge amounts of rock and ash are ejected into the atmosphere. This ballistic debris is called "tephra." Tephra from felsic eruptions can reach miles in altitude, blot out the sun, cause acid rain and smother human settlements on the ground. Felsic volcanoes generally occur near convergent plate boundaries, where heavy ocean crust descends into the upper mantle, melts, then sends a plume of magma up through the silica-rich continental crust. Felsic volcanism produces tall, conical mountains of tephra accumulated over the ages.
Mafic Eruptions: Rivers of Melted Rock
In contrast to felsic rock, mafic rock is richer in heavy elements like iron and magnesium. Types of mafic rock include basalt and gabbro. More dense and rich in metal than felsic rock, mafic rock flows easily when it melts. So, mafic magma/lava doesn't form blockages over volcanic vents like felsic magma, and doesn't trap pressure. Therefore, mafic eruptions aren't explosive. Basaltic magma simply rises out of the earth and flows quickly out of the vent until it cools.
The Hawaiian Islands are all formed from basalt rich, mafic volcanoes: Over millions of years, basaltic magmas rose up through the sea floor and accumulated to form massive, broad mountains. We see the very tops of these colossal mountains as the Hawaiian Islands. Mafic volcanoes are typically formed over mantle plumes or "hotspots," where convection currents inside the earth cause magma to rise up through the basalt rich ocean crust. Also known as "shield volcanoes," mafic volcanoes are the largest, tallest mountains on Earth in terms of their distance from base to summit. Most of their mass, however, remains underwater.
In general, mafic/basaltic eruptions are less dangerous to humans than felsic eruptions. There is little, if any, explosive force, and there is virtually no tephra.
Tags: mafic rock, Basaltic Lava, felsic rock, basalt rich, Basaltic Lava Tephra, Different Types
