Felsic magma is viscous and does not flow easily. Most felsic magma will stay deeper in the crust and will cool to form igneous intrusive rocks such as granite and granodiorite. If felsic magma rises into a magma chamber, it may be too viscous to move and so it gets stuck. Dissolved gases become trapped by thick magma and the magma chamber begins to build pressure.
The type of magma in the chamber determines the type of volcanic eruption. A large explosive eruption creates even more devastation than the force of the atom bomb dropped on Nagasaki at the end of World War II in which more than 40, people died. A large explosive volcanic eruption is 10, times as powerful. Felsic magmas erupt explosively because of hot, gas-rich magma churning within its chamber. The pressure becomes so great that the magma eventually breaks the seal and explodes, just like when a cork is released from a bottle of champagne.
Magma, rock, and ash burst upward in an enormous explosion creating volcanic ash called tephra. That is why it is so dangerous to inhale the air following an eruption. Pyroclastic flows knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,oC 1, degrees F.
Prior to the Mount St. Helens eruption in , the Lassen Peak eruption on May 22, , was the most recent Cascades eruption. A column of ash and gas shot 30, feet into the air. This triggered a high-speed pyroclastic flow, which melted snow and created a volcanic mudflow known as a lahar. Lassen Peak currently has geothermal activity and could erupt explosively again.
Shasta, the other active volcano in California, erupts every to years. An eruption would most likely create a large pyroclastic flow, and probably a lahar. Now we explore them in greater depth.
Definitions: Igneous Rocks : Rocks that form through the solidification of magma. The geotherm - a graph of the relationship of temperature and depth, is a useful means of visualizing the processes the cause rocks to melt. At right, a schematic geotherm tracks the big patterns of Earth's temperature gradient all the way to its center.
Temperature does not increase evenly with depth. Rather, there are sharp discontinuities at: The asthenosphere The km transition of olivine the major component of the mantle rock peridotite to wadsleyite.
The km transition of wadsleyite the major component of the mantle rock peridotite to ringwoodite. The core-mantle boundary For now we focus on Earth's upper km - down to the zone of partial melting in the asthenosphere.
The following graphs track: The terrestrial geotherm only to the depths of the asthenosphere right. The melting curve for peridotite below. The melting curve shows the boundary of temperature and pressure beyond which peridotite melts. How do magmas form?
Three factors influence melting point: Temperature Pressure Volatiles. In detail: Temperature. All other things being equal, every mineral has a distinct melting point. In the mantle, heat is brought upward by convection.
As hot rocks convect upward they transfer heat to cooler rocks lying above them, which may melt. Pressure : All other things being equal, the greater the pressure, the less likely materials are to melt.
This explains why the asthenosphere is limited to a shallow region of the mantle and the inner core is solid despite being hotter than the liquid outer core.
When rocks experience decompression without losing their heat, they can experience decompression melting. Consider the fate of hot rocks rising through the mantle from a hot spot. Volatile substances : Generally, the addition of substances like water or CO 2 to a mineral lowers its melting point. In this case, the shape of the melting curve for peridotite changes. Where does magma form on Earth? Mid-ocean ridges : Rising rocks in mantle convection cell bring heat near the surface, transfering heat to overlying rocks.
At the same time, the hot rising mantle rocks experience decompression melting. The motion of lithospheric plates away from the mid-oceanic ridge further diminishes pressure yielding more melting.
Mantle plumes : Those enigmatic localized upwellings of hot mantle rock from hot spots very deep in the mantle, expressed on the surface as hot spots. As in mid-ocean ridges, hot spot rocks transfer heat to overlying rocks and experience decompression as they come up.
Subduction zones : As oceanic crust sits at bottom of ocean, it becomes charged with sea water. Subducting slabs, although relatively cold, dive into hot surrounding rock. The slab acts as conveyors drawing water into the hotter, drier asthenosphere. When the water percolates into the surrounding hot rocks, melting due to the infusion of volatiles occurs. This leads to some interesting consequences: Subduction zone magmas tend to be low temperature magmas compared to those from the other regions.
Because they are, they are compositionally enriched in SiO 4. These magmas are the ultimate origin of continental crust.
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