Which magma type is most explosive

If the viscosity is low, non-explosive eruptions usually begin with fire fountains due to release of dissolved gases. When magma reaches the surface of the earth, it is called lava. Since it its a liquid, it flows downhill in response to gravity as a lava flows. Different magma types behave differently as lava flows, depending on their temperature, viscosity, and gas content. Pahoehoe Flows - Basaltic lava flows with low viscosity start to cool when exposed to the low temperature of the atmosphere.

This causes a surface skin to form, although it is still very hot and behaves in a plastic fashion, capable of deformation. Such lava flows that initially have a smooth surface are called pahoehoe flows. Initially the surface skin is smooth, but often inflates with molten lava and expands to form pahoehoe toes or rolls to form ropey pahoehoe. See figure 6. Pahoehoe flows tend to be thin and, because of their low viscosity travel long distances from the vent.

A'A' Flows - Higher viscosity basaltic and andesitic lavas also initially develop a smooth surface skin, but this is quickly broken up by flow of the molten lava within and by gases that continue to escape from the lava. This creates a rough, clinkery surface that is characteristic of an A'A' flow see figure 6.

Pillow Lavas - When lava erupts on the sea floor or other body of water, the surface skin forms rapidly, and, like with pahoehoe toes inflates with molten lava.

Eventually these inflated balloons of magma drop off and stack up like a pile of pillows and are called pillow lavas. Ancient pillow lavas are readily recognizable because of their shape, their glassy margins and radial fractures that formed during cooling. Lava Domes or Volcanic Domes - result from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava. Since the viscosity is so high, the lava does not flow away from the vent, but instead piles up over the vent.

Blocks of nearly solid lava break off the outer surface of the dome and roll down its flanks to form a breccia around the margins of domes. The surface of volcanic domes are generally very rough, with numerous spines that have been pushed up by the magma from below. Explosive eruptions are favored by high gas content and high viscosity andesitic to rhyolitic magmas.

Explosive bursting of bubbles will fragment the magma into clots of liquid that will cool as they fall through the air. These solid particles become pyroclasts meaning - hot fragments and tephra or volcanic ash, which refer to sand- sized or smaller fragments.

If the gas pressure inside the magma is directed outward instead of upward, a lateral blast can occur. Directed blasts often result from sudden exposure of the magma by a landslide or collapse of a lava dome. Pyroclastic Deposits. Pyroclastic material ejected explosively from volcanoes becomes deposited on the land surface. The process of deposition leaves clues that allow geologists to interpret the mode of ejection from the volcano.

Pyroclastic flows are also sometimes called pyroclastic density currents PDCs. They can range from surges which can have a range of clast densities from low to high with generally low concentration of of solid clasts high amonts of gases to high clast concentration clouds of ash and gas pyroclastic flows.

As defined above, block and ash flows consist of an unsorted mixture of blocks and ash with the blocks being mostly rock fragments. Surges tend to hug the ground as they flow over the surface and thus tend to produce thicker deposits in valleys with thinner deposits over ridges.

This helps to distinguish surge deposits from flow deposits and fall deposits. Volcanic eruptions, especially explosive ones, are very dynamic phenomena.

That is the behavior of the eruption is continually changing throughout the course of the eruption. This makes it very difficult to classify volcanic eruptions. Nevertheless they can be classified according to the principal types of behavior that they exhibit. An important point to remember, however, is that during a given eruption the type of eruption may change between several different types.

Hawaiian - These are eruptions of low viscosity basaltic magma. Gas discharge produces a fire fountain that shoots incandescent lava up to 1 km above the vent. The lava, still molten when it returns to the surface flows away down slope as a lava flow.

Hawaiian Eruptions are considered non-explosive eruptions. Very little pyroclastic material is produced. Strombolian - These eruptions are characterized by distinct blasts of basaltic to andesitic magma from the vent. These blasts produce incandescent bombs that fall near the vent, eventually building a small cone of tephra cinder cone.

Sometimes lava flows erupt from vents low on the flanks of the small cones. Strombolian eruptions are considered mildly explosive, and produce low elevation eruption columns and pyroclastic fall deposits. Vulcanian - These eruptions are characterized by sustained explosions of solidified or highly viscous andesite or rhyolite magma from a the vent. Eruption columns can reach several km above the vent, and often collapse to produce pyroclastic flows.

Widespread pyroclastic falls are common that contain mostly angular blocks. Vulcanian eruptions are considered very explosive. They may also produce surges with resulting surge deposits.

Pelean eruptions are considered violently explosive. Plinian - These eruptions result from a sustained ejection of andesitic to rhyolitic magma into eruption columns that may extend up to 45 km above the vent.

Eruption columns produce wide-spread fall deposits with thickness decreasing away from the vent, and may exhibit eruption column collapse to produce pyroclastic flows and surges. Plinian ash clouds can circle the Earth in a matter of days.

Plinian eruptions are considered violently explosive. Phreatomagmatic - These eruptions are produced when magma comes in contact with shallow groundwater causing the groundwater to flash to steam and be ejected along with pre-existing fragments of the rock and tephra from the magma. Because the water expands so rapidly, these eruptions are violently explosive although the distribution of pyroclasts around the vent is much less than in a Plinian eruption. Temperature: Magma temperatures reflect the melting points of their mineral components.

Not surprisingly, magmas formed by partial melting of mantle rocks are much hotter — well over o C for some Hawaiian basalts — than is the case for crustally derived melts. Rhyolites may reach the surface at temperatures of less than o C, and so have much higher viscosity. Volatile Content: Magma invariably contains small amounts of dissolved gas water, CO 2 etc which is released as pressure is removed. It should first be noted that magma is molten material inside the earth, whereas lava is molten material on the surface of the earth.

Volcanoes do not always erupt in the same way. Each volcanic eruption is unique, differing in size, style, and composition of erupted material. One key to what makes the eruption unique is the chemical composition of the magma that feeds a volcano, which determines 1 the eruption style, 2 the type of volcanic cone that forms, and 3 the composition of rocks that are found at the volcano.

Different minerals within a rocks melt at different temperatures and the amount of partial melting and the composition of the original rock determine the composition of the magma. Magma collects in magma chambers in the crust at kilometers miles beneath the surface of a volcano. The words that describe composition of igneous rocks also describe magma composition. Mafic magmas are low in silica and contain more dark, magnesium and iron rich mafic minerals, such as olivine and pyroxene. Felsic magmas are higher in silica and contain lighter colored minerals such as quartz and orthoclase feldspar.

The higher the amount of silica in the magma, the higher is its viscosity. Viscosity determines what the magma will do. Mafic magma is not viscous and will flow easily to the surface. 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. This is sometimes referred to as the first boiling. Alternatively, as magma cools and anhydrous minerals begin to crystallize out of the magma, the residual liquid will become increasingly enriched in gas.

In this case, the increased vapor pressure in the residual liquid can also lead to gas exsolution. This is sometimes referred to as second or retrograde boiling. Both mechanisms can trigger an explosive volcanic eruption. The amount of dissolved gas in the magma provides the driving force for explosive eruptions. The viscosity of the magma, however, is also an important factor in determining whether an eruption will be explosive or nonexplosive.

A low-viscosity magma, like basalt, will allow the escaping gases to migrate rapidly through the magma and escape to the surface. However, if the magma is viscous, like rhyolite, its high polymerization will impede the upward mobility of the gas bubbles. As gas continues to exsolve from the viscous melt, the bubbles will be prevented from rapid escape, thus increasing the overall pressure on the magma column until the gas ejects explosively from the volcano.

As a general rule, therefore, nonexplosive eruptions are typical of basaltic-to-andesitic magmas which have low viscosities and low gas contents, whereas explosive eruptions are typical of andesitic-to-rhyolitic magmas which have high viscosities and high gas contents. There are, however, two exceptions to this general rule. Andesitic-to-rhyolitic lavas that have been degassed often erupt at the surface nonexplosively as viscous lava domes or obsidian flows.

Similarly, many of the so-called hydrovolcanic eruptions involve basaltic-to-andesitic magmas that erupt explosively in the presence of groundwater or surface water.

For more information on the variability of explosivity, see the Volcano Explosivity Index. Intermediate to advanced users may be interested in the following programs for calculating viscosity. Click image to download. Viscosity for Windows. Developed by Dr.