What makes magma less viscous

Nonexplosive eruption with effusive lava flows Explosive eruption with voluminous plume of tephra. Whether or not an eruption falls into one of these end-member types depends on a variety of factors, which are ultimately linked to the composition of the magma molten rock underlying the volcano. Magma composition is discussed below, followed by a description of the controlling factors on explosivity -- viscosity , temperature , and the amount of dissolved gases in the magma. Because oxygen and silicon are by far the two most abundant elements in magma, it is convenient to describe the different magma types in terms of their silica content SiO 2.

The magma types vary from mafic magmas , which have relatively low silica and high Fe and Mg contents, to felsic magmas , which have relatively high silica and low Fe and Mg contents. Mafic magma will cool and crystallize to produce the volcanic rock basalt , whereas felsic magma will crystallize to produce dacite and rhyolite.

Intermediate-composition magmas will crystallize to produce the rock andesite. Because the mafic rocks are enriched in Fe and Mg, they tend to be darker colored than the felsic rock types. There also exists more unusual magmas that erupt less commonly on the Earth's surface as ultramafic, carbonatite, and strongly alkaline lavas.

The viscosity of a substance is a measure of its consistency. Viscosity is defined as the ability of a substance to resist flow. In a sense, viscosity is the inverse of fluidity. Cold molasses, for example, has a higher viscosity than water because it is less fluid.

A magma's viscosity is largely controlled by its temperature, composition, and gas content see downloadable programs at the bottom of this page. The effect of temperature on viscosity is intuitive. Like most liquids, the higher the temperature, the more fluid a substance becomes, thus lowering its viscosity.

Composition plays an even greater role in determining a magma's viscosity. A magma's resistance to flow is a function of its "internal friction" derived from the generation of chemical bonds within the liquid. Chemical bonds are created between negatively charged and positively charged ions anions and cations , respectively. Of the ten most abundant elements found in magmas see above , oxygen is the only anion. Silicon, on the other hand, is the most abundant cation. Thus, the Si-O bond is the single most important factor in determining the degree of a magma's viscosity.

These two elements bond together to form "floating radicals" in the magma, while it is still in its liquid state i. These floating radicals contain a small silicon atom surrounded by four larger oxygen atoms SiO 4. This atomic configuration is in the shape of a tetrahedron.

The radicals are therefore called silicon-oxygen tetrahedra , as shown here. Volatile Content: Magma invariably contains small amounts of dissolved gas water, CO 2 etc which is released as pressure is removed.

Magmas formed by melting of mantle rocks have generally low volatile contents, but those formed by partial melting of crustal rocks are often volatile-rich. A high volatile content decreases viscosity like adding water to treacle , and is probably the main factor in enabling some highly viscous but also volatile-rich melts to reach the surface at all.

The release of gas during eruption is particularly likely to be explosive if the magma is both viscous as gas is released, so viscosity is immediately increased and volatile rich. Crystal Content: Some magmas have already begun to crystallise by the time they reach the surface. Again, this applies particularly to the cooler, more viscous magmas typical of destructive plate margins.

Most shield volcanoes are much smaller. In fact, the island of Hawaii itself is composed of several volcanoes, not one, though each is itself very large. Basaltic lava flows on Hawaii and elsewhere produce may have two kinds of surface. One type has a smooth surface with ridges that look like coiled ropes. This type of volcanic flow is called by the Hawaiian name pahoehoe.

Thick lava flows may remain molten in the interior and continue to move even while the surface solidifies. But continued movement breaks up the surface into jagged rock fragments. These broken, jagged volcanic flows are called aa.

Sometimes basaltic lava erupts from a series of fissures , or cracks in the Earth and spreads widely over the landscape, rather than erupting from a singe volcanic vent.

These kinds of eruptions produce flood basalts. Cinder Cones. Sometimes, in the late stage of basaltic volcanic activity, small cinder cones become active.

When its supply of magma from deep in the Earth slows or stops, the magma chamber beneath a volcano will cool and begin crystallizing. The first-formed minerals will be high-temperature, mafic minerals like olivine which are rich in iron and magnesium and poor in silica. The result is that the remaining magma becomes depleted in iron and magnesium and enriched in silica.

Consequently, the viscosity increases in the remaining magma and it does not flow out of the ground as easily. Rather, it builds up enough back-pressure to eject a spray of lava into the air.

The lava droplets cool and crystallize rapidly or at least the surface of large blobs of lava crystallizes rapidly and then they fall as volcanic cinders, or pyroclasts , on the flanks of the volcano and tumble down its side. Most are less than 1, ft high. They usually have a large crater where the pyroclastics were ejected. Sunset Crater in Arizona is an example of a cinder cone. At the angle of repose, gravity which drives the particles downslope and friction which prevents them from sliding down are balanced.

Composite Cones - Stratovolcanoes. Intermediate composition andesitic lavas are common where two plates converge to produce a subduction zone, especially along continental margins like western South America. Intermediate lavas are more viscous than the mafic lava which forms shield volcanoes see above and normally contain more volatiles.

Sometimes the lava flows out as a thick, slowly-flowing fluid which is not able to flow too far before crystallizing. Oftentimes, the pressure builds up enough to produce large pyroclastic eruptions. Alternating eruptions of lava and pyroclasts produce large, layered volcanoes with steep slopes.