What kind of magma produces violent eruptions

The style of eruption depends on a number of factors, including the magma chemistry and content, temperature, viscosity how runny the magma is , volume and how much water and gas is in it, the presence of groundwater, and the plumbing of the volcano. For information on volcanic hazards which can be produced by our volcanoes, click here.

Hydrothermal eruption An eruption driven by the heat in a hydrothermal systems. Hydrothermal eruptions pulverise surrounding rocks and can produce ash, but do not include magma. These are typically very small eruptions. Phreatic eruption An eruption driven by the heat from magma interacting with water. The water can be from groundwater, hydrothermal systems, surface runoff, a lake or the sea.

Phreatic eruptions pulverise surrounding rocks and can produce ash, but do not include new magma. 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.

These floating tetrahedra are electrically charged compounds. As such, they they are electrically attracted to other Si-O tetrahedra. The outer oxygen atoms in each tetrahedron can share electrons with the outer oxygen atoms of other tetrahedra.

The sharing of electrons in this manner results in the development of covalent bonds between tetrahedra. In this way Si-O tetrahedra can link together to form a variety shapes: double tetrahedra shown here, C , chains of tetrahedra, double chains of tetrahedra, and complicated networks of tetrahedra. As the magma cools, more and more bonds are created, which eventually leads to the development of crystals within the liquid medium. Thus, the Si-O tetrahedra form the building blocks to the common silicate minerals found in all igneous rocks.

However, while still in the liquid state, the bonding of tetrahedra results in the polymerization of the liquid, which increases the "internal friction" of the magma, so that it more readily resists flow. Magmas that have a high silica content will therefore exhibit greater degrees of polymerization, and have higher viscosities, than those with low-silica contents. The amount of dissolved gases in the magma can also affect it's viscosity, but in a more ambiguous way than temperature and silica content.

When gases begin to escape exsolve from the magma, the effect of gas bubbles on the bulk viscosity is variable. 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. Of course, Mt. Shasta could explode and collapse like Mt. They can travel so rapidly that few humans can escape. Poisonous Gas Emissions , as discussed above. Tsunami - Debris avalanche events, landslides, caldera collapse events, and pyroclastic flows entering a body of water may generate tsunami.

During the eruption of Krakatau volcano, in the straits of Sunda between Java and Sumatra, several tsunami were generated by pyroclastic flows entering the sea and by collapse accompanying caldera formation.

The tsunami killed about 36, people, some as far away from the volcano as km. In the discussion we had on igneous rocks and how magmas form, we pointed out that since the upper parts of the Earth are solid, special conditions are necessary to form magmas. These special conditions do not exist everywhere beneath the surface, and thus volcanism does not occur everywhere. If we look at the global distribution of volcanoes we see that volcanism occurs four principal settings.

Along divergent plate boundaries, such as Oceanic Ridges or spreading centers. In areas of continental extension that may become divergent plate boundaries in the future.

Along converging plate boundaries where subduction is occurring. And, in areas called "hot spots" that are usually located in the interior of plates, away from the plate margins. Since we discussed this in the lecture on igneous rocks, we only briefly review this material here. Active volcanism is currently taking place along all of oceanic ridges, but most of this volcanism is submarine volcanism. One place where an oceanic ridge reaches above sea level is at Iceland, along the Mid-Atlantic Ridge.

Here, most eruptions are basaltic in nature, but, many are explosive strombolian types or explosive phreatic or phreatomagmatic types. As seen in the map to the right, the Mid-Atlantic ridge runs directly through Iceland. Volcanism also occurs in continental areas that are undergoing episodes of rifting. The extensional deformation occurs because the underlying mantle is rising from below and stretching the overlying continental crust. Upwelling mantle may melt to produce magmas, which then rise to the surface, often along normal faults produced by the extensional deformation.

Basaltic and rhyolitic volcanism is common in these areas. In the same area, the crust has rifted apart along the Red Sea, and the Gulf of Aden to form new oceanic ridges. This may also be the fate of the East African Rift Valley at some time in the future. Other areas where extensional deformation is occurring within the crust is Basin and Range Province of the western U.

These are also areas of recent basaltic and rhyolitic volcanism. All around the Pacific Ocean, is a zone often referred to as the Pacific Ring of Fire, where most of the world's most active and most dangerous volcanoes occur.

The Ring of Fire occurs because most of the margins of the Pacific ocean coincide with converging margins along which subduction is occurring. These are all island arcs. The Hawaiian Ridge is one such hot spot trace.

Here the Big Island of Hawaii is currently over the hot spot, the other Hawaiian islands still stand above sea level, but volcanism has ceased. Northwest of the Hawaiian Islands, the volcanoes have eroded and are now seamounts. Plateau or Flood basalts are extremely large volume outpourings of low viscosity basaltic magma from fissure vents. The basalts spread huge areas of relatively low slope and build up plateaus.

Many of these outpourings appear to have occurred along a zone that eventually developed into a rift valley and later into a diverging plate boundary. Examples of questions on this material that could be asked on an exam. Physical Geology. Volcanoes and Volcanic Eruptions. Magmas and Lava Since volcanic eruptions are caused by magma a mixture of liquid rock, crystals, and dissolved gas expelled onto the Earth's surface, we'll first review the characteristics of magma that we covered previously.

Viscosity of Magmas Viscosity is the resistance to flow opposite of fluidity. Higher SiO 2 content magmas have higher viscosity than lower SiO 2 content magmas Lower Temperature magmas have higher viscosity than higher temperature magmas. Solidified Volcanic Rock. Solidified Plutonic Rock.

Intermediate or Andesitic. 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 9. 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 9. Lava Tubes - Once the surface skin becomes solid, the lava can continue to flow beneath the surface in lava tubes.

The surface skin insulates the hot liquid lava form further cooling. When the eruption ends, liquid lava often drains leaving an open cave see figure 9. 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 see figure 9.

Columnar Jointing - When thick basaltic or andesitic lavas cool, they contract. The contraction results in fractures and often times results in a type of jointing called columnar jointing. The columns are usually hexagonal in shape. This often happens when lavas pool in depressions or deep canyons see figure 9.

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.

Pyroclastic Material If the magma has high gas content and high viscosity, the gas will expand in an explosive fashion and break the liquid into clots that fly through the air and cool along their path through the atmosphere.

Blocks are angular fragments that were solid when ejected. Volcanic Landforms Volcanic landforms are controlled by the geological processes that form them and act on them after they have formed. Shield Volcanoes A shield volcano is characterized by gentle upper slopes about 5 o and somewhat steeper lower slopes about 10 o. Most shield volcanoes have a roughly circular or oval shape in map view. Long periods of repose times of inactivity lasting for hundreds to thousands of years, make this type of volcano particularly dangerous, since many times they have shown no historic activity, and people are reluctant to heed warnings about possible eruptions.

Cinder Cones Cinder cones are small volume cones consisting predominantly of ash and scoria that result from mildly explosive eruptions. They usually consist of basaltic to andesitic material. They are actually fall deposits that are built surrounding the eruptive vent. Slopes of the cones are controlled by the angle of repose angle of stable slope for loose unconsolidated material and are usually between about 25 and 35 o.

On young cones, a depression at the top of the cone, called a crater, is evident, and represents the area above the vent from which material was explosively ejected. Craters are usually eroded away on older cones.

Craters and Calderas Craters are circular depressions, usually less than 1 km in diameter, that form as a result of explosions that emit gases and ash. Calderas are much larger depressions, circular to elliptical in shape, with diameters ranging from 1 km to 50 km. Calderas form as a result of collapse of a volcanic structure.