The Australian Museum respects and acknowledges the Gadigal people as the First Peoples and Traditional Custodians of the land and waterways on which the Museum stands. Image credit: gadigal yilimung shield made by Uncle Charles Chicka Madden. This website uses cookies to ensure you get the best experience on our website. Learn more. Temperature The temperature of a magma is hard to measure. Density The density of chilled magma volcanic glass is usually measured. Viscosity Viscosity is the resistance of a fluid to motion.
Gas content Many lavas are vesicular have cavities , indicating former gas bubbles which escaped from within the magma as it erupted at the surface. Abundance The most common igneous rocks by far are granites and basalts. 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.
Surge deposits are usually produced. Phreatic also called steam blast eruptions - result when magma encounters shallow groundwater, flashing the groundwater to steam, which is explosively ejected along with pre-exiting fragments of rock.
No new magma reaches the surface. Surge deposits may result from these eruptions. Questions on this material that could be asked on an exam. Natural Disasters. Volcanoes, Magma, and Volcanic Eruptions. Characteristics of Magma Types of Magma Types of magma are determined by chemical composition of the magma. Temperature of Magmas Temperature of magmas is difficult to measure due to the danger involved , but laboratory measurement and limited field observation indicate that the eruption temperature of various magmas is as follows: Basaltic magma - to o C Andesitic magma - to o C Rhyolitic magma - to o C.
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. Although the growing gas bubbles will exhibit low viscosity, the viscosity of the residual liquid will increase as gas escapes. The overall bulk viscosity of the bubble-liquid mixture depends on both the size and distribution of the bubbles.
Although gas bubbles do have an effect on the viscosity, the more important role of these exsolving volatiles is that they provide the driving force for the eruption. This is discussed in more detail below. As dissolved gases are released from the magma, bubbles will begin to form. Bubbles frozen in a porous or frothy volcanic rock are called vesicles , and the process of bubble formation is called vesiculation or gas exsolution.
The dissolved gases can escape only when the vapor pressure of the magma is greater than the confining pressure of the surrounding rocks. The vapor pressure is largely dependent on the amount of dissolved gases and the temperature of the magma. Gas escape through vertical vesicle cylinders Vesicle-rich flow top. Explosive eruptions are initiated by vesiculation, which in turn, can be promoted in two ways: 1 by decompression , which lowers the confining pressure, and 2 by crystallization, which increases the vapor pressure.
In the first case, magma rise can lead to decompression and the formation of bubbles, much like the decompression of soda and the formation of CO 2 bubbles when the cap is removed.
Such magmas erupt as andesites and rhyolites or are intruded as granite masses. The more extensive silicate chain molecules render these magmas highly viscous, so when eruption occurs it is usually explosive e. Mt St Helens. These magmas erupt as basalts or intrude as gabbro, and are far less viscous.
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