Today, we are going to delve into some characteristics that make each volcano unique by looking at certain features that cause them to behave differently. Volcanoes have long fascinated humans with their immense—sometimes incredibly destructive—power and impact. Would any of you like to live near a volcano? Expect students to respond by mentioning the dangers of being close to an active volcano, including the potential for death and destruction caused by eruptions.
Heat, fire, hot magma, etc. Commonly, people associate volcanoes with eruptions of a violent nature, but many volcanoes do not pose a catastrophic risk to their surrounding environments because they do not erupt violently.
During some eruptions, lava, as well as volcanic ash, gas and rock fragments, are sent miles into the air and surrounding areas, posing a large environmental hazard. Volcanoes are dangerous because of the extreme temperatures of the molten melted rock involved in eruptions.
Complex processes happen inside magma, and that is our focus today. Figure 1. Myers and S. Brantley, U. First, it is important to note that pressure is incredibly high under the surface of volcanoes. We measure pressure in units of Pascals, just like we measure length in meters.
At that point, a volcano can erupt in one of two main ways: an effusive eruption or an explosive eruption. Explosive eruptions pose huge dangers to nearby communities. Eruption intensity can also vary depending on how much pressure is inside the bubbles. We must understand these two concepts to understand violent volcanic eruptions and ultimately what we can do to engineer solutions to the inevitable hazards they pose.
A thorough understanding of volcanoes made this feat possible in the form of engineered tools that monitored and predicted the volcanic eruption. Today we will learn about the fundamentals of what makes a violent eruption so catastrophic.
In truth, eruptions come in a wide variety of forms and the explosive and damaging class is just one of a wide range of types. For example, many volcanoes in Hawaii are characterized by free-flowing magma, but rarely are as destructive as violent eruptions that spew large amounts of rock and ash. Mount St. Helens, one of the most well-known volcanoes in American history, is characterized by eruptions on the opposite end of the spectrum.
The Mount St. Helens pyroclastic eruption killed 57 people and remains the most destructive in U. In this activity, students explore two of the main concepts behind explosive eruptions as opposed to the less-damaging, effusive eruptions : viscosity and the degassing rate. The first part of the activity is a simplified procedure based on a lab assay that is regularly performed by researchers who study fluid mechanics.
Figure 2. Using a marble dropped in liquid to explore viscosity. In the case of volcanoes, freeing gas from magma. Plinian eruption: The largest and most violent type of volcanic eruptions.
Usually associated with very viscous magma. Example: Mount. Helens eruption. A bubble encounters more resistance as it rises through honey than water. Discussion of Volcanic Eruptions: Engage students in an open discussion to help them to ponder why volcanoes can be dangerous. Encourage brainstorming and sharing of ideas. Ask students:.
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.
Viscosity of Magmas Viscosity is the resistance to flow opposite of fluidity. Higher SiO 2 silica content magmas have higher viscosity than lower SiO 2 content magmas viscosity increases with increasing SiO 2 concentration in the magma. Lower temperature magmas have higher viscosity than higher temperature magmas viscosity decreases with increasing temperature of the magma.
How Magmas Form in the Earth As we have seen the only part of the earth that is liquid is the outer core. If the mineral contains no water H 2 O or carbon dioxide CO 2 and there is no water or carbon dioxide present in the surroundings, then melting occurs at a single temperature at any given pressure and increases with increasing pressure or depth in the Earth. This is called dry melting. If water or carbon dioxide are present within or surrounding the mineral, then melting takes place at a single temperature at any given pressure, but first decreases with increasing pressure.
Since rocks are mixtures of minerals, they behave somewhat differently. Unlike minerals, rocks do not melt at a single temperature, but instead melt over a range of temperatures. Thus, it is possible to have partial melts, from which the liquid portion might be extracted to form magma. The two general cases are:. Melting of dry rocks is similar to melting of dry minerals, melting temperatures increase with increasing pressure, except there is a range of temperature over which there exists a partial melt.
Melting of wet rocks is similar to melting of wet minerals, except there is range of temperature range over which partial melting occurs. Again, the temperature of beginning of melting first decreases with increasing pressure or depth, then at high pressure or depth the melting temperatures again begin to rise.
Three ways to Generate Magmas From the above we can conclude that in order to generate a magma in the solid part of the earth either the geothermal gradient must be raised in some way or the melting temperature of the rocks must be lowered in some way. Chemical Composition of Magmas The chemical composition of magma can vary depending on the rock that initially melts the source rock , and process that occur during partial melting and transport.
Initial Composition of Magma The initial composition of the magma is dictated by the composition of the source rock and the degree of partial melting. Magmatic Differentiation But, processes that operate during transportation toward the surface or during storage in the crust can alter the chemical composition of the magma.
Assimilation - As magma passes through cooler rock on its way to the surface it may partially melt the surrounding rock and incorporate this melt into the magma.
Because small amounts of partial melting result in siliceous liquid compositions, addition of this melt to the magma will make it more siliceous. Mixing - If two magmas with different compositions happen to come in contact with one another, they could mix together.
The mixed magma will have a composition somewhere between that of the original two magma compositions. Evidence for mixing is often preserved in the resulting rocks. Crystal Fractionation - When magma solidifies to form a rock it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change. Depending on how many minerals are lost in this fashion, a wide range of compositions can be made.
The processes is called magmatic differentiation by crystal fractionation. Crystals can be removed by a variety of processes.
If the crystals are more dense than the liquid, they may sink. If they are less dense than the liquid they will float. If liquid is squeezed out by pressure, then crystals will be left behind. Removal of crystals can thus change the composition of the liquid portion of the magma. Let me illustrate this using a very simple case.
Imagine a liquid containing 5 molecules of MgO and 5 molecules of SiO 2. Volcanic Eruptions In general, magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rocks.
Effusive Non-explosive Eruptions Non explosive eruptions are favored by low gas content and low viscosity magmas basaltic to andesitic magmas.
Lava Flows Pahoehoe Flows - Basaltic lava flows with low viscosity start to cool when exposed to the low temperature of the atmosphere. Explosive Eruptions Explosive eruptions are favored by high gas content and high viscosity andesitic to rhyolitic magmas. Blocks are angular fragments that were solid when ejected.
Bombs have an aerodynamic shape indicating they were liquid when ejected. Bombs and lapilli that consist mostly of gas bubbles vesicles result in a low density highly vesicular rock fragment called pumice. Clouds of gas and tephra that rise above a volcano produce an eruption column that can rise up to 45 km into the atmosphere.
Eventually the tephra in the eruption column will be picked up by the wind, carried for some distance, and then fall back to the surface as a tephra fall or ash fall. If the eruption column collapses a pyroclastic flow will occur, wherein gas and tephra rush down the flanks of the volcano at high speed.
This is the most dangerous type of volcanic eruption. The deposits that are produced are called ignimbrites if they contain pumice or pyroclastic flow deposits if they contain non-vesicular blocks.
Pyroclastic Deposits Pyroclastic material ejected explosively from volcanoes becomes deposited on the land surface. Fall Deposits. Material ejected into an eruption column eventually falls back to the earth's surface and blankets the surface similar to the way snow blankets the earth. The thickest deposits occur close to vent and get thinner with distance from the vent. By measuring the thickness at numerous locations one can construct an isopach map.
Such isopach maps help to locate the source volcanic vent if it is not otherwise known and provides information about wind direction in the upper levels of the atmosphere during the eruption. Fall deposits are usually fairly well-sorted, meaning that the clast size does not vary too much within the individual deposit. The clast size can be ash as in a cinder cone. They may also contain clasts of rock fragments called lithic fragments that are pieces of the volcanic structure ripped from the sides of the conduit during the explosive eruption.
If the pyroclastic flows consist of solid clasts with high density along with ash fragments, they are called block and ash flows. If the pyroclastic flows have low density clasts pumice along with ash, they are called ignimbrites. There are no definitive boundary between pyroclastic flows and surges as they grade into one another continuously.
Similarly, ignimbrites grade into block and ash flows as the clast density increases. Pyroclastic Flow Deposits Pyroclastic flows tend to follow valleys or low lying areas of topography. The material deposited, thus tends to fill valleys, rather than uniformly blanket the topography like fall deposits. 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. Experiements The "Treacle Test" experiment is designed to enable students to investigate how temperature, volatiles and crystals may affect viscosity.
It is suggested that viscosity is roughly measured by how long it takes the treacle to flow from one end of a boiling tube to the other.
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