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VOLCANISM

VOLCANISM

A volcano occurs where lava erupts at the surface and solidifies into rock. This section describes volcano location, type, hazards, and monitoring.

MAGMA GENERATION

Magma and lava contain three components – melt, solids, and volatiles (dissolved gases). The liquid part, called melt, is made of ions from minerals that have already melted. The solid part, called solids, are crystals of minerals that have not melted (higher melting temperature) and are floating in the melt. Volatiles are gaseous components dissolved in the magma such as water vapor, carbon dioxide, sulfur, and chlorine. The presence and amount of these three components affect the physical behavior of the magma.

GEOTHERMAL GRADIENT

Although it is scorching under the Earth’s surface, the crust and mantle are mostly solid. This heat inside the Earth is caused by residual heat left over from the original formation of Earth and radioactive decay. The rate at which temperature increases with depth is called the geothermal gradient. The average geothermal gradient in the upper 100 kilometers of the crust is generally about 25 degrees Celsius per kilometer (km). So, for every kilometer of depth, the temperature increases by about 25 degrees Celsius.

Pressure-temperature diagrams illustrate the geothermal gradient together with the behavior of rock by graphing depth (pressure) and temperature (see figure). The figure shows the geothermal gradient changing with depth through the crust into the upper mantle. The diagram shows the geothermal gradient as a red line, and at 100 km depth, the temperature is about 1,200°C. Also, the pressure at the bottom of the crust (shown here as depth at 35 km deep) is about 10,000 bars. Bar is a measure of pressure, 1 bar being normal atmospheric pressure at sea-level. At these pressures and temperatures in the Earth, the crust and mantle rocks are solid. On the P-T diagram, the green solidus line shows the pressures and temperatures at which rocks start to melt. Since the geothermal gradient (red line) is always left of the solidus (green line) to a depth of 150 km, the rocks of the upper mantle are solid. This relationship continues through the mantle to the core-mantle boundary at about 2880 km. The solidus line slopes to the right because the melting temperature of any substance depends on the pressure. Higher pressure at greater depth requires a higher temperature to melt rock. In another example, water boils at 100°C at an atmospheric pressure close to 1 bar. However, if the pressure is lowered, as shown in the video below, then water boils at a much lower temperature.

The P-T diagram shows that there are three main ways that pressure and temperature conditions can change to cause rock to cross the green solidus line to the right to induce melting and create magma: 1) lower the pressure (decompression melting), 2) add volatiles (flux melting),and 3) increase the temperature (heat). Bowen’s work and the Bowen’s Reaction Series diagram show that minerals melt at different temperatures, so one can visualize that the green solidus line is a fuzzy zone in which some minerals are melting, and some remain solid. This is called partial melting and represents real magmas containing solid, liquid, and volatile components.

The figure below uses P-T diagrams to show how melting can occur at three different plate tectonic settings. Setting A is a typical situation in the middle of a stable plate in which no magma is generated. Setting B is at a mid-ocean ridge (decompression melting). Setting C is a hotspot (decompression melting plus the addition of heat), and setting D is a subduction zone (flux melting).

DECOMPRESSION MELTING

Magma is created at the mid-ocean ridge by decompression melting. The mantle is solid but is slowly flowing under enormous pressure and temperatures due to convection. Rock is not a good conductor of heat so as mantle rock rises, the pressure is reduced along with the melting point (the green line) but the rock temperature remains about the same and the rising rock begins to melt. Pressure changes instantaneously as the rock rises but temperature changes slowly because of the low heat conductivity of rock. On the figure above, setting B: mid-ocean ridge shows a mass of mantle rock at a pressure-temperature location X on the P-T diagram as well as its geographical location on the cross section under a mid-ocean ridge. At this location, the P-T diagram shows the red arrow increasing to the right. Thus, hotter rock is now shallower, at a lower pressure, and the new geothermal gradient (red line) shifts past the solidus (green line) and melting starts. As this magma continues to rise at divergent boundaries and encounters seawater, it cools and crystallizes to form new lithospheric crust.

FLUX MELTING

Another way that rocks melt is when volatiles gases (e.g., water vapor) are added to mantle rock from a descending subducting slab in a process called flux melting (or fluid-induced melting). The subducting slab contains oceanic lithosphere and hydrated minerals. As the slab descends and slowly increases in temperature, volatiles is expelled from these hydrated minerals, like squeezing water out of a sponge. The volatiles then rises into the overlying asthenospheric mantle lowering the melting point of the peridotite minerals (olivine and pyroxene). The pressure and temperature of the overlying mantle rock do not change, but the addition of volatiles lowers the melting temperature. This is analogous to adding salt to an icy roadway. The salt lowers the melting/crystallization temperature of the solid water (ice) so that it melts. Another example is welders adding flux to lower the melting point of their welding materials.

Flux melting is illustrated in setting D: island arc (subduction zone) of the P-T diagram above. Volatiles added to mantle rock at location “Z“ act as a flux to lower the melting temperature. This is shown in the P-T diagram by the solidus (green line) shifting to the left. The solidus line moves past the geothermal gradient (red line) and melting begins. Magmas producing the volcanoes of the Ring of Fire, associated with the circum-Pacific subduction zones are a result of flux melting. As introduced in the minerals chapter, water ions can bond with other ions in the crystal structures of amphibole (and other silicates), and this is important in considering how magmas form in subduction zones by “flux melting.” Such hydrated minerals in subducting slabs contribute water to the flux melting process.

MAGMA COMPOSITION

In 1980, Mount St. Helens blew up in the costliest and deadliest volcanic eruption in United States history. The eruption killed 57 people, destroyed 250 homes and swept away 47 bridges. Mount St. Helens today still has minor earthquakes and eruptions, and now has a horseshoe-shaped crater with a lava dome inside. The dome is formed of viscous lava that oozes into place.

It should first be noted that magma is molten material inside the earth, whereas lava is molten material on the surface of the earth. The reason for the distinction is because lava can cool quickly from the air and solidify into rock rapidly, whereas magma may never reach the earth’s surface. Volcanoes do not always erupt in the same way. Each volcanic eruption is unique, differing in size, style, and composition of the 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 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 160 kilometers (100 miles) beneath the surface of a volcano.

The words that describe the 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 is a liquid’s resistance to flow.

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 intrusive igneous 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. Dissolved gases become trapped by thick magma, and the magma chamber begins to build pressure.

EXPLOSIVE ERUPTIONS

The type of magma in the chamber determines the type of volcanic eruption. A massive explosive eruption creates even more devastation than the force of the atom bomb dropped on Nagasaki at the end of World War II in which more than 40,000 people died. A large explosive volcanic eruption is 10,000 times as powerful. 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. It should be noted that when looked under a microscope, the volcanic “ash” is actual microscopic shards of glass. That is why it is so dangerous to inhale the air following an eruption.

Scorching hot tephra, ash, and gas may speed down the volcano’s slopes at 700 km/h (450 mph) as a pyroclastic flow. Pyroclastic flows knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,000oC (1,800 degrees F).

Before the Mount St. Helens eruption in 1980, the Lassen Peak eruption on May 22, 1915, was the most recent Cascades eruption. A column of ash and gas shot 30,000 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. Mt. Shasta, the other active volcano in California, erupts every 600 to 800 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. Mazama in Oregon.

Volcanic gases can form toxic and invisible clouds in the atmosphere that could contribute to environmental problems such as acid rain and ozone destruction. Particles of dust and ash may stay in the atmosphere for years, disrupting weather patterns and blocking sunlight.

EFFUSIVE ERUPTIONS

Mafic magma creates gentler effusive eruptions. Although the pressure builds enough for the magma to erupt, it does not erupt with the same explosive force as felsic magma. People can usually be evacuated before an effusive eruption, so they are much less deadly. Magma pushes toward the surface through fissures and reaches the surface through volcanic vents. Click here to view a lava stream within the vent of a Hawaiian volcano using a thermal camera.

Low-viscosity lava flows down mountainsides. Differences in composition and where the lavas erupt result in lava types like a ropy form pahoehoe and a chunky form called aa. Although effusive eruptions rarely kill anyone, they can be destructive. Even when people know that a lava flow is approaching, there is not much anyone can do to stop it from destroying a building, road, or infrastructure.

LICENSE

 

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Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Kategoria: Moje artykuły | Dodał: kolo (2019-04-04)
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