Volcanic mudflows are called lahars. These can occur accompanying an eruption or occur long after an eruption. Lahars may be hot or cold and move at high velocity as they fill stream valleys that drain the volcano.
At the base of the volcano, they spread out and cover wide areas. In general, they dev estate anything in their path, carrying away homes, buildings, bridges, and destroying roads, and killing livestock and people. In a lahar produced by a mild eruption of Nevado de Ruiz volcano in Colombia wiped out the village of Armero, about 60 km away from the volcano and killed about 23, people.
Although the predominant gas erupted from volcanoes is H 2 O vapor, other gases are erupted can have disastrous effects on life. The Chlorine, Sulfur. In an CO 2 gas emission from Lake Nyos in Cameroon killed more than people and cattle.
The gases can also have an effect on the atmosphere and climate. Much of the water on the surface of the earth was produced by volcanoes throughout earth history.
The Eruption of Mount. Helens, Prior to , Mount St. Helens last erupted in On March 21, a 4. Small eruptions took place through mid April and the summit of the mountain developed a new crater due to the explosions.
By the end of April surveys showed that the north face of the mountain had begun to bulge upwards and outwards at rates up to 1 m per day. By May 12, the bulge had displaced parts of the northern part of the volcano a distance of about m. Geologists now recognized that this bulge could soon develop into a landslide.
At AM on May 18, a magnitude 5. This led to a violent eruption that took place over about the next minute. The earthquake triggered a large landslide that began to slide out to the north, initially as three large blocks. In all, 62 people lost their lives, either by being buried by the debris avalanche deposit, or suffocating by breathing the hot gases and dust of the blast.
Over the next several days melted snow combined with the new ash to produce lahars that roared down the North and South Forks of the Toutle River and drainages to the south of the volcano. In general, the eruption had been much larger than most anticipated, but the fact that a hazards study had been carried out, that public officials were quick to act and evacuate the danger zone, and that the volcano was under constant monitoring, resulted in the minimization of loss of life to only 62 instead of a much larger number that could have been killed had not these efforts been in place.
Since the eruption, several volcanic domes have been emplaced in the crater and some have been blasted out. In the future, it is expected that new domes will continue to form, eventually building the volcano back to a form that will look more like it did prior to the eruption. Predicting Volcanic Eruptions. Before discussing how we can predict volcanic eruptions, its important to get some terminology straight by defining some commonly used terms.
Active Volcano - An active volcano to volcanologists is a volcano that has shown eruptive activity within recorded history. Thus an active volcano need not be in eruption to be considered active. Extinct Volcano - An extinct volcano is a volcano that has not shown any historic activity, is usually deeply eroded, and shows no signs of recent activity.
How old must a volcano be to be considered extinct depends to a large degree on past activity. Dormant Volcano - A dormant volcano sleeping volcano is somewhere between active and extinct. A dormant volcano is one that has not shown eruptive activity within recorded history, but shows geologic evidence of activity within the geologic recent past. Because the lifetime of a volcano may be on the order of a million years, dormant volcanoes can become active volcanoes all of sudden.
These are perhaps the most dangerous volcanoes because people living in the vicinity of a dormant volcano may not understand the concept of geologic time, and there is no written record of activity. These people are sometimes difficult to convince when a dormant volcano shows signs of renewed activity.
Such hazards maps delineate zones of danger expected from the hazards discussed above: lava flows, pyroclastic flows, tephra falls, lahars, floods, etc. Short - term prediction of volcanic eruptions involves monitoring the volcano to determine when magma is approaching the surface and monitoring for precursor events that often signal a forthcoming eruption.
Earthquakes - As magma moves toward the surface it usually deforms and fractures rock to generate earthquakes. Thus an increase in earthquake activity immediately below the volcano is usually a sign that an eruption will occur.
Ground Deformation - As magma moves into a volcano, the structure may inflate. This will cause deformation of the ground which can be monitored. Instruments like tilt meters measure changes in the angle of the Earth's surface.
Other instruments track changes in distance between several points on the ground to monitor deformation. Changes in Heat Flow - Heat is everywhere flowing out of the surface of the Earth. As magma approaches the surface or as the temperature of groundwater increases, the amount of surface heat flow will increase. Although these changes may be small they be measured using infrared remote sensing.
The main types of volcanic hazards have been discussed above, so here we only briefly discuss them. You should make sure you understand what each of these are, and what effects each type of hazard can have. We will not likely have time to discuss these again in detail, so the following material is mostly for review.
Pyroclastic Flows - Pyroclastic flows are one of the most dangerous aspects of volcanism. They cause death by suffocation and burning. 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.
Andesite magma can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Facts: The word andesite is derived from the Andes Mountains, located along the western edge of South America, where andesite rock is common.
Andesite was the main rock type erupted during the great Krakatau eruption of Holocene Hydrology Hydrothermal explosion. Igneous ignimbrite Intrusive Isopach. K-Ar ka. Which crystals does andesite have? Andesites commonly have abundant plagioclase feldspar along with pyroxenes clinopyroxene and orthopyroxene , often mica mostly biotite , and sometimes small amounts of hornblende. Silica-rich andesites may contain small amounts of quartz, while very silica-poor andesites, close to basaltic compositions, still may have some olivine in them.
Characteristics of andesite lavas Andesite lava is still hot between deg C and fluid enough to be able to form lava flows, but already sticky viscous enough to prevent lava flowing fast and being "runny". Instead, andesitic flows are rather thick and tall, with a high aspect ratio between thickness and length.
They often form broad, thick tongues, that resemble giant toes. In rare cases, their length exceed a few km. Due to their stickyness, gasses can not easily escape from the magma, and when erupted, a gas-rich andesitic magma can drive very explosive eruptions, as the gasses suddenly fragment the magma.
Andesitic lava can also form so-called domes rather than lava flows - accumulations of thick lava masses above or near the vent that have been unable to flow away.
Andesites in Greece Andesite is a characteristic rock on the volcano peninsula Methana, on Santorini and on Nisyros island. Most lava domes on Methana are composed of andesitic rocks. Interesting is the phenomenon of magma-mixing that is thought to be a driving force in many explosive eruptions where the relatively cool andesitic lava is involved: basaltic intrusions in such a magma-chamber heat up the magma and chemical reactions activate such a magma.
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