1. You are a geologist trying to determine the volcanic nature of a past eruptio
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Question
1. You are a geologist trying to determine the volcanic nature of a past eruption. Using the information provided; determine if the volcanic eruption was explosive or effusive.
Volcanic Deposit Information:
Tephra Deposits
Andesite Lava Deposits
A. Explosive, because they tephra indicates a higher viscosity eruption
B. Explosive, because we found lava deposits
C Effusive because of the lava deposits
D. Effusive because of the tephra deposits
2. What do the locations of volcanoes reveal about magma generation?
A. The mantle is made of magma is this magma creates volcanism at the surface of earth.
B. Plate boundaries are mainly associated with the generation of magma b/c 2/3 of volcanism is found here.
C. Magma is made in a volcano tube that all volcanoes across the globe have.
D. Divergent plate boundaries display the only evidence for the location for magma generation.
3. Which rock from the igneous classification chart would most likely represent volcanic rocks from Mt. St. Helens.
A. Andesite
B. Rhyolite
C. Diorite
D. Basalt
4. A parasitic cinder cone found near Mt. St. Helens produced a vesicular rock, black color, and no visible minerals. This rock would most likely be termed what?
A. Scoria
B. Ah’Ah’
C. Pumice
D. Obsidian
5. Pyroclastic material includes the following except?
A. Lava
B. Gas
C. Pumice, Ash, Bombs, welded Tuffs
D. Obsidian
6. A Rhyolite magma composition would most likely involve pyroclastic.
A. True
B. False
Explanation / Answer
(1) the volcanic eruption was effusive because we found lava deposits
Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. Since "supervolcano" calderas can have eruptive lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct.
For example, the Yellowstone Caldera in Yellowstone National Park is at least 2 million years old and hasn't erupted violently for approximately 640,000 years, although there has been some minor activity relatively recently, with hydrothermal eruptions less than 10,000 years ago and lava flows about 70,000 years ago. For this reason, scientists do not consider the Yellowstone Caldera extinct. In fact, because the caldera has frequent earthquakes, a very active geothermal system (i.e., the entirety of the geothermal activity found in Yellowstone National Park), and rapid rates of ground uplift, many scientists consider it to be an active volcano.
(2) The locations of volcanoes reveal about magma generation is divergent plate boundaries display the only evidence for the location for magma generation.Most volcanoes in the world erupt on divergent and convergent boundaries of plates and hot spots. For example, volcanoes of mid-ocean ridges are distributed along the divergent boundaries, those on island arcs are arranged along the convergent boundaries, and in Hawaii, volcanoes are formed on a hot spot. Since tectonic settings and the mechanism of magma generation vary with location, various types of volcanic activity are observed in these regions. In Japan in a subduction zone, many different types of island arc volcanoes have been formed. This section explains Japanese Quaternary volcanoes including magma, volcanic landforms, and distribution.
Magma is a mixture of molten or semi-molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals, dissolved gas and sometimes gas bubbles. Magma often collects in magma chambers that may feed a volcano or solidify underground to form an intrusion. Magma is capable of intrusion into adjacent rocks (forming igneous dikes and sills), extrusion onto the surface as lava, and explosive ejection as tephra to form pyroclastic rock.
(3) Basalt rock from the igneous classification chart would most likely represent volcanic rocks from Mt. St. Helens.
Only three common minerals can form at temperatures between 1200 and 900°C; olivine, pyroxene and calcium plagioclase. As these minerals are all dark in color, mafic rocks also tend to be relatively dark (black to dark-gray), but they may exhibit iron staining on weathered surfaces. Mafic rocks can be distinguished from the ultramafic variety on the asis that they almost always contain plagioclase feldspar. The striated cleavage faces offeldspar crystals can easily be identified with a 10X hand lens in intrusively mafic rocks (e.g.,gabbro). The extrusive mafic rocks (e.g.,basalt) will appear as a smooth, black rock. Only under microscopic examination can the minerals olivine, pyroxene and Ca-plagioclase be observed in basalt.
(4) A parasitic cinder cone found near Mt. St. Helens produced a vesicular rock, black color, and no visible minerals. This rock would most likely be termed as a Obsidian.
Obsidian...even the name is exotic. Sharp and shiny, obsidian is so different from other rocks. But until a few years ago when I made my first obsidian collecting trip to Glass Buttes, Oregon, I thought obsidian was pretty much just black glass. That amazing trip really opened my eyes. The ancient volcanic hills called Glass Buttes hold a dazzling variety of gem-quality obsidian, including: mahogany, red, flame, midnight lace, jet black, pumpkin, brown, rainbow, gold sheen, silver sheen, green, lizard skin, snowflake and more. My goal in this article is to increase your awareness of some of the more fascinating aspects of this incredible stone.
Obsidian consists of about 70 percent or more non-crystallized silica (silicon dioxide). It is chemically similar to granite and rhyolite, which also were originally molten. Because obsidian is not comprised of mineral crystals, technically obsidian is not a true "rock." It is really a congealed liquid with minor amounts of microscopic mineral crystals and rock impurities. Obsidian is relatively soft with a typical hardness of 5 to 5.5 on the mineral hardness scale. In comparison, quartz (crystallized silicon dioxide) has a hardness of 7.0.
Obsidian occurs only where geologic processes create volcanoes and where the chemical composition of the magma is rich in silica. Obsidian-bearing volcanoes are typically located in or near areas of crustal instability or mountain building. In North America, obsidian is found only in localized areas of the West, where the processes of plate tectonics have created geologic conditions favorable to volcanism and the formation of obsidian. Obsidian typically forms near the end of a volcanic cycle and is often associated with domes of volcanic rock, such as the hills of Glass Buttes, Oregon.
(5) Except Obsidian all are Pyroclastic materials.
Pyroclastic rocks or pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic. Commonly associated with unsieved volcanic activity—such as Plinian or krakatoan eruption styles, or phreatomagmatic eruptions—pyroclastic deposits are commonly formed from airborne ash, lapilli and bombs or blocks ejected from the volcano itself, mixed in with shattered country rock.
Pyroclastic rocks may be a range of clast sizes, from the largest agglomerates, to very fine ashes and tuffs. Pyroclasts of different sizes are classified as volcanic bombs, lapilli, and volcanic ash. Ash is considered to be pyroclastic because it is a fine dust made up of volcanic rock. One of the most spectacular forms of pyroclastic deposit are the ignimbrites, deposits formed by the high-temperature gas-and-ash mix of a pyroclastic flow event.
(6) A Rhyolite magma composition would most likely involve pyroclastic.This statement is false because of pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic. Commonly associated with unsieved volcanic activity—such as Plinian or krakatoan eruption styles, or phreatomagmatic eruptions—pyroclastic deposits are commonly formed from airborne ash, lapilli and bombs or blocks ejected from the volcano itself, mixed in with shattered country rock.
Obsidian consists of about 70 percent or more non-crystallized silica (silicon dioxide). It is chemically similar to granite and rhyolite, which also were originally molten. Because obsidian is not comprised of mineral crystals, technically obsidian is not a true "rock." It is really a congealed liquid with minor amounts of microscopic mineral crystals and rock impurities. Obsidian is relatively soft with a typical hardness of 5 to 5.5 on the mineral hardness scale. In comparison, quartz (crystallized silicon dioxide) has a hardness of 7.0.
Obsidian occurs only where geologic processes create volcanoes and where the chemical composition of the magma is rich in silica. Obsidian-bearing volcanoes are typically located in or near areas of crustal instability or mountain building. In North America, obsidian is found only in localized areas of the West, where the processes of plate tectonics have created geologic conditions favorable to volcanism and the formation of obsidian. Obsidian typically forms near the end of a volcanic cycle and is often associated with domes of volcanic rock, such as the hills of Glass Buttes, Oregon.
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