guided notes on volcanoes section 18.1 magma. 1.in the last 10,000 years, more than 1500 different...
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Guided Notes on Volcanoes
Section 18.1Magma
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1. In the last 10,000 years, more than 1500 different volcanoes have erupted, providing evidence that the Earth is indeed geologically active.
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2. All volcanoes are fueled by magma deep beneath the Earth’s surface.
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3. What is the asthenosphere?
• The asthenosphere is the plasticlike portion of the mantle directly beneath the lithosphere. It is hot enough to contain molten magma.
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Basaltic Magma
• 50% silica• Low viscosity• Least violent eruptions• Low amounts of trapped gases• Occur in Hawaii, Iceland, and areas
with oceanic crust
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Andesitic Magma
• 60% silica• Intermediate viscosity• Some violent eruptions, some
quiet ones• Are found along subduction zones• An example is Mt. St. Helens, in
Washington
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Rhyolitic Magma
• 70% silica• High viscosity• Usually have violent eruptions• Occur in areas with continental
crust• An example is Yellowstone Park
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5. The viscosity of magma depends upon its temperature and composition. The hotter the magma, the lower the viscosity. Magmas high in silica have higher viscosities than magmas low in silica.
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Section 18.2
Intrusive Activity
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1. Why does magma come into contact with overlying rock?
• Magma is less dense, so it moves upward where it eventually comes in contact with overlying rock.
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3 Ways That Intruding Magma Can Affect the Crust
• It can force the rock apart and enter the newly formed fissures
• It can cause blocks of rock to break off and sink into the magma, where they melt
• It can melt the rock into which it intrudes
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Define Pluton
• A pluton is an intrusive igneous rock body formed through mountain-building processes and oceanic-oceanic collisions. It can be exposed at the Earth’s surface due to uplift and erosion.
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Types of Plutons
Sills• When magma intrudes into parallel
rock layers• Between a few centimeters and
hundreds of meters thick• An example is the Palisades Sill in
New York
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Types of Plutons
Dikes• When magma cuts across pre-
existing rock layers, magma invades the cracks in rock
• Usually 10 km. long and a few meters wide
• An example is the Great Dike in Zimbabwe
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Types of Plutons
Laccoliths• Mushroom-shaped plutons with
rounded tops and flat bottoms• Less than 16 km. wide• An example is the Black Hills of
South Dakota
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Types of Plutons
Batholiths• Large plutons that are at least 100
km2
• Irregularly shaped• Coarse-grained• An example is the Coast Range
Batholith in British Columbia
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Types of Plutons
Stocks• Similar to batholiths, but smaller
than 100 km2
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5. What texture do most plutons have, and why?
• Most plutons are coarse-grained because they have cooled slowly under the earth’s surface.
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6. How are plutons usually formed?
• Plutons are usually formed as a result of mountain-building processes.
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2 Types of Convergence that Create Batholiths
• Continental-Continental: creates chains of mountains
• Oceanic-Oceanic: create batholiths that are later uplifted and eroded
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Section 18.3
Guided Notes about Volcanoes
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How Volcanoes Form
Step 1: magma chambers deep within the earth fuel volcanoes.
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How Volcanoes Form
Step 2: magma that reaches the surface is called lava. Lava erupts through an opening called a vent.
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How Volcanoes Form
Step 3: Lava cools and solidifies around the vent
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How Volcanoes Form
Step 4: over time, lava accumulates to form a mountain called a volcano
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How Volcanoes Form
Step 5: a crater forms at the top of the volcano near the vent.
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2. What is a caldera?
• A caldera is a large volcanic crater that forms when the summit or side of a volcano collapses into the magma chamber of the volcano
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3. The appearance of a volcano depends upon:
• The type of material that forms the volcano
• The type of eruptions that occur
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The 3 Types of Volcanoes
Cinder-Cone• Forms when tephra is ejected high
in the air, falls back to earth, and piles up around the vent
• They are usually small, with steep sides
• They have viscous magma, high in silica and trapped gases.
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The 3 Types of Volcanoes
Composite• Forms when layers of volcanic
fragments alternate with lava• The magma contains high amounts
of water, with moderate levels of silica and trapped gases
• Are larger than cinder-cone volcanoes
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The 3 Types of Volcanoes
Shield• They have broad, gently sloping
sides and a nearly circular base• They are made when basaltic lava
accumulates during quiet eruptions
• They are the largest volcanoes
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5. What is tephra?
• Tephra are rock fragments thrown into the air during a volcanic eruption.
• Tephra can be newly cooled and hardened lava, mineral grains that have crystallized, or pieces of the volcanic cone.
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The 5 Classifications for Tephra
• Dust: less than .25 mm in diameter• Ash: between .25 and 2 mm in diameter• Lapilli: between 2 and 64 mm in
diameter• Blocks: angular, greater than 64 mm.• Bombs: rounded, greater than 64 mm.
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7. Differences between blocks and bombs
• Blocks are angular fragments of lava. Bombs are blobs of lava that are squeezed out of a vent and form a rounded, streamlined shape.
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8. Describe a pyroclastic flow.
• Clouds of gas, ash, and other tephra that move down a volcanic slope at tremendous speeds is a pyroclastic flow.
• Speeds can be 200 km. per hour and temperatures can be 700 degrees Celcius.
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9. Worldwide Distribution of Volcanoes
• 80% of volcanoes are found at convergent boundaries
• 15% are found at divergent boundaries
• Only 5% are found away from plate boundaries
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10. How does convergence lead to the formation of a volcano?
• Convergence causes the crust to descend into the mantle and melt. The magma generated is forced upward and forms volcanoes when it reaches the surface.
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11. How does divergence lead to the formation of a volcano?
• Magma rises into the fractures and cracks formed when two plates separate. Usually this occurs at ocean ridges.
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12. What is a hot spot?
• A hot spot is an unusually hot region of the mantle, where plumes of magma rise to the surface, creating volcanoes
• An example is Hawaii
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13. How do hot spots create volcanoes?
• The intense heat of the plumes melt crustal rock, which is forced through a vent to form volcanoes
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14.How do the chains of volcanoes form over hot spots?
• As the earth’s plates move across hot spots, they create chains of volcanoes
• The rate and direction of motion can be calculated from the positions of volcanoes formed.