Guided Notes on Volcanoes Section 18.1 Magma. 1.In the last 10,000 years, more than 1500 different volcanoes have erupted, providing evidence that the.

Guided Notes on Volcanoes

Section 18.1Magma

1. In the last 10,000 years, more than 1500 different volcanoes have erupted, providing evidence that the Earth is indeed geologically active.

2. All volcanoes are fueled by magma deep beneath the Earth’s surface.

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.

Basaltic Magma

• 50% silica• Low viscosity• Least violent eruptions• Low amounts of trapped gases• Occur in Hawaii, Iceland, and areas

with oceanic crust

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

Rhyolitic Magma

• 70% silica• High viscosity• Usually have violent eruptions• Occur in areas with continental

crust• An example is Yellowstone Park

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.

Section 18.2

Intrusive Activity

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.

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

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.

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

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

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

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

Types of Plutons

Stocks• Similar to batholiths, but smaller

than 100 km2

5. What texture do most plutons have, and why?

• Most plutons are coarse-grained because they have cooled slowly under the earth’s surface.

6. How are plutons usually formed?

• Plutons are usually formed as a result of mountain-building processes.

2 Types of Convergence that Create Batholiths

• Continental-Continental: creates chains of mountains

• Oceanic-Oceanic: create batholiths that are later uplifted and eroded

Section 18.3

Guided Notes about Volcanoes

How Volcanoes Form

Step 1: magma chambers deep within the earth fuel volcanoes.

How Volcanoes Form

Step 2: magma that reaches the surface is called lava. Lava erupts through an opening called a vent.

How Volcanoes Form

Step 3: Lava cools and solidifies around the vent

How Volcanoes Form

Step 4: over time, lava accumulates to form a mountain called a volcano

How Volcanoes Form

Step 5: a crater forms at the top of the volcano near the vent.

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

3. The appearance of a volcano depends upon:

• The type of material that forms the volcano

• The type of eruptions that occur

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.

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

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

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.

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.

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.

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.

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

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.

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.

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

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

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.