Do Now

• How do volcanoes form? Where does vocanism occur near?

Chapter 18: Volcanism

• The locations of volcanoes are mostly determined by plate tectonics.

• Volcanism: al the processes associated with the discharge of magma, hot fluids, and gases.

Most volcanoes form at plate boundaries. The majority form at convergent boundaries and divergent boundaries.Circum Pacific = Ring of Fire

Convergent Volcanism • These

volcanoes are characterized by explosive eruptions and Felsic lavas (thick and chunky).

USA West Coast Volcanism

These volcanic mountains are the result of subduction between the Pacific Plate and the North American Plate.

Divergent Volcanism

• Eruptions at divergent boundaries tend to be nonexplosive and Mafic, or low in silicon.

• Divergent boundaries account for 2/3 of earth’s volcanism.

• Ex- Iceland on the Mid-Atlantic ridge.

A Hot Spot

• Some volcanoes form far from plate boundaries over hot spots.

• A Hot Spot is an unusually hot area in Earth’s mantle where high-temperature plumes of mantle material rise towards the surface

Resume 3-8-77

Hawaiian Hot Spot

• The Hawaiian Islands are located over a plume of magma.

• The hop spot formed by the magma plume remained stationary while the Pacific Plate slowly moved northwest.

• The world’s most active volcano, Kilauea, is currently located over the hot spot.

Flood basalts

• Formed when lava flows out of long cracks in Earth’s crust: Cracks called fissures.

• The Columbia River basalts, located in the northwestern United States, were formed this way.

• About 65 m.y.a in India, a huge flood of lava created an enormous plateau called the Deccan Traps.

• The volume of basalt in the Deccan Traps is estimated to be about 512, 000 km3.

Anatomy of a Volcano

• Lava reaches the surface by traveling through a tubelike structure called a conduit.

• The lava emerges through an opening called a vent

• A Magma chamber

Resume 12 23 2010

• Over time, layers of solidified lava can accumulate to form a mountain known as a volcano.

• At the top of a volcano, around the vent, is a bowl-shaped depression called a crater.

• Volcanic craters are usually less than 1 km in diameter.

• Larger depressions are called calderas. These can be up to 50 km in diameter or more: Yellowstone.

Resume 3-10-11

Diamond Head: Crater

Types of Volcanoes

Shield Volcano • A shield volcano is a mountain with broad, gently sloping sides and a nearly circular base.

• Form when layers of lava accumulate during nonexplosive eruptions and are the largest type of volcano.

Cinder Cones• When eruptions eject

small pieces of magma into the air, cinder cones form as this material, called tephra, falls back to earth and piles up around the vent: ash.

• Cinder comes have steep sides and are the smallest type of volcano.

Composite Volcano or Stratovolcano

• Formed of layers of hardened chunks of lava from violent eruptions alternating with layers of lava that oozed downslope before solidifying.

• Generally cone-shaped with concave slopes.

Two Major Belts

• The volcanoes associated with convergent plate boundaries for two major belts.

• 1 the larger belt, the Circum – Pacific Belt, is also called the Pacific Ring of Fire. The outline of the belt corresponds to the outline of the Pacific Plate.

• 2 the smaller is the Mediterranean Belt. Its general outlines correspond to the boundaries between the Eurasian, African and Arabian plates.

Volcanic Dome

• Commonly occurs adjacent to craters of composite volcanoes.

• Very thick lava, but small• Can be very explosive: a plug• Generally forms from side vents off the

main conduit.

A Caldera

• Formed after a major eruption

• Magma chamber releases most of its pressure and magma

• Overhanging ceiling cant support weight

• Ceiling collapses into vacant chamber.

Do Now

• Where would you find volcanoes with explosive eruptions?

• At what plate boundary would you find the majority of the earth’s volcanism?

• What plate is Hawaii on?• What is the scientific name for the ring of

fire?• In your opinion, are flood basalts

volcanoes?

18.2 ERUPTIONS:

Thar she blows!

• OBJECTIVES• Explain how magma

type influences volcanic activity.

• Describe the role pressure and dissolved gases in eruptions.

• Recognize classifications of material ejected by eruptions.

COMPOSITION DETERMINES CHARACTERISTICS

• The composition of magma determines the characteristics of a volcanic eruption.

• Basaltic: also Mafic: relates to a group of rocks rich in dark-colored minerals containing magnesium and iron: generally low silicon content.

• Felsic or Silicic: relates to a group of rocks low in dark-colored minerals containing magnesium and iron: generally High silicon content.

Making Magma

• The activity of the volcano depends on the composition of the magma.

• Lava from an eruption can be thin and runny (mafic) or thick and lumpy (felsic).

• The composition of magma determines a volcano’s explosivity, which is how it erupts and how its lava flows.

• Understanding the factors that determine the behavior of magma can aid scientists in predicting the explosivity of volcanic eruptions.

Temperature

• Depending on their composition, most rocks begin to melt at temperatures between 800°C and 1200°C.

• In addition to temperature, pressure and the presence of water also affect the formation of magma.

• Temperature keeps magma movable.

Pressure

• Pressure increases with depth because of the weight of overlying rock.

• As pressure increases, the temperature at which a substance melts also increases, which explains why most of the rocks in Earth’s lower crust and upper mantle do not melt.

Dissolved Gases

• In general, as the amount of gases in magma increases, the magma’s explosivity also increases.

• Important gases in magma include water vapor, carbon dioxide, sulfur dioxide, and hydrogen sulfide.

• Minerals in the mantle, such as albite, melt at high temperatures.

• The presence of dissolved water vapor lowers the melting temperatures of minerals, causing mantle material to melt into magma.

• Hence, although pressure prevents materials from melting, the presence of water helps materials to melt.

Composition of Magma

• The physical property that describes amaterial’s resistance to flow is called viscosity.

• Temperature and silica content affect the viscosity of magma.

• The silica content of magma determines not only its explosivity and viscosity, but also which type of volcanic rock it forms as lava cools.

Types of Magma: Basaltic

• When rock in the upper mantle melts, basaltic magma typically forms.

• Basaltic magma contains less than 50% silica which produces low-viscosity magma.

• The resulting volcano is characterized by quiet eruptions (relatively speaking).

Pahoehoe of Hawaii

Vesuvius and a Pahoehoe flow of late 1800”s

Andesitic Magma

• 50% to 60% silica and is found along oceanic – continental subduction zones.

• The source material for this magma can be either oceanic crust of oceanic sediments.

• The Andes Mountains of South America• The higher silica content results in a

magma that has intermediate viscosity• Intermediate explosivity.

Rhyolitic Magma

• When molten material rises and mixes with the overlapping continental crust rich in silica and water, it forms rhyolitic magma.

• Contains more than 60% silica: Silicic• High viscosity, along with the large volume

of gas trapped, makes the volcanoes fueled by this magma explosive: makes plugs.

The Eruption

• As magma rises due to plate tectonics and hot spots, it mixes with Earth’s crust.

• This mixing caused differences in temperature, silica content, and gas content as it reaches the Earth’s surface.

These properties of

magma determine how

volcanoes erupt.

Explosive Eruptions

• When lava is too viscous to flow freely from the vent, pressure builds up in the lava until the volcano explodes, throwing lava and rock into the air.

• The erupted materials are called tephra.• This includes Hollywood’s ash.• But this is not all.

Ejecta• Tephra are classified by

size.• The smallest fragments,

with diameters less than 2mm, are called ash.

• The largest tephra thrown from a volcano are called blocks.

• Large explosive eruptions can disperse tephra over much of the planet.

• In 1991, the eruption of Mount Pinatubo in the Philippines sent so much ash into the stratosphere that it lowered global temperatures for two years.

Resume 3-16-11

Pyroclastic Flows• Violent volcanic

eruptions can send clouds of ash and other tephra down a slope at speeds of nearly 200 km/hr.

• Rapidly moving clouds of tephra mixed with hot, suffocating gases are called pyroclasitc flows.

Eruption Column

Eruption Column

• They occur when ash and material from the volcano is light enough to be suspended in the air like smoke.

• Once the column is either cooled or more material is in the air than can be suspended, the column will collapse.

• This results in the material flowing down the sides of the volcano like water.

• In 1902, a pyroclastic flow from Mount Pelee on the island of Martinique in the Caribbean Sea was so powerful that it destroyed the entire town of St. Pierre in only a few Minutes.

• In 79 A.D. the towns of Pompeii and Herculaneum were covered by many such flows.

Pompeii revisited after 1800 years

18.3 Intrusive Activity

Resume 3-3-10

18.3 Intrusive Activity

Objectives

• Contrast and compare features formed from magma the solidifies near the surface with those that solidify deep underground.

• Classify the different types of intrusive rock bodies.

• Describe how geologic processes result in intrusive rock that appear at Earth’s surface.

• Magma that solidifies below ground forms geologic features different from those formed by magma that cools at the surface.

Plutons• Most of Earth’s volcanism happens below the

surface because not all magma emerges at the surface.

• Before it gets to the surface, rising magma can interact with crust in several ways

Plutons

• Magma can force the overlying rock apart and enter the newly formed fissures

• Magma can cause blocks of rock to break off and sink into the magma.

• It can melt its way through the rock into which it intrudes

• Plutons are intrusive igneous rock bodies, formed through mountain-building processes and oceanic-oceanic collisions.

• They can be exposed due to uplift and erosion and are classified based on their size, shape, and relationship to surrounding rocks.

Batholiths

• The largest plutons, are irregularly shaped masses of coarse-grained igneous rocks that cover at least 100 km2 and take millions of years to form.

• They are common in the interior of mountains.

Stock

• Irregularly shaped plutons that are similar to batholiths but smaller in size are called stocks.

• Both batholiths and stocks cut across older rocks and generally form 5 to 30 km beneath the Earth’s surface.

Laccoliths

• A lens-shaped pluton with a rounded top and a flat bottom.

• Compared to batholiths and stocks, laccoliths are relatively small; at most, they are 16 km wide.

Sill and Dyke

• A sill forms when magma intrudes parallel to the layers of rock.

• Because it takes great amounts of force to lift entire layers of rock, most sills form relatively close to the surface

• A dyke is a pluton that cuts across preexisting rocks and often forms when magma invades cracks is surrounding rock bodies.

• A volcanic neck occurs when the magma in a volcano conduit solidifies.

• Dykes are often associated with the conduit but do not always form the neck.

• Coarse grained texture means they formed deep: magma cooled slowly for large grains.

• Fine grained texture formed closer to the surface with fast crystallization.

Plutons and Tectonics

• Many plutons from as a result of mountain –building processes.

• In fact, batholiths are found at the cores of many of Earth’s mountain ranges.

• Scientists think that some of the collisions along continental – continental convergent plate boundaries might have forced continental crusts down into the upper mantle where it melted, intruding into the overlying rocks and eventurally cooled to form batholiths

• Plutons are also thought to form as a result of oceanic plate convergence.

• When an oceanic plate converges with another plate, water from the subducted plate causes the overlying mantel to melt.

• Plutons often form when the melted material rises but does not erupt at the surface.

Practice Questions

1. What is magma called when it reaches Earth’s surface?

a. Plutonb. Calderac. Tephrad. Lava

2. Which type of volcano usually produces the most violent eruptions?

a. Shieldb. Underwaterc. Cinder coned. Composite

3. Where are most of Earth’s active volcanoes?

a. Circum-Pacific Beltb. East African Riftc. Mediterranean Beltd. Cascade Range

4. Where do volcanic eruptions tend to be nonexplosive?

a. Over hot spotsb. At divergent boundariesc. In the upper mantled. At convergent boundaries

5. What is a pyroclastic flow?___________________________________

________________________________________________________________________________________________________________________________

6. Where do most volcanoes form?a. Over hot spotsb. In the Pacific Oceanc. At plate boundariesd. In mountain ranges

7. Materials erupted explosively from volcanoes is called…

a. Tephrab. Rhyolitec. Andesited. Basalt

8. What kind of volcanism occurs from a subduction zone: oceanic-continental boundary?

A. DivergentB. ConvergentC. Hot SpotD. Flood Basalt

9. What effect does the viscosity of a volcano’s magma have on how it erupts?

___________________________________________________________________________________________________________________________________

10.Which structure connects a volacno’s crater to its magma chamber?

A. cinder coneB. Lava tubeC. VentD. Conduit

Questions

1. D. Lava2. D. Composite3. A. Circum-Pacific Belt4. At divergent boundaries5. A could of tephra mixed

with hot, suffocating gases moving rapidly down the side of a volcano

6. C. At plate boundaries

7. A. Tephra8. B. Convergent9. Higher viscosity magmas

produce more explosive eruptions.

10.D. Conduit