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Unit 3: Earth’s Dynamic Geosphere
Section 2: Volcanoes
In 1883, on the island of Krakatoa in the East Indies, one of the most violent eruptions of
recorded time took place. Half of the island was blown away by a volcanic eruption. Over a
cubic mile of rock was hurled into the air. The sound of the explosion was heard in
Australia, over 2000 miles away! Volcanoes are one of nature’s most feared, yet
spectacular events. Many motion pictures are based on exciting geologic events such as
this. On average, about 60 of Earth’s 550 historically active volcanoes erupt each year.
Volcano – an opening in the Earth’s crust that releases molten rock and gases.
The Global Distribution of Volcanoes (G7)
Geologists have long known that volcanoes are abundant along the edges of certain
continents. However, the presence of volcanic rocks on the floors of all ocean basins
indicates that volcanoes are far more abundant under water than on land.
1. Volcanoes beneath the Sea
All of Earth’s ocean basins have a continuous mountain range, called a mid-ocean ridge
extending through them. Magma (molten rock) from deep in the Earth rises up into the rift
valley to form submarine volcanoes.
2. Volcanoes on Land
Volcanoes that erupt on land are much more dangerous than volcanoes beneath the
ocean. Eruptions along the western edge of the United States have formed the Cascades
volcanic mountain range. They also form island chains, like the Aleutians in Alaska.
Volcanoes like these form a narrow belt all around the Pacific Ocean called the “Ring of
Fire”.
The Ring of Fire
Around the edges of the Pacific Ocean, the plates of the slide down beneath the
continents. As a plate is subducted, it becomes heated as it sinks into much hotter rocks
of the deep Earth. The heat causes fluids, especially water, to leave the plate and rise into
overlying hot rocks. The added water lowers the melting point of the solid rock and if
enough water is added the rock melts and magma is formed. Magma is less dense the
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rock around it so it rises to the surface forming a volcano. About 80% of all land volcanoes
form this way.
Volcanoes Formed by Rifting on the Continents
When two continents move apart, rift valleys are formed just like under the ocean. The
continental plates are stretched and broken during this process. Magma sometimes takes
advantage of this weakness in the crust and rises up.
Volcanoes at Hot Spots
A small percentage of volcanoes occur in
the interior of a plate. The Hawaiian
Islands are one such example. Only the
youngest island has an active volcano.
Beneath the ocean there is a fixed source
of rising magma called a hot spot. As the
Pacific Plate moves northwest, it passes
over the fixed hot spot. Magma from the hot
spot punches its way through the moving
plate to form a chain of islands. Seamounts
are submerged mountains that never broke
the surface of the water.
Map Projections
There is always a problem with drawing a map of the world, because you have to try to
show the curved surface on a flat sheet of paper. Multiple map projections have been
developed to show the Earth’s surface, however, they all have some sort of distortion.
The Mercator projection (like the one showed above) becomes more distorted as you
move further away from the equator. The larger the area, the less detail we can show so
some volcanoes seem to overlap.
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Define the following terms using your textbook.
Magma – Naturally occurring molten rock material generated within the Earth.
Lava – Molten rock that issues from a volcano or fissure.
Hot Spot – A fixed source of abundant rising magma that forms a volcanic center that has
persisted for tens of millions of years.
Map Projections – The process of systematically transforming positions on the Earth’s
spherical surface to a flat map while maintaining spatial relationships.
Mercator Projection – A map projection in which the Equator is represented by a straight
line true to scale, and as you move further away from the Equator, more distortion occurs.
Seamount – A peaked or flat-topped underwater mountain rising from the ocean floor.
Review Questions
1. What evidence do geologists have that volcanoes occur on the ocean floor?
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2. What is the Ring of Fire and where is it located?
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3. Where do most volcanoes on land form?
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4. What are hot spots? Provide one example of a hot spot.
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5. What is the problem with flat maps?
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6. Where on Earth do most volcanoes occur? __________________________________
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When most people think about volcanoes, they probably have in mind a steep-sided cone.
Many volcanoes, however, have very gentle slopes and some look like craters.
Topography of Volcanic Regions (G17)
Topographic maps have contour lines. These are curves that connect all points at the
same elevation. The contour interval is the difference in elevation between adjacent
contour lines. A topographic map shows how steep or gentle a slope is. It also shows the
elevation and shape of the land. Relief is the difference in elevation between the highest
and lowest points on the map.
Important Points to Remember when Interpreting
Topographic Maps
 Contour lines never cross, but two or more can
run together, where there is a vertical cliff.
 The closer together the contour lines, the steeper
the slope.
 Contour lines for closed depressions, such as a
volcanic crater, are marked with “tick marks”
pointing downslope, into the depression.
 On most topographic maps, every fifth contour
line on a map is darker and its elevation is always
marked.
Magma Composition
Volcanoes come in many shapes and sizes. The forces of nature can change the shape of
a volcano, both between eruptions and after the volcano becomes dormant. The chemical
composition of magma can have an even greater effect on the shape the volcano takes as
it forms.
Magma is a mixture of liquid, melted rock, and dissolved gasses. The most common
elements in magma are silicon and oxygen which together are called silica. As the magma
cools it combines with other elements to form silicate minerals.
Rock that forms when molten materials become solid are called igneous rock. Magmas
that are rich in silica do not flow as easily as magmas that are poor in silica. Therefore,
silica-rich magmas are more likely to stay underground at shallow depths, but are more
explosive. Dissolved gasses cannot escape slowly, so they continue to build until an
explosion becomes necessary to release the pressure.
Viscosity – the property of a fluid to offer internal resistance to flow or a measurement of
how much a particular substance behaves like a fluid.
High viscosity = less fluid, low viscosity = more fluid.
Types of Volcanic Landforms
1. Shield Volcanoes – When low-silica magma erupts, lava tends to flow freely and far. It
erupts from a single opening (or vent) or closely spaced vents. Creates a gently sloping
hill.
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3. Dome Volcanoes – When silica-rich volcanoes erupt, they ooze slowly like toothpaste
squeezed out of a vertical tube. Sometimes the magma doesn’t even make it to the
surface. The stiff lava forms volcanic domes with steep slopes. If the volcano’s vent gets
plugged, gasses cannot escape and pressure builds. The pressure can be released in a
volcanic eruption that blasts pieces of lava and rock into the atmosphere. (pyroclastics)
A composite cone forms by many eruptions of material with medium or high-silica
content. They erupt violently when pressure builds up in the magma. After the
initial explosion, lots of gooey (viscous) lava oozes out of the top. The volcano
becomes quiet for a while, but pressure builds and repeats the cycle all over again.
4. Caldera Volcanoes – When a very large volume of magma is erupted, the overlying rock
may collapse all around the original vent. The collapse produces a hole or depression at
the surface called a caldera. A caldera is much larger that the original vent from which the
magma erupted. Sometimes these depressions collect water.
Define the following terms using your textbook.
Contour Lines –
Contour Interval –
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Topographic Map –
Relief –
Igneous Rock –
Silica –
Shield Volcano –
Composite Cone –
Caldera –
Review Questions
1. What is the purpose of a topographic map?
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2. Arrange the following substances from low viscosity to high viscosity: water, syrup,
vegetable oil.
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3. What is the silica content of magma with a low viscosity? (low or high content) _______
4. Why do silica-poor magmas produce broad volcanoes with gentle slopes?
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5. Why are silica-rich magmas more prone to explosive eruptions?
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6. How is a caldera formed?
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Only one person in the entire city of St. Pierre, on Martinique in the Caribbean, survived
the hot ash and rock fragments that swept over the city from the explosive eruption of Mt.
Pelée in 1902. He was a prisoner in a dungeon deep underground. Over 29,000 people
were reported dead.
Volcanic Hazards: Flows (G26)
Lava flows are streams of molten rock that come from vents and fissures in the Earth’s
crust. Lava flows destroy almost everything in their path. Fortunately, most lava flows
move slowly enough that people and animals can move out of the way.
The speed and overall distance covered by lava is influenced by the steepness of the
slope, the viscosity of the lava, and the temperature at which it leaves the volcano.
Factors Affecting Speed and Distance of Lava
Slope
Temperature
Silica Content
Increase in Factor
Decrease in Factor
+
+
-
+
It is sometimes possible to control the flow of lava. In 1973, lava flows threatened to cut
off a vital harbor at Heimaey, Iceland. Citizens sprayed water onto the lava from ships in
the harbor and stopped the flow. Lava flows can also be diverted away from populated
areas by carving a new channel or pathway through the landscape for the lava to flow.
Pyroclastic Flows
These types of flows occur in explosive eruptions and are composed of high-density
mixtures of hot ash and rock fragments with hot gases. They move away from the vent at
speeds of up to 350 km/hr. They often have two parts: a lower flow of coarse fragments
and a turbulent cloud of ash rising above. They are very dangerous and destroy
everything in their path.
Lahar
Lahar is a wet mixture of water, mud, and volcanic rock fragments, with the consistency of
wet concrete. Lahars often are filled with small rock fragments but can carry huge
boulders as well. Heat from eruptions may melt snow and ice, rain may fall, or flows
displace water from a mountain river adding water to the ash and other debris which then
act like a landslide that can cover whole towns.
Volcanic Hazards: Airborne Debris (G31)
Tephra is a term for pieces of volcanic rock and lava that are ejected into the air.
Particle types according to size:
Volcanic Bombs (greater than 64 mm)
Lapilli (between 2 and 64 mm)
Ash (less than 2 mm)
Bombs and lapilli usually fall close to the
volcano, but ash can travel hundreds to
thousands of kilometers. The height of
the ash and the wind speed control how
far the ash travels.
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Ash that falls on homes, factories, and schools
can collapse roofs. More than 300 people died
after the 1991 eruption of Mt. Pinatubo in the
Philippines and most of these deaths were
caused by roof failure. At ground level, fine ash
causes breathing problems in humans and
animals by coating the inside of the lungs and
throat and mixing into a cement-like material. It
can also damage automobile, truck, and plane
engines. Ash that coats the leaves of plants
interferes with photosynthesis.
Volcanic Explosivity Index
The Volcanic Explosivity Index or VEI is based in the volume of erupted material and the
height it reaches. This chart can be found on page G35 of your text. The size of the
eruption depends on several factors such as the composition of the magma and the
amount of gas dissolved in the magma. Just because the volcano is judged to be more
explosive does not necessarily mean that it is more dangerous.
For example, Tambora was measured to be a very explosive volcano (VEI of 7) and it
erupted in 1815 with tephra fall, a tsunami, and pyroclastic flows killing about 10,000
people. Later on, famine from the destroyed crops and shortened growing season killed
more than 82,000 people.
Define the following terms using your textbook.
Lava Flow –
Viscosity –
Pyroclastic Flow –
Lahar –
Tephra –
Volcanic Bomb –
Lapilli –
Ash –
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Review Questions
1. Name the 3 factors that influence the distance and speed of lava flows.
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2. Describe two ways in which lava flows can be controlled.
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3. Name two factors that can affect the distance that volcanic ash can flow.
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4. Why is ash so dangerous to inhale?
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5. What is the number one reason people die from a high amount of tephra fall?
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6. a. On the VEI of the deadliest eruptions since 1500 A.D. on page G36 of your text,
which eruption killed the most people? ____________________________
b. Which one was measured to be the most explosive? _________________________
Section 2 Vocabulary
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Mid-Ocean Ridge
Rift Valley
Magma
Hot Spot
Map Projections
Lava
Mercator Projections
Seamount
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Contour Lines
Contour Interval
Topographic Map
Relief
Igneous Rock
Silica
Shield Volcano
Composite Cone
Caldera
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Lava Flow
Viscosity
Pyroclastic Flow
Lahar
Tephra
Volcanic Bomb
Lapilli
Ash
Volcanic Explosivity Index
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