Download Volcanic Activity and Formation: The Case of Mount St. Helens and more Assignments Logic in PDF only on Docsity!
U.S. Department of the Interior U.S. Geological Survey
clIVAr t IWO THE Eli II
tii
fig. 1
Inside the Earth
Inner Core (solid)
Outer Core (liquid)
Mantle (solid)
Upper Mantle (soft)
Crust (hard )
Magma is generated in the Earth's lithosphere, which is made up of the crust and upper mantle.
Until the spring of 1980, most people thought of Mount St. Helens as a serene, snow-capped mountain and not as a lethal volcano. The mountain had given little evidence that it posed a hazard for more than a century—a long time in human terms but a blink of an eye in terms of the mountain's 40,000-year geo- logic history. A series of earthquakes that began in mid-March of 1980 sound- ed the alarm that Mount St. Helens was awakening from its sleep. In other words, Mount St. Helens, which had
been dormant, (^) became active and likely to erupt. Its catastrophic eruption 2 months later was a reminder that a fiery world lies beneath the Earth's surface.
Why Volcanoes Occur The roots of Mount St. Helens are 110 to 330 kilometers (70 to 200 miles) below the Earth's surface. Here in the Earth's mantle (fig. 1) temperatures arc hot enough to melt rock and form a thick, flowing substance called magma. Lighter than the solid rock that surrounds it,
LESSON 1
magma is buoyant much like a cork in water; being buoyant, it rises. As the magma rises, some of it col- lects in large reservoirs, or magma cham- bers (poster fig. I) that fuel volcanoes. As the rising magma nears the Earth's sur- face, pressure decreases, which causes the gases in the magma to expand. This expansion propels the magma through openings in the Earth's surface: a vol- canic eruption occurs. Once magma is erupted, it is called lava.
Where Volcanoes Occur Volcanic eruptions occur only in certain places and do not occur randomly. That's because the Earth's outermost shell— the lithosphere—is broken into a series of slabs known as lithospheric or tectonic plates. These plates are rigid, but they float on the hotter, softer layer in the Earth's mantle. (poster fig. 2) As the plates move about, they spread apart, collide, or slide past each other. Volcanoes occur most frequently at plate boundaries. Some volcanoes, like those that form the Hawaiian Islands, occur in the interi- or of plates at areas called hot spots. (poster fig. 2) Although most of the active volcanoes we see on land occur where plates collide, the greatest number of the Earth's volcanoes are hidden from view, occurring on the ocean floor along spreading ridges. Mount St. Helens is typical of more than 80 percent of the volcanoes that have formed on land. Known as subduc- tion zone volcanoes, they occur along the edges of continents where one plate
fig. 2
Inside the Earth
Mount St. Helens
North American Plate
Magma
Juan De Fuca Plate
About 240 kilometers (150 miles) west of the northwest coast of the United States, the Juan de Fuca Plate plunges beneath the North American Plate. Mount St. Helens is among the volcanoes that have formed as a result.
dives, or subducts, beneath a second plate. (fig. 2). When the subducting plate reaches about 100 kilometers (60 miles) into the Earth's hot mantle, it triggers partial melting of the overlying plate and forms new magma. Some of the magma rises and erupts as volcanoes.
Why Some Volcanoes Erupt Sonic volcanoes, like Mount St. Helens, tend to be explosive when they erupt, whereas others, like Hawaii's Kilauea, tend to be effusive (loosely flowing) and nonexplosive. How explosive an eruption is depends on the magma's chemical composition and gas content, which in turn affect the magma's stickiness, or vis- cosity. All magma contains gases that escape as the magma travels to the Earth's sur- face. If magma is fluid (as is Kilauea's), gases can escape relatively rapidly. As a result, lava flows instead of exploding during an eruption. If magma is viscous
(as is Mount St. Helens), the gases can- not escape easily; pressure builds inside the magma until the gases sometimes escape violently. In an explosive eruption, the sudden expansion of gases blasts magma into air- borne fragments called tephra, which can range in size from fine particles of ash to giant boulders. After the initial explosive phase of the eruption, however, quieter lava flows can follow. In both explosive and nonexplosive (effusive) eruptions, volcanic gases, including water vapor, are released into the atmosphere.
Three Types of Volcanoes Repeated volcanic eruptions build vol- canic mountains of three basic types, or shapes, depending on the nature of the materials deposited by the eruption. Shield volcanoes (poster fig. 5), such as Kilauea, form by effusive eruptions of fluid lava. Lava flow upon lava flow slowly builds a broad, gently sloping vol-
canic shape that resembles a warrior's shield. Stratovolcanoes (poster fig.3), such as Mount St. Helens, build from both explo- sive and effusive eruptions. Layers of tephra alternating with layers of viscous lava flows create steep-sided, often sym- metrical cones that we think of as the classic volcano shape. In his log of the Lewis and Clark Expedition, William Clark wrote: "Mount St. Helens is per- haps the greatest pinnacle in America." The smallest volcanoes, cinder cones (poster fig. 4), such as Sunset Crater in Arizona, form primarily from explosive eruptions of lava. Blown violently into the air, the erupting lava breaks apart into fragments called cinders. The fallen cinders accumulate into a cone around the volcano's central vent. Cinder cones can form on the flanks of shield and stratovolcanoes.
Activity 2 Ring of rim Actlrlty Sheet 2
30 minutes Students locate some of the 1,500 active volcanoes on a world map. Then by com- paring their maps with a map of the world's tectonic plates, they discover that volcanoes occur because of the dynamic nature of the Earth's lithosphere-the crust and upper mantle.
Key teaching points
- Volcanoes are windows into how the Earth works. They occur because the Earth's rigid outer shell, the crust and upper mantle, is broken into a mosaic of plates that are in constant motion.
- Most volcanoes occur along the boundaries of the Earth's tectonic plates.
- More than half of the volcanoes that are exposed on land form a chain along the converging plates that encircle the Pacific Ocean. This chain is called the "Ring of Fire."
- Mount St. Helens is located in the "Ring of Fire."
Materials
- A photocopy of Activity Sheets 1. and 1.2b for each student
- Master Sheets 1.4 and 1.
- Two 8 1 / 2 " x 11" transparencies for overhead projection
- Colored pencils
Procedures
Preparation
- Make transparencies of Master Sheets 1.4-1.5.
- (^) Distribute Activity Sheets 1.2a-1.2b and ask each student to read the Activity Sheets.
Discussion
- After the students have completed their maps, lead a discussion. Are there volcanoes on every continent? (yes) How many of the volcanoes on their map are located within the shaded area, "The Ring of Fire?" (14)
plate outlines. Tell students that the outer layer of the Earth is broken into a series of 16 major plates and that the colored lines indicate the boundaries between these plates.
- Next, superimpose the transparency of Master Sheet 1.4, "Volcanoes Map," on Master Sheet 1.5, the "Plate Tectonics Map."
- Ask students to observe the location of the volcanoes in relationship to the plates. Where do most of the volcanoes occur? They should observe that most occur near plate boundaries. Is the "Ring of Fire" located near plate boundaries?
- Use (poster fig. 2) to explain that rigid plates float on top of a softer layer of rock. As the plates move, they push together, pull apart, or slide past each other. Along two plate boundaries, magma comes to the surface and volca- noes can occur.
- Explain that there are some volcanoes that occur in the interior of plates and not at plate boundaries. They occur over "hot spots" in the plates. Scientists do not know exactly why hot spots develop, but hot spots are places in the Earth's interior from which magma rises and erupts through the plate as it moves over the hot spot. Ask students which U.S. State is composed primarily of shield volcanoes? (Hawaii)
- Is Mount St. Helens located in the "Ring of Fire"? (Yes)
- Yes
- 66.6%
- 33.3%
- 14 stratovolcanoes; 1 shield; 8 cindercone
- Stratovolcano
- Answers will vary
Ia.
- Show the class the transparency of Master Sheet 1.5, the "Plate Tectonics Map." With a colored marker, trace the
VOLCANOES!
There are more than 1,500 active volcanoes in the world. An active volcano is one that has erupted at least once in the past 10,000 years and is likely to erupt again. Because most of the Earth's volcanoes are hidden under the oceans, people have not been able to witness their eruptions. Every
What to do
Locate and label each of the volcanoes listed on the blank map. Use a different colored marker for stratovolcano, shield, and cinder cone volcanoes.
- Are most of the volcanoes located in the Ring of Fire?
- What percentage of the volcanoes are located in the Ring of Fire? To find out use the following formula:
in shaded area
total # x 100 =^ % of volcanoes in the Ring of Fire
3.What percentage of the volcanoes are located outside of the Ring of Fire? To find out use the following formula:
not in shaded area
total # x 100 =^ % outside of volcanoes in the Ring of Fire
4.Types of volcanoes in the Ring of Fire
of stratovolcanoes
of shield volcanoes
of cinder cones
- What type of volcano is most common in the Ring of Fire?
- How many of the volcanoes listed have erupted since you were born?
Activity Sheet 1.2a
The "Ring of Fire"
year, about 50-60 volcanoes erupt on land where people might be able to see them. Scientists estimate that there are about 200 volcanic eruptions under the oceans. The shaded area on your map is called the "Ring of Fire." Do the exercise below and you will discover why.
This is a list of some active, or recently active, volcanoes.
Name Type Last Erupted
1 Azul Stratovolcano 1967 2 Bezymianmy Stratovolcano 1993 3 Cerro Negro Cinder cone 1971 4 Cotopaxi Stratovolcano 1942 5 Erebus Stratovolcano 1980 6 Katmai Stratovolcano 1912 7 Kilauea Shield 1995 (^8) Krakatau Stratovolcano 1894 9 Ksudach Shield 1907 10 La Palma Stratovolcano 1954 11 Lassen Peak Stratovolcano 1914 12 Mt. Etna Shield 1993 13 Mt. Fuji Stratovolcano 1709 14 Mt. Pelee Stratovolcano 1932 15 Mt. Rainier Stratovolcano 1894 16 Mount St. Helens Stratovolcano 1991 17 Nevada del Ruiz Stratovolcano 1991 18 OlDoinyo Lengai Stratovolcano 1993 19 Paricutin Cinder cone 1952 20 Pinatubo Stratovolcano 1992 21 Sunset Crater Cinder cone 1065 22 Surtsey Shield 1967 23 Tambora Stratovolcano 1967 24 Vesuvius Stratovolcano 1944
This volcano erupts explosively. It is built from layers of tephra and layers of lava.
Label the following:
Vent Crater Lava flow layer
Lava flow Tephra
Ma
s
ter
Shee
t^
1.
1
This volcano erupts explosively. Lava blasted into the air breaks into small pieces called cinders.
Label the following:
Vent Crater Cinders
Cinders
\
\
/
/
/
Mas
te
r
S
he
e
t 1.
2
ai forth America11; Mt. Rainier
Sunset Crater
Africa
001 Doinya Lenga
Pidtubo
t. Pelee
Nevada del Ruiz
Mas
te
r
S
hee
t 1.
4
Antarctica
Eurasian Plate North American Plate
Caribbean Plate
African Plate
Iri t•
CD CD = C/ as
CI)
CC
E
Antarctic Plate
Antarctic Plate
