Physical Geology Slides-Volcanoes

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay  
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Cinder Cones

Small volcanoes are often just conical piles of loose rubble called cinder cones with interspersed lava flows. This cone is Wizard Island in Crater Lake, Oregon.

This is an unfamiliar view of a famous landmark. Most people are familiar with just the rightmost side of it. This is Diamond Head, near Honolulu. It got its name because 19th century sailors mistook olivine crystals weathering out of the volcanic rocks for diamonds.

Seen from the air, Diamond Head is a broad, low ring. This is the sort of cinder cone that forms from shallow submarine eruptions; the eruption spends most of its energy sloshing water around and the water deposits the fragmentary debris in a ring around the vent.

Shield Volcanoes

Shield volcanoes, like Haleakala, on the island of Maui, Hawaii, erupt basalt, a very fluid lava. They build immense, broad domes with gentle slopes. The foreground is nearly at sea level; the summit of the volcano is over 10,000 feet. Only the central third of the volcano is shown, and there's another 18,000 feet of it below sea level. Shield volcanoes are the largest volcanoes by any measure; height, volume, mass, or diameter.

Stratovolcanoes

Mount Shasta, in northern California, is a very typical stratovolcano. The scale in this picture is very similar to the preceding picture; the valley is at about 4,000 feet and the summit is at 14,000 feet. Stratovolcanoes erupt more viscous lava than shield volcanoes, and there is more fragmentary material because of the more violent eruptions. The cones tend to be smaller and steeper than shield volcanoes, though still pretty impressive. Note the rounded hills just visible in the middle distance. We'll see them from a different vantage point in the next picture. By the way, what are the v-shaped UFO's above the summit? They're not clouds - a small real cloud is also visible.

Answer: they are probably static electricity marks. It was a very hot, dry day when the picture was taken. Winding film under such conditions sometimes generates static electricity, tiny lightning bolts inside the camera.

Collapsing Volcanoes

This is taken from the summit of Mount Shasta, looking toward the place from which the previous photo was taken. Very old volcanoes frequently find it easier to break through the side of the cone, building a new vent often called a parasitic cone. Shasta's parasitic cone is called Shastina. At 12,000 feet it would be a major Cascade Range summit in its own right; it just looks small compared to Mount Shasta. (It's not obvious in the preceding picture because of the lighting and because it's directly in front of Mount Shasta).

Shastina has a well-formed crater, indicating it's been active recently. There is a possible report of an eruption of Shasta or Shastina in 1786 seen by a ship off the coast.

Note the rounded hills beyond Shastina. After Mount Saint Helens collapsed in 1980, geologists realized how easily stratovolcanoes can collapse and rapidly turned up other cases. The hills had been interpreted as old lava flows covered by glacial deposits, but are now considered to be the relics of a great collapse of Mount Shasta about 300,000 years ago. It is probably no accident that Shastina formed in the collapsed sector, since it would be very easy for lava to reach the surface there.

Volcanoes, despite their impressive size, are poorly-consolidated piles of lava flows interspersed with loose debris. In addition, they eat themselves away from within as acidic fluids weaken the rocks. Even shield volcanoes can collapse. More than 50 huge submarine landslides are recognized around Hawaii. They show up as bright in these sonar images. In sonar and radar images, bright means rough, since signals are reflected back at all angles. Smooth areas show up as dark since signals mostly bounce away from the source.

Calderas

Crater Lake, in Oregon, marks a different sort of volcanic collapse, a caldera. About 6,000 years ago, a huge eruption blasted out so much material that the volcano could no longer support its own weight and caved in. The pre-eruption volcano, called Mount Mazama, was probably about 12,000 feet high. It was high enough to support glaciers, because glacial deposits are found beneath Llao Rock, the cliff on the left side. The cliff actually marks a former glacial valley filled by a lava flow that also spread out on either side of the valley.

Although only six miles across, Crater Lake is 1,932 feet deep just outside the picture to the right. It is the deepest lake in the United States and the second-deepest in North America (Great Slave Lake in Canada is deeper). Wizard Island is visible to the left, but there are two more volcanoes beneath the surface of the lake. The lake has no outlet but drains by seepage through the walls.

Interestingly, the Indian legends about Crater Lake tell of a great war between Llao, who lived in Crater Lake, and an evil god Skell, who lived in Mount Shasta. The war devastated the countryside with fire. Is this an oral tradition going back to the actual eruption? Is it a story made up by people who were smart enough to figure things out long afterward?

The largest caldera in the United States is little-known except to geologists: the Jemez Caldera in northern New Mexico. It should be a national park but has long been private ranch land and closed to the public. In a excellent display of corporate citizenship, the owners offered the Federal government the right of first purchase. The deal nearly collapsed when the Federal government violated an agreement to keep the terms of the purchase secret, but was finally concluded. The city of Los Alamos is located on the eastern rim of the caldera, a site chosen precisely because of its limited access.

Volcanic Hazards in the U.S.

Until Mount Saint Helens erupted in 1980, this was the most recently-active volcano in the continental United States. Mount Lassen, California is the southernmost major peak in the Cascade Range and erupted in 1914-17. Although the photo is taken in August, new snow dusts the summit. Mount Lassen is really a mass of overlapping viscous lava flows called lava domes. The crags in the right middle distance are another lava dome. The lave emerges as an almost solid pasty mass and does not flow very far.

Because of copyright restrictions, the best-known images of the 1980 eruption of Mount Saint Helens are not reproduced here. Refer to a site specifically about the eruption to access those images.

A tree carried downriver by the Mount Saint Helens mudflow.

Mudflow deposits, actually more like very coarse sand than mud. The channel was dredged by the Army Corps of Engineers to prevent flooding.

These trees were partially buried by a Mount Saint Helens mudflow. The mudflow compacted after it stopped moving. The marks on the trees show the original depth of the mudflow.

Mount Saint Helens as it appeared from the northwest in December, 1980. The view is diagonally across the amphitheater. Note the volcanic gases above the summit.

Risking life and limb in the red zone of Mount Saint Helens, the effect only slightly ruined by the people walking up the road beyond the sign. The red zone has since been abolished and the road to within a few miles of the crater is now open.

The highest summit of Mount Rainier is a smooth volcanic cone but the rest of the mountain is rugged. In fact, the summit of Mount Rainier collapsed about 6,000 years ago. If we extend the remains of the volcano upward, the original height would have been perhaps 16,000 feet instead of the present 14,400. Suburbs of Olympia and Tacoma are built on former mudflow deposits from Mount Rainier.

Vesuvius and Pompeii

Pompeii, buried by Mount Vesuvius in 79 A.D.

Victims of the eruption were buried in the ash, which hardened around them. The bodies decayed but the cavity remained. The cavities are filled with plaster to reveal even such details as clothing and facial expressions.

Of the 20,000 people estimated to have lived in Pompeii, only a couple of thousand bodies were found, leading most historians and scientists to assume the eruption was fairly non-violent. That picture changed dramatically in the 1980's.

The lowest deposit consists of popcorn-like pumice, which would not have been very dangerous. The two upper layers are finer-grained and contain cross-bedding, indicative of turbulent flow. The brick protruding from one layer tells the story: a violent ash flow powerful enough to fling bricks. The eruption was much more lethal than anyone had previously surmised.

The high peak is the present summit of Vesuvius (4800 feet). To the right is part of Monte Somma, a ring of ridges that surrounds most of Vesuvius. Long thought to be prehistoric, Monte Somma now seems to be the remains of Vesuvius after the 79 A.D. eruption. Roman paintings of Vesuvius show only a single cone, without a hint of Monte Somma. Also, drawings and paintings of medieval and Renaissance eruptions of Vesuvius, without exception, show the summit of Vesuivius lower than Monte Somma.

The present crater of Vesuvius. The last major eruption of Vesuvius was in 1944 (as if Italy didn't have enough problems then!). Before then, the crater was shallow with a small, constantly fuming cone in the center. The white band about 2/3 of the way up the picture marks the pre-1944 crater floor.

Intrusions

The cliff is a nice example of a sill, a sheet of magma intruded between rock layers. The sill and the surrounding layers tilt to the right. In the background is the city of Edinburgh, Scotland. This same view is visible in the background in one scene in the film Chariots of Fire.

Mount Rushmore was picked for the sculpture because of its granite, but the granite is actually a relatively thin lens on the top of the mountain. Beneath Washington's lapel, the much more roughly textured rocks are metamorphic rocks. A thick dike of granite, one of the feeders of the pod of granite on top of the peak, can be seen cutting through the metamorphic rocks.

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Created 13 July 1998, Last Update 17 August 1998

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