They tied pillows on top of their heads as protection against the shower of rock. It was daylight now elsewhere in the world, but there the darkness was darker and thicker than any night…(t)hen came a smell of sulfur, announcing the flames, and the flames themselves…he stood up, and immediately collapsed…his breathing was obstructed by the dust-laden air.
—Account by Pliny the Younger of the death of Pliny the Elder in the eruption of Mount Vesuvius in A.D. 79 that destroyed Pompeii and Herculaneum in Italy. [Living with a volcano in your backyard: an educator's guide with emphasis on Mount Rainier [1]]
Supposedly, Atlantis was an island civilization "outside the Pillars of Hercules" and thus located in the Atlantic Ocean, where it was destroyed by an earthquake or tsunami (giant wave) about 11,000 years ago. The source of this information (according to Wikipedia) is an account that Plato wrote in 360 BC of information reportedly given to Solon two hundred years earlier by priests he visited in Egypt. Now, if someone told you that 200 years ago someone else had received information by yet another person regarding something that happened 9000 years earlier, would you immediately believe it? A lot of people apparently do; a search of Google for "Atlantis Plato" finds about 327,000 2.8 million matches, and not all of them are academic discussions.
A better question might be whether there really are islands that disappear below the sea. The answer is yes; many do. Some slide slowly downhill, at about the same rate as your fingernails grow, and disappear first beneath the waves and finally beneath the continents. Others suddenly explode, scattering themselves across the world.
Before we go any further, take a look at the following short video introduction by Dr. Anandakrishnan...
You will have one week to complete Unit 3. See the course calendar for specific due dates.
As you work your way through the online materials for Unit 3, you will encounter a video lecture, several vTrips, some animated diagrams (called GeoMations and GeoClips), additional reading assignments, a practice quiz, a "RockOn" quiz, and a "StudentsSpeak" Survey. The chart below provides an overview of the requirements for this unit.
REQUIREMENTS | SUBMITTED FOR GRADING? |
---|---|
Read /view all of the Instructional Materials |
No, but you will be tested on all this material. |
Submit Exercise #1: Scientific Literature | Yes, this is the first of 6 Exercises and is worth 5% of your total grade. |
Begin Exercise #2: Geology is All Around You | Yes, this is the second of 6 Exercises and is worth 5% of your total grade. |
Take the Unit 3 "RockOn" quiz | Yes, this is the second of 12 end of unit RockOn quizzes and is worth 4.5% of your total grade. |
Complete the "StudentsSpeak #4" survey | Yes, this is the third of 12 weekly surveys and is worth 1% of your total grade. |
If you have any questions, please feel free to email "All Teachers" and "All Teaching Assistants" through Canvas conversations.
On the following pages, you will find all of the information you need to successfully complete Unit 3, including the online textbook, a video lecture, several vTrips and animations, and two overview presentations.
Students who register for this Penn State course gain access to assignment and instructor feedback, and earn academic credit. Information about registering for this course is available from the Office of the University Registrar [4].
Crater Lake, at 1932 feet (about 600 m) deep, is the deepest and probably the cleanest lake in the United States, and surely among the most beautiful. Crater Lake sits in a great volcanic crater or caldera, 5 miles (8 km) across, formed when Mt. Mazama experienced a cataclysmic eruption about 6600 years ago. That massive eruption laid down ash that is 200-300 feet thick (almost 100 m) on the flanks of the volcano; the ash forms a recognizable layer hundreds of miles away in Yellowstone, and has been identified in Greenland ice cores.
The peak of the volcano had risen more than a mile above its mile-high base on the highlands of southwestern Oregon, but the great eruption removed about 4000 feet (1200 m) from the mountain’s height. About 16 cubic miles (40 cubic km) of rock were blown away. Glaciers had flowed down from the mountain peak; today, the glacial valleys can be followed upward until they disappear at the caldera rim. Although 50 feet (15 m) of snow falls in a typical year now, melting in the summer is sufficient to remove all this snow, and no glaciers exist. A tongue-in-cheek Christmas celebration on Aug. 25 substitutes for the snowbound December event.
After the great eruption, lava flows began building Wizard Island. If the water were removed from the lake, you could see that Wizard Island is roughly 0.5 mile (0.8 km) high. No permanent streams feed into the lake; the great rainfall and snowfall in the crater are balanced by evaporation, and by seepage through the rocks and eventually out the sides of the volcano as springs. With no streams supplying sediment, the lake is exceptionally clear and clean. Aquatic moss receives enough sunlight to grow 425 feet (130 m) below the water surface. When trout were stocked in the lake, freshwater shrimp were stocked first because otherwise biologists feared that the trout would have nothing to eat.
We will discuss volcanoes soon. For now, note that Crater Lake sits atop one of a string of volcanic peaks: Lassen Volcanic National Park and Mt. Rainier National Park preserve other peaks in the Cascades range. Mt. St. Helens, Glacier Peak, and several others are protected federally. These peaks line up in a row, called a volcanic arc, parallel to the coast. A similar arc sits along much of Central America, and forms the Andes of South America. And similar arcs also occur as island chains—the Aleutians, Japan, and others. In fact, the Pacific Ocean is almost entirely encircled by such volcanic arcs, forming the “Ring of Fire”.
Sitting offshore of the Ring of Fire is a ring of trenches, which include the greatest depths of the ocean. The trenches parallel the arcs. Some trenches, which sit near continents, are nearly filled with sediments dumped off the continents, but other trenches are almost free of sediment and so have very deep water, to almost 7 miles (11 km) deep. (Figuring out depths often is complicated by sediment. The surface of Death Valley sits more than two miles lower than the adjacent peaks of the Sierra Nevada. But below the salt flats of Death Valley there are sediments as much as three miles thick, materials that were eroded off the tops of the peaks, so the valley has dropped by much more than three miles relative to the peaks.)
The trenches and volcanoes that ring the Pacific are clues to help solve a problem that might have been bothering you from last time. If the sea floor is made at spreading ridges and then moves away, where does it go? The earth is not getting bigger. (Well, meteorites are adding a tiny, tiny bit, but not nearly enough to account for sea-floor spreading.) So, the sea floor must be disappearing somewhere. The oldest sea floor we know of is only about 160 million years old, but the continents contain rocks as old as almost 4 billion years, suggesting that the sea floor is being consumed before it gets very old. (Remember, before the class ends, we’ll discuss how geologists date rocks.) (And remember that when a geologist dates a rock, it involves physics or chemistry but not dinner or a movie.)
Now, a few more clues. Sea floor is made of basalt. This is just the kind of rock that would be made if you melted a little bit of the deep, convecting rocks of the Earth’s mantle, and let that melt float up to the surface and “freeze” (cool until it solidifies). If you take basalt, a little ocean sediment, and some ocean water, and heat them enough to cause a little melting, let that melt come to the surface and freeze, you obtain a rock called andesite with a little more silica and a little less iron and magnesium than basalt, lighter in color and lower in density than basalt. Interestingly, the dominant rock in the walls of Crater Lake, and in the other Cascades and Ring-of-Fire volcanoes, is andesite (named after the Andes, which are part of the Ring of Fire).
If sea floor were plunging under the continents and melting to make andesite, you might expect that occasionally the down going rocks would get stuck and then break free, making earthquakes. Indeed, a three-dimensional map of earthquakes shows that shallow ones occur near the trenches, and the quakes are progressively deeper inland beneath the volcanic arcs. The great 1964 Alaska earthquake was such an earthquake, which happened where rocks of the Pacific Ocean floor plunge to the north under coastal Alaska and the Aleutian chain. The earthquakes there are shallow to the south and deepen to the north, along the down going rocks. The disastrous 2011 Tohoku earthquake in Japan was of the same type.
Careful work on the speed of earthquake waves, which is affected by the temperature of rocks, even provides a picture of cold slabs going down into the hotter mantle. As these down going slabs of rock are heated, with their water and sediment, a little melt is produced. (Interestingly, wet rocks melt at a lower temperature than do dry rocks, just as adding a little water to flour and yeast speeds cooking in the oven.) When the melt rises to the surface and cools, andesite forms, such as is seen around Crater Lake, in the Andes, or in the Aleutian volcanoes.
So, sea floor is made at the spreading ridges. It is hot and low-density initially, but cools and contracts as it gets older and loses heat to the colder ocean water. When the sea floor becomes cold and dense enough, it can sink back into the mantle, and we call the place where it sinks a “subduction zone”. The sinking sea-floor slab drags along a little sediment and water, warms up and melts a little of the slab and some of the sediment, and feeds the volcanic arcs. Old sea floor is going down around much of the Pacific Ocean, and in a few other places, such as beneath the Caribbean and beneath portions of the Alps. Wherever this happens, andesitic volcanic arcs form (see Figure 1).
The Olympic Peninsula juts out into the Pacific Ocean, separated from Seattle by Puget Sound. Moisture-laden winds off the Pacific dump more rain and snow on the Olympic than anywhere else in the lower-48 United States. Great old-growth forest trees—Sitka spruce, Douglas fir, etc.—tower up to 300 feet (almost 100 m) above the forest duff, where butterflies flit past crystalline streams and cascading waterfalls. Along the coast, sea lions bask on offshore stacks, while urchins and starfish populate tidal pools. On the “high peaks,” numerous glaciers form and flow downhill. More snow accumulates than melts above about 6000 feet (2000 m). On most mountains, you have to go much higher to find summertime snow, but the huge winter snowfall on the Olympic allows the peaks to be snow-clad year-round despite rising less than 8000 feet (about 2500 m) above sea level.
Olympic National Park is a bit unusual in that it was established as much for biological reasons as for geological—to protect the Roosevelt elk that live on the peninsula. (The elk, named after Theodore Roosevelt, were critical in obtaining national monument status, which was signed by President Theodore Roosevelt. Later, the upgrade to national park status was signed by President F.D. Roosevelt. The Roosevelt elk is the largest of the elk subspecies in the country. Some consideration was given to naming the park Elk National Park before Olympic was chosen.)
The geologic story of the Olympic is somewhat shorter and less dramatic than for most of the national parks. The rocks of the Olympic are almost all young--less than 40 million years. (Again, please bear with us—we will justify these numbers before the course ends!) Before that, the coastline must have been farther to the east, perhaps in North Cascades National Park, and before that even farther east.
You’re standing at the grocery-store checkout. You put a bag of potato chips on the conveyor, and off they go, followed by a case of Pepsi, three loaves of bread, a watermelon, a box of Ho-Hos, and a sack of potatoes. Then, you realize that there is no bagger working, and that everything is piled up at the end, in a BIG mess. That mess is a good model for the Olympic Peninsula, and the whole coast from there up to Alaska.
The rocks of the Olympic Peninsula are a mixture of sea-floor basalts and of the sorts of sediments that accumulate today off the coast and fill the trench there. Rivers draining the peninsula, and much of the West Coast, carry great loads of sediment down to the ocean. Much of that sediment piles onto sea floor that is slowly moving beneath the continent, a conveyor belt that tries to pull the sediment down to melt and be erupted. Most of those sediments are “scraped off” on the way down, just as at the grocery store. The Olympic Peninsula is the off scrapings. Most of the rocks have been bent and twisted from the attempt to shove them under the continent (think of the potato chips after the Pepsi hits them!). Some of the Olympic rocks have been heated a good bit—the conveyor belt took them down a ways, but they then were squeezed back out.
Our emerging picture of plate tectonics is that the earth is heated inside, softening the deep rocks of the asthenosphere enough that they can move in great, slow convection currents that transfer heat from deep in the earth to near the surface. Heat is conducted through the upper rocks, or is erupted through them by volcanoes, and eventually is lost to space. But, the upper rocks in most places are cold enough that they tend to break rather than flow—they are brittle. These brittle rocks form the lithosphere, which includes the crust and the uppermost mantle. The rocks of the crust in continents are rich in silica (often like andesite in composition), making them light in color and low in density, so that they float on the deeper rocks and are rafted around on them by the moving convection currents. The sea floor rocks in the crust fall between the continents and the mantle in composition, and typically are basalt. The sea-floor rocks are usually intermediate between continents and mantle in density as well, but if the sea-floor rocks are cold enough, they will be slightly denser than hot mantle. Then, the sea-floor lithosphere consisting of the sea-floor crust plus a little attached mantle will sink into the asthenosphere of the deeper mantle.
The lithosphere is broken into a few big rafts, called plates—eight big ones plus some smaller ones, depending a little on how you define “big” and “small”—that float around on the convection cells below. Plate boundaries include spreading ridges where the plates move apart (remember Death Valley and the mid-ocean ridges), and subduction zones where the plates come together and one side sinks under the other. You might imagine that if plates can come together or pull apart, they must be able to slide past each other as well, which is what happens at the San Andreas Fault in California (we met it when we were discussing earthquakes); such slide-past boundaries are often called transform boundaries or transform faults (see Figure 1). You might worry that sometime, two continents would run together; we’ll meet that soon when we visit the Great Smoky Mountains.
The lithosphere and asthenosphere are solids, but a little melted rock may occur in the asthenosphere, and leak out where plates are pulled apart, feeding basaltic volcanoes. And the water taken down subduction zones can stimulate a little melting, feeding andesitic volcanoes that line up in arcs above the downgoing slabs of the subduction zones; examples of these volcanic arcs include the Cascades, Aleutians, and Andes. Continents are a collection of scum from melting of the mantle, too low in density to sink back into the mantle. Continents grow as the conveyor belt from the mid-ocean ridge to the subduction zone brings in sediments and islands and what-not, or when andesitic volcanoes are erupted on continents, or when andesitic volcanoes form an arc offshore that then collides with a continent (sometimes the site of subduction moves, and the volcanoes find themselves on the conveyor belt, or they hit a different continent). Because much of the sediment comes from the continents themselves, the growth of continents is not fast—material eroded from the continents falls on the conveyor and is added back at Olympic or erupted back at Crater Lake.
Mt. St. Helens, in southwestern Washington, was in some ways the queen of the Cascades Range. Beautifully symmetric, snow-capped, it had been called the Fujiyama of the Pacific Northwest. Scores of people flocked to St. Helens’ flanks to hike, camp, ski, and generally enjoy the environment. And all that changed in 1980.
Mt. St. Helens has also been the most active of the Cascades volcanoes over the most recent centuries. In early 1980, the volcano clearly was “waking up”. Earthquakes shook it almost continuously, including special “harmonic tremors”, similar to those sometimes caused by fluid flow in pipes, which showed that liquid rock was moving up from below. Small eruptions occurred, and hot springs and fumaroles (steam or gas vents) became increasingly active. The north side of the mountain was bulging, blowing up like a balloon as the magma moved into it. Scientists were scrambling to study the volcano, and predict its course. They recommended evacuation for safety, and most people (but not all, including some scientists) were moved out of the way. Penn State professor Barry Voight warned that the huge bulge on the north side of the volcano would fail, unleashing a giant landslide and a devastating eruption.
On the morning of May 18, 1980, Professor Voight’s prediction came frighteningly, awesomely true. The bulge failed. A large earthquake either caused, or was caused by, failure of the north side of the mountain in a giant landslide. Like pulling the cap off a hot, well-shaken soda bottle, the liquid beneath flashed into froth, driving an eruption 12 miles (20 km) high. A shock wave knocked over full-grown trees in an area 20 x 10 miles (32 x 16 km). The landslide eventually poured more than 100 million cubic yards of rock material down the Toutle and Cowlitz Rivers, raising the floor of the North Fork of the Toutle as much as 600 feet (200 m), and sweeping roads and houses downstream, with the debris reaching and clogging the shipping channels of the Columbia River. The Toutle floor now sat higher than the smaller streams that fed it, and lakes began to form; only quick work by the Army Corps of Engineers prevented those lakes from overtopping the mud damming them, cutting quickly down through it and releasing further floods.
All told, the Corps of Engineers spent $250 million clearing shipping channels and doing other critical work. Approximately 60 people were killed in the blast and landslide; some were buried under hundreds of feet of steaming mud and their bodies were never recovered.
President Jimmy Carter scowled at the disaster from a helicopter. Disaster planners pontificated. And in the shadows of the other Cascades volcanoes, people continued building houses in regions of known volcanic hazard.
The Mt. St. Helens Volcanic Memorial today has little in common with conditions pre-1980. The center of the volcano was more than 1/2 mile (nearly 1 km) lower after the eruption than before, with the missing rock spread over the surrounding countryside, forming a visible layer as far as 900 miles (1500 km) away. (Professor Alley and his wife Cindy were driving in Alberta, Canada during the summer of 1980, on a great, seven-week, see-the-national-parks-in-a-Chevette-with-a-tent honeymoon. A secondary eruption of Mt. St. Helens put enough ash in the air to halt traffic because of reduced visibility, hundreds of miles from the volcano.) Many of the trees knocked over by the blast still lie there—hundred-foot-long toothpicks pointing in the direction of the searing winds of the blast. Among these dead trees, however, salmonberry and fireweed and young firs are pushing skyward, elk are grazing, and coyotes search for rodents. In some places, salvage-logging of the downed trees was allowed. In some of those places, it appears that erosion has run amok, large gullies have developed, and the return of vegetation has been greatly slowed. In the crater of the volcano, a new lava dome is forming, squeezing slowly upwards like toothpaste from a very hot tube, and amazingly, a glacier sits behind it, fed by the great snowfall and the avalanches down into the crater. At night, the dome glows dull red. (It may seem weird that we focus on an event from before most of you were born, from 1980, when larger eruptions have happened more recently. But, St. Helens is the largest recent eruption in the lower-48 of the US, the easiest eruption site to get to and observe, and it really is awesome. The elder Alley daughter, Janet, was a ranger there one summer, and recommends that you take in Ape Cave if you visit, but the real goal is to see just how immense the eruption's effects really were.)
Volcanoes occur where melted rock rises to the Earth’s surface. Almost all volcanoes are associated with one of three settings—pull-apart margins (spreading ridges), push-together subduction zones, and hot spots. We’ve already met the volcanoes at spreading ridges, where low-silica basalt is erupted, and those producing higher-silica andesite at subduction zones.
Hot spots are creatures of another type in the zoo of Earth’s features, and especially interesting creatures at that. Deep in the warm, soft, convecting mantle of the planet, in some places a rising tower of hot rock forms and then lasts for quite a while. Some of these rising towers may come all the way from the base of the mantle where it meets the core; others may start shallower. (To see something that looks vaguely like the formation of such a hot spot, go back and view again the “lava lamp” film of Dr. Anandakrishnan in the introductory material to this unit.)
As the lithosphere drifts overhead, the hot spot may “punch through” to make a volcano. Then as the lithosphere carries that volcano away, the hot spot punches through a new place to make a new volcano. Rising melt behaves a little bit like people driving cars, who use one road or the other but not the lawn in-between; hot spots often make a string of separate volcanoes rather than a continuous line or ridge, by coming up through one hole in the lithosphere for a while and then switching to a different one. Hot spots bring melt from deep in the mantle, and so normally make basaltic volcanoes (it takes fairly subtle and sophisticated chemistry to tell the difference between hot-spot basalt and sea-floor basalt from spreading ridges). However, where a hot spot pokes through a continent rather than through sea floor, silica from the continental rocks may mix with the melt to increase its silica content, as at Yellowstone.
When a new hot-spot first rises from below, the top must push through the mantle and crust, and the resistance of the stuff in the way of the rising column causes its top to spread out like the head of a thunderhead, or of a mushroom cloud from an atomic bomb, or of a blob in a lava lamp, and for the same reasons. When that wide head reaches the surface, immense lava flows can be produced that spread across state-sized areas and bury them hundreds of feet deep. Much of central and eastern Washington and Oregon is buried by the “flood basalts” from the head of the Yellowstone hotspot. As the continent has moved across the hot spot after it reached the surface, a string of volcanoes erupted, including Craters of the Moon National Monument in Idaho. The hot spot now fuels Yellowstone (which is why it is called the Yellowstone hot spot…a lot of this stuff isn’t that difficult!).
The flood basalts from the Hawaiian hot spot have been subducted beneath the Aleutians, but the string of volcanoes that has formed since from the hot spot can be traced from the Aleutians south all the way to the volcanically active big island of Hawaii. A new volcano, Loihi Seamount, is growing undersea just off Hawaii as the big island slowly drifts away, and Loihi will someday (not in our lifetimes!) be the new, volcanically active “big island” while the volcanoes of the current big island die and the island slowly erodes away.
So, melt can leak up from below to feed volcanoes at spreading ridges, at hot spots, and above subduction zones. But very different volcanoes develop: sea floor from spreading ridges; flood basalts and then wide, not-very-steep Hawaii-shaped volcanoes from hot spots; and, steep Mt. St. Helens-type volcanoes above subduction zones. The type of volcano that develops at a place depends on a host of factors: temperature, composition, supply of melt, duration of supply, and several others. We will focus on two: composition (how much silica) and volatile content (mostly how much water, although carbon dioxide, hydrogen sulfide, and other compounds that are gaseous under earth-surface conditions may be present and classified with volatiles). Silicon and oxygen get together in melt to form the material we call silica. Left to itself, each silicon atom will be surrounded by four oxygen atoms, which form a tetrahedron (a little pyramid). But, give them a little time, and the tetrahedra will start sticking together, or polymerizing, into chains and sheets and bigger clumps, with some oxygens serving in more than one tetrahedron. If these lumps get big enough, they are minerals and the melt has solidified.
When the lumps are present but not too big, the melt is like lumpy oatmeal—it doesn’t flow very well. There are three ways to get rid of the lumps: make the melt really hot; fill the melt with iron, magnesium and other elements that interfere with the tetrahedra polymerizing; or, fill the melt with volatiles that interfere with the tetrahedra polymerizing. When polymerization is low, the melt flows easily. Lava comes out of the volcano quietly, without making big explosions, and flows easily and far from the mouth of the volcano. In extreme cases, flows may be nearly horizontal and cover much of a state, as in the flood basalts. If the melt spreads almost as easily as flood basalts, the lava will have very slight slopes of only a few degrees, forming shield volcanoes (they look like a warrior’s shield lying on its side) such as Hawaii. Hawaiian lavas and flood basalts flow easily because they are hot and are high in iron and magnesium.
When volatiles remove the lumps, a different situation develops. This is because the volatiles will only stay in the melt under high pressure. Just as a bottling company can force CO2 into the water of a soft drink under high pressure, but the CO2 escapes as the pressure falls when you open the can, the water and CO2 and other volatiles stay in the melt under high pressure down in the Earth but escape when the melt gets close to the surface.
Silica-rich melts usually form with many volatiles. Remember that in subduction zones, wet sediment dragged down the trench releases water (and carbon dioxide and others) that promote melting. When the melt (called magma when it is in the Earth and lava when it reaches the surface) nears the surface, the lower pressure allows the volatiles to bubble off and escape into hot springs, geysers, etc. (Note that most of the fluids that come out of such hot springs are rainwater that has circulated down into the earth, but some of the fluids may be “juvenile” waters from the magma below.) Silica-rich, relatively cool lava that has lost its volatiles flows only with great difficulty. It may emerge from the volcano and flow a little ways as a very thick, slow-moving, steep flow. It may not even flow, but simply form a dome directly over the volcanic vent. And, it may “plug the system” when it solidifies. Then the stage may be set for a big explosion.
The next melt that rises in the volcano cannot follow the same path, because hardened lava above prevents escape. The gases are trapped, and pressure builds up. The volcano is like a hot, shaken pop bottle. If the top is removed, either by an opener (say, a landslide as in the case of Mt. St. Helens, or a crack opened by an earthquake) or just because the pressure becomes great enough to blow the top off, the sudden release allows the soda, or the magma, to come foaming out. A good champagne may fountain to many times the bottle’s height, and blast the cork across the room. A powerful volcano may blast ash higher than jet flight paths. The melt really does get foamy, and that foam hardens into little glass shards. The ash layer deposited by Mt. St. Helens, which stopped drivers hundreds of miles away, was mostly of such little glass shards, although torn-up bits of the former volcano were also included.
The andesitic volcanoes of the Ring of Fire are typically stratovolcanoes, formed of alternating layers of thick lava flows and of pyroclastics—things thrown through the air by the volcano. The steepness comes from the flows, which cannot get far from the vent. Some of the andesitic volcanoes, including the rebuilding Mt. St. Helens, include plug-dome elements, the oozing lava staying right above the vent.
So, the major volcanoes for our purposes are the quiet, basaltic shield volcanoes of hot spots, the quiet basaltic rift volcanoes of spreading ridges, and the steep, scenic, explosive, andesitic volcanoes of the Ring of Fire. Other types exist, notably cinder cones thrown up by typically minor eruptions tossing pyroclastics short distances. Also, hot spots or rifts trying to poke through continental rather than oceanic crust may produce explosive silica-rich volcanoes. But if you understand shields and stratovolcanoes, you will be a long way toward understanding volcanism.
People who live near volcanoes should be worried about them. Volcanoes can do much damage. The volcanic-triggered landslide that buried Armero, Colombia in 1985, and the eruption of Mt. Pelee on the island of Martinique in the Caribbean in 1902, each killed about 30,000 people. Other volcanic disasters bring the human death toll to perhaps 200,000 over the last few centuries. Compared to war, disease, or even automobile accidents, this is not a terribly high toll; however, the 200,000 people directly involved almost certainly would have appreciated enough warning to get out of harm’s way. One of the goals of modern geology is to predict volcanic hazards and to save lives and property by doing so.
There are many hazards to worry about. These include:
So, we wish to predict when and where volcanoes will endanger people. Various things can be done. For problems such as climate change, the best we can do is to know that every few years or decades some region is likely to experience difficulties with crop production because of eruptions. The solutions are either to maintain a little excess food to feed those endangered people, or to ignore them and figure that some will starve to death. (Many other climate changes, including droughts, give us the same choice. Despite the apparent silliness—either we stockpile food and figure out how to distribute it to the needy, or we let people starve to death—it is surprising how often starving to death is the outcome.)
For tsunamis, an operational warning system now exists around the Pacific and in some other places. If a tsunami is detected, or if seismographs detect the shaking caused by a large earthquake, landslide, or volcanic eruption, warnings are relayed to coastal regions likely to be affected, in time to allow evacuation. Such a system is being assembled for the Indian Ocean following the tragic 2004 earthquake-generated tsunami that killed approximately 250,000 people.
One way to avoid volcanic hazards is to stay out of harm’s way. Geologists can map regions where large pyroclastic chunks have fallen, or where landslides have occurred, with great confidence. Using carbon dating of logs caught in debris flows, or tree-ring dating of those growing on landslides (just hang on; explanation of such things will come later), we can determine the recurrence interval—how often do such disasters happen? Today, whole subdivisions are being built around Mt. Rainier National Park in the growing Seattle-Tacoma region that have a danger of destruction by landslide many, many times higher than their danger of destruction by fire. Roughly 200,000 people work, and more than 100,000 people live, on debris-flow deposits less than 10,000 years old, with more people coming. The homeowners will all carry fire insurance, but few if any are insured against the volcano; presumably, if they survive the next volcanic disaster, these people are counting on disaster aid from the rest of the country to bail them out.
(Much argument is attached to sending disaster aid for predictable events, even if they are not very common. Should those who wish to live in beautiful but risky areas carry insurance to pay for their gambles? Increasingly, planners are saying “yes,” and much effort is being devoted to quantifying the hazards so that insurance rates can be set wisely. This applies to such things as hurricanes along coasts, earthquakes along faults, and floods along rivers. Geologists have an important role to play in learning hazards and thus setting rates.)
With sufficient care, volcanic eruptions can be predicted with some confidence. Volcanoes usually give off many signals before an eruption: the ground swells as magma moves up; the moving magma and the swelling ground create earthquakes and especially the distinctive harmonic tremors of fluid flowing in a pipe; small eruptions occur; gaseous emissions increase as the magma nears the surface and then cease if the system becomes plugged and builds up pressure for an explosion. A monitoring program of seismographs to detect earthquakes, repeat surveying of laser reflectors set on the mountain to watch for deformation patterns, gas sampling, and perhaps photographic or other sensors to watch for landslides, can track a volcano’s behavior and allow timely warning. Monitoring of ground shape from space can even see the changes in volcanoes as magma moves under them. The eruption of Mt. St. Helens was predicted well enough to save hundreds of people including the residents of a YMCA camp. The eruption of Mt. Pinatubo in the Philippines in 1991, which heavily damaged the U.S. military bases there, was predicted accurately, allowing timely evacuation and saving tens or hundreds of thousands of lives of residents and military personnel.
The burden of predicting eruptions is very high. Imagine telling an Air Force general to abandon his or her assigned duty post, spend a few hundred thousand dollars to move tens of thousands of people, and then having nothing happen—the general, and all of those people, would be very unhappy. Imagine instead deciding to wait another day to be sure, and having all of those people (possibly including you) killed. Important as this is, predicting disasters is not for the faint of heart.
The Mt. St. Helen’s eruption was a small one compared to many others. Each of the major eruptions of Yellowstone moved about 1000 times more material than Mt. St. Helens did, and Yellowstone’s eruptions were not the largest known. Small eruptions are more common than large ones. But, eruptions ten times as big as Mt. St. Helens are perhaps five times as rare, but not ten times as rare. This means that, as for earthquakes, most of the “work” done by volcanoes is achieved by the few big ones, not the many little ones.
Join Dr. Alley and his team as they take you on "virtual tours" of National Parks and other locations that illustrate some of the key ideas and concepts being covered in Unit 3.
TECH NOTE
Click on the first thumbnail below to begin the slideshow. To proceed to the next image, move the mouse over the picture until the "next" and "previous" buttons appear ON the image or simply use the arrow keys.Here are some optional vTrips you might also want to explore! (No, these won't be on the quiz!)
Hawaii Volcanoes National Park and a second Crater Lake Slideshow [7]
(Provided by Dr. Alley)
Hawaii [8]
(Provided by USGS)
Crater Lake National Park [9]
(Provided by UCGS)
Crater Lake National Park - 3D version [10]
(Provided by UCGS - red/cyan stereo viewing glasses required)
Mount St. Helens National Volcanic Monument [11]
(Provided by UCGS)
Olympic National Park [12]
(Provided by UCGS)
There are three Unit 3 GeoClips (movies) linked below. We hope they help you understand Unit 3 just a little bit better, and that you enjoy them.
The hot spot of Hawaii erupts runny lava to the surface, giving some very interesting features, such as the lava tubes you will see forming in the first video, and formed in the second video. The hike out to the flowing lava was, in spring of 2007, over three miles across rough, often broken and glassy lava that solidified from glowing hot flows over the last couple of decades. Whales were spouting offshore when Dr. Alley and family made the trip. Tag along, and see what they saw.
Lava was erupting in the Southwest Rift of Kilauea not that long ago. Sometimes, the lava erupts with a little force, throwing pieces that freeze to glass in the air and rain down. Other times, the lava flows even more quietly along the surface. Here, you can see evidence of both.
Optional Videos, for your enjoyment (and education, but you won't be quizzed on them.) Volcanoes are just too interesting to leave so quickly, so here are some more looks at these important, and dangerous, pieces of our planet. First, visit Hawaii again, and see some strange things. Then, head over to Sunset Crater Volcano National Monument, Arizona, with the CAUSE class. Have fun, and keep your feet cool!
Hawaiian lava flows engulf whatever is in their way, including trees. What happens when hot lava hits a cold, wet tree? Find out here, your chance to look down on Dr. Alley.
Kilauea Volcano is a wonderful place to visit. Stay in the lodge on the rim, and you'll wake up to the view shown here behind Dr. Alley.
Cinder cones are rather odd volcanoes, formed of pyroclastic bits tossed through the air to pile up near the vent. If you let the spaghetti sauce boil on the stove, without a lid, you would soon have a lot of tomato-sauce blobs around the pot. Let those build up, and you are heading for a cinder cone. Here, see three different versions of the cinder cones at Sunset Crater.
An explanation of cinder cone volcano formation by CAUSE student Sam A.
Another, slightly "dramatized" explanation of cinder cone volcano formation by CAUSE students Stephanie S. and Raya G.
A third explanation of cinder cone volcano formation, by Dr. Alley himself.
There are TWO Unit 3 lectures both featuring Dr. Sridhar Anandakrishnan.
Start with the first lecture. It is 40:50 minutes long.Now watch the second lecture (36:46 minutes long).
Check out the Unit 3 PowerPoint Presentation [14] used in the online lecture here.
Did you catch all of that? Review the chapter with another Johnny Cash tune not sung by Johnny Cash, "Ring of Fire.", Mt. St. Helens by the subduction zone—it really is a burning thing!
The eruption from Mt. St. Helens in 1980 was not especially big—each of the main Yellowstone eruptions moved about 1000 times more material, for example. But, the Mt. St. Helens eruption killed more people (57; before that only 2 deaths in the US were blamed on volcanic eruptions, with none since), and did more property damage (almost $1 billion), than any other eruption in the United States since the country was formed. Novarupta, in Alaska in 1912, blasted more material than the Mt. St. Helens eruption, but was so far from most people that no one died and damages were small; also, Alaska, at that time, was a territory of the US but not yet a state.
The 1980 Mt. St. Helens eruption probably involved the largest landslide ever observed by humans, in the US or anywhere else, at least since we started writing down what we saw. So, by many measures, in 1980, Mt. St. Helens gave us "The Biggest Eruption in the Whole USA." Here's a Rock-Video parody to show you what happened. If an eruption this big happened from Mt. Rainier, which is close to many more people, the damages would be far, far greater. And, Rainier might just do it some day...
We've seen that most of the Earth's crust is made of minerals with silica (silicon and oxygen) in them. Melting these minerals feeds volcanoes, and freezing the melt clogs volcanoes and makes new minerals in new rocks. When we get to the Redwoods and the Badlands in a couple of weeks, you'll see how the weather attacks the new minerals, and you'll meet a few of the minerals being attacked. Here, just for fun, you can see some truly beautiful minerals, and learn a bit about the wonderful ways they are put together—Lego blocks and erector sets have nothing on nature! Meet the main minerals of the crust, in a parody of Bill Haley and the Comets' Rock around the Clock. This Rock Video has a bit more detail than we'd ever ask in this class, so don't worry about actually learning the difference between nesosilicates and inosilicates unless you're really interested.
You have reached the end of Unit 3! Double-check the list of requirements on the Unit 3 Introduction page and the Course Calendar to make sure you have completed all of the activities listed there.
Following are some supplementary materials for Unit 3. While you are not required to review these, you may find them interesting and possibly even helpful in preparing for the quiz!
Please feel free to send an email to ALL of the teachers and TAs through Canvas conversations with any questions. Failure to email ALL teachers and TAs may result in a delayed or missed response. See "How to send email in GEOSC 10 [18]" for instructions.
Links
[1] https://www.usgs.gov/educational-resources/living-volcano-your-backyard-educators-guide-emphasis-mount-rainier
[2] http://commons.wikimedia.org/wiki/File:Hamilton-Campi_Phlegraei.jpg
[3] http://creativecommons.org/licenses/by-sa/3.0/
[4] http://www.registrar.psu.edu/
[5] http://craterlake.wr.usgs.gov/bathy_images.html
[6] http://www.fs.fed.us/gpnf/mshnvm/digital-gallery/25yearsofrecoverybeforeandafter.htm
[7] https://www.e-education.psu.edu/geosc10/node/1890
[8] https://www.e-education.psu.edu/geosc10/sites/www.e-education.psu.edu.geosc10/files/downloads/u03/u03vt6_hawaiiusgs.ppt
[9] https://www.usgs.gov/media/galleries/crater-lake-national-park
[10] https://www.usgs.gov/media/galleries/crater-lake-national-park-gallery
[11] https://www.usgs.gov/volcanoes/mount-st-helens
[12] http://3dparks.wr.usgs.gov/olym/index.html
[13] https://www.e-education.psu.edu/geosc10/sites/www.e-education.psu.edu.geosc10/files/geosc10/Unit3/Unit 3 Vtripsa.docx
[14] https://www.e-education.psu.edu/geosc10/sites/www.e-education.psu.edu.geosc10/files/downloads/u03/u03_volcanoes.ppt
[15] http://www.nps.gov/crla/
[16] http://www.nps.gov/olym/
[17] https://www.fs.usda.gov/visit/destination/mount-st-helens-national-volcanic-monument-0
[18] https://www.e-education.psu.edu/geosc10/node/2023