Welcome to Week 3 of Pacific Northwest Geology. The topics of this week's lecture are:

1. Volcanism in the Pacific Northwest
   1. Cascade Range composite cones
   2. Shield Volcanoes of the Pacific Northwest
   3. Yellowstone Hotspot
   4. Bimodal Volcanism of the Basin and Range
   5. The Columbia River Basalts
2. Rocks and Minerals
3. Web Links
4. Glossary Terms

Related Basics Pages: Rocks and Minerals; Volcanoes

Volcanism in the Pacific Northwest

The Pacific Northwest is rich in volcanoes and volcanic landscapes, including many active volcanoes. Some landscape regions in the Pacific Northwest contain large volumes of volcanic material erupted during earlier epochs of geologic time. In some parts of the Pacific Northwest, older volcanic rocks have been changed into metamorphic rocks. In other places the volcanic rocks are not very changed from when they originally erupted. To read about the different types of volcanoes and volcanic landforms go to the Basics page on volcanoes.

Cascade Range composite cones

The main set of volcanoes that are active in the Pacific Northwest are the composite cones that crown the Cascade Range The northernmost composite cones are Meager Mountain and Mt. Garibaldi in British Columbia and the southernmost are Mt. Shasta and Lassen Peak in California.

In Washington the active volcanoes are Mt. Baker, Glacier Peak, Mt. Rainier, Mt. St. Helens and Mt. Adams. All of them are tall, glacier-clad cones over 10,000 feet in elevation above sea level; or rather, all of them were until Mt. St. Helens exploded in 1980, losing about 1500 feet of its peak, beheading its glaciers, and leaving a gaping crater.

Composite cone volcanoes erupt lava flows, ash flows, and ash falls. Typically about 90% of the volume of a composite cone is solidified lava. Ash flows, ash falls, and other tephra blown into the air during a pyroclastic eruption may form only a modest amount of a composite cone's volume, but they can cause damage farther from the volcano than a lava flow can reach, and with less warning. Another hazard from composite cones such as Mt. Rainier is the lahar, a water-laden debris flow that surges from the side of the volcano down into its river drainages.

Andesite is the type of rock that is most typical of composite cones like those in the Cascade Range. If andesite is found, the area was was probably in or near a volcanic arc, part of a subduction zone, when the andesite erupted.

The Cascade Range has been a zone of active volcanoes for tens of millions of years, although it may be only within the last 10 million years that the location of the current crest of the range and chain of composite cones has shifted to where it now. Composite cones are usually active for for 0.5-2 million years. Over the millions of years of existence of the Cascade volcanic arc, numerous earlier volcanoes have come and gone, with the oldest ones mostly to entirely eroded away and younger volcanic rocks partly covering the older rocks. Older, inactive volcanoes of the Cascade Range are being eroded into jagged ridges and lower hills even as the young, active volcanoes keep their cones high and steep through repeated eruptions.

Next week, we will see that the theory of plate tectonics provides a detailed and elegant explanation of the Cascade volcanic arc. We will also see, through the theory of plate tectonics, how the volcanoes are related to much of the other geologic activity in the Pacific Northwest today, from major earthquakes to uplift of the non-volcanic Coast Ranges.

Shield Volcanoes of the Pacific Northwest

Just east of the Cascade Range there are three large shield volcanoes: Medicine Lake Volcano in northern California, Newberry Volcano in central Oregon, and Simco Volcano in southern Washington. Simco Volcano is on the Yakama Reservation northwest of the town of Goldendale. It is several million years old and has not been active for millions of years, so it has lost most of its shield shape and is being eroded by stream valleys. Newberry Volcano and Medicine Lake Volcano have been volcanically active during the Holocene and retain most of their shield-like shape. They may have a chance of becoming active again.

Unlike composite cones, which are mostly andesite, shield volcanoes are made primarily of basalt. Late in the history of a shield volcano it may also erupt rhyolite and obsidian, but the dominant volume of the shield is basalt flows.

In the Snake River Plain, running from near Boise in southwestern Idaho to near Yellowstone National Park in southeastern Idaho, there are many small to medium-sized, extinct shield volcanoes, which are thought to be part of the Yellowstone hotspot track.

Yellowstone Hotspot

Most of the southern half of Yellowstone National Park, including Old Faithful geyser, is in a volcanic crater, a type of volcanic crater which is technically called a caldera because it is many kilometers (miles) wide and formed by collapse. The Yellowstone caldera formed several hundred thousand years ago, during the Pleistocene epoch, when a huge volume of felsic magma erupted from the ground as ash and pumice and other pyroclastic debris. This hot ash fell to the ground to form ash flow tuff that covered thousands of square miles at depths up to a thousand feet or so, piling up deepest near and in the caldera. This tuff is the yellow rock that gave the national park its name.

After the magma blasted out of its underground chamber, the ground above collapsed into the void that was left, forming the main caldera of Yellowstone National Park. Geologic studies have shown that a chain of calderas, similar to the one that makes up a third of Yellowstone National Park, trends west across the Snake River Plain of southern Idaho and into southeastern Oregon and northern Nevada. The calderas get progressively older to the west.

The path of the calderas from Yellowstone across the Snake River Plain, and their age sequence, are consistent with the motion of the North American Continent across the earth's mantle during the last 20 million years. It is theorized that these calderas indicate the presence of a stationary hotspot beneath the moving continent. The hotspot is now located beneath Yellowstone National Park and is considered very likely to cause more eruptions in the future.

Bimodal Volcanism of the Basin and Range

In the Basin and Range landscape region the earth's crust apparently is being stretched apart. It is breaking along faults that run along the front of the mountain ranges. The faults cut deep and the stretched crust in the area has become thinner, bringing the mantle closer to the surface The deep faults, thin crust, and upwelling of the mantle underneath may be why basalt flows have erupted from fissures in the Basin and Range region. Basalt forms from mafic magma, which comes from molten rocks in the upper mantle.

There are some places in the Basin and Range region where felsic magma has erupted to form either rhyolite or felsic tuff. Melting of continental crust can produce felsic magmas, perhaps due to heat from hotter mafic magmas that rises into the crust from the mantle and melt the crust.

In sum, the Basin and Range region has been subjected to a combination of mafic and felsic volcanism, with a complete lack of intermediate volcanic rocks such as andesite. The term bimodal volcanism is used to describe this type of volcanic activity. It contrasts with the volcanism of a volcanic arc like the Cascades, which, despite having eruptions of basalt, rhyolite, and felsic tuff, is dominated by eruptions of intermediate magma, forming andesite.

The Columbia River Basalt

A great volume of basalt stacked in layers like wide, thin pancakes, one on top of another, forms the Columbia Plateau. This set of lava flows is named the Columbia River Basalt Group, or Columbia River Basalts (CRB) for short. Some of the CRB flows have been traced from the Idaho border across the width of the Columbia Plateau and all the way to the coast of Oregon. Such large, widespread, high-volume flows are called flood basalts. There are only a few flood basalt provinces in the world: one in South America, one in Africa, one in India, one in Siberia, and one in eastern Washington.

The CRB eruptions began 17 or 18 million years ago, and most of the volume of the flows had erupted by 14 millions years ago. This was during the Miocene epoch. A few small flows occurred as recently as 5 to 6 million years ago.

The time spans between CRB flows was as much as several hundred thousand to even millions of years in some cases, although early in the eruption history the flows tended to occur more closely together in time. During the intervals between flows, plants and animals reoccupied the land covered by lava, soil developed, lakes formed, rivers re-established valleys to flow through, and ash and sediments from the Cascade Range volcanoes to the west was deposited onto the nearby basalt flows to the east. At Gingko Petrified Forest State Park by Vantage, Washington, petrified wood preserves a record of a tall, thick forest that existed on wet ground in a time between Columbia River Basalt flows. After a basalt flow covered the trees, silica in groundwater slowly replaced the cell walls in the wood, turning it into petrified wood. Recent studies suggest that most of the trees at Gingko petrified forest washed into the area in a lahar or tree-and-mud-saturated flood from nearby highlands.

Discussion and debate among geologists as to what could have caused so much basaltic lava to erupt across the region during the Miocene epoch has yielded contradictory hypotheses. One hypothesis attributes the CRB eruptions to a meteorite impact. The meteorite hypothesis is a testable hypothesis. Large meteorite impacts produce a wide variety of evidence in the geologic record including "shocked" quartz grains and widespread drops of solidified molten material from the impact site. Large meteorites also form an impact crater structure that can be detected by indirect geologic evidence even if the crater is filled. None of these items of evidence for a meteorite is associated with the CRB eruptions, so the meteorite hypothesis can be ruled out.

The hotspot hypothesis is now accepted by many geologists as the explanation for what caused the CRB eruptions. This hypothesis relates the CRB eruptions to the same hotspot in the mantle that has shifted, as North America moved over it, to its current location underneath Yellowstone National Park. The hotspot has left a volcanic track across the Snake River Plain. The hotspot would have been near (though south of) the source area of the CRB at the time most of the basalt erupted. Hotspots are hypothesized to arise from deep in the mantle, deeper than the tectonic plates, most likely from the base of the mantle where they receive extra high heat from the outer core. According to the calculations that have been made for how such mantle plumes would work, the initial rise of a hotspot from deep within the earth to the earth's surface would produce an initial stage of widespread, high-volume basalt flows. The hotspot hypothesis is favored because the mantle plume model accounts well for the CRB eruptions and the volcanic track that leads from the initial hotspot center to the currently hot volcanic center at Yellowstone National Park.

Rocks and Minerals

Rocks are the foundation of the science of geology. Most of what we know about the geology and geologic history of the Pacific Northwest comes from studying rocks. Although geologic knowledge also comes from studying the geologic structures such as folds and faults that have bent or broken the rocks, and from studying fossils and minerals in the rocks, it all starts with rocks themselves.

To start learning about rocks, and the minerals that rocks are composed of, go to the Basics page on Rocks and Minerals.

Web Links

For more information about the Yellowstone caldera and Yellowstone-Snake River hotspot, visit the USGS CVO Web page: http://vulcan.wr.usgs.gov/Volcanoes/Yellowstone/description_yellowstone.html

For more information about Newberry shield volcano, visit the USGS CVO Web page: http://vulcan.wr.usgs.gov/Volcanoes/Newberry/framework.html

For more information about the Cascade Range volcanoes, visit the USGS CVO Web page: http://vulcan.wr.usgs.gov/Volcanoes/Cascades/framework.html

Glossary terms that appear on this page: composite cone; lava; ash flow; ash fall; tephra; pyroclastic; lahar; andesite; volcanic arc; subduction zone; plate tectonics; shield volcano; basalt; rhyolite; obsidian; caldera; felsic; tuff; mantle; mafic; bimodal volcanism

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