Welcome to Week 3 of Pacific Northwest Geology. The topics of this week's lecture are:
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. There are also landscape regions in the Pacific Northwest that are filled with lava or ash from earlier epochs of geologic time. Even in those regions where the volcanoes are extinct, the volcanic rocks still dominate the landscapes and their geologic history. To read about the different types of volcanoes and volcanic landforms go to the Basics page on volcanoes.
Cascade Range composite cones
The volcanoes that are known to be 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 flows that surges from the side of the volcano down into its river drainages.
The type of rock that is most typical of composite cones like those in the Cascade Range is andesite. If andesite is found in the geology of an area, it can be inferred that in the geologic history of that area there were once volcanoes a lot like the modern Cascade volcanoes, even if the volcanoes themselves have long since eroded away.
The Cascade Range has been a zone of active volcanoes for tens of millions of years. Individual volcanoes have come and gone. Composite cones are usually active for about a million years. Some last longer and some last a shorter time. Lava and ash started piling up to form Mt. Rainier roughly a million years ago. In contrast, the oldest volcanic rocks associated with Mt. St. Helens are only about 80,000 years old.
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, the Pacific Northwest has three 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.
Yellowstone Hot Spot
Most of the southern half of Yellowstone National Park, including Old Faithful geyser, is in a volcanic crater. It formed several hundred thousand years ago, during the Pleistocene, 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 and had depths up to a thousand feet or so. 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 caldera of Yellowstone National Park. Geologic studies have shown that a chain of calderas, like 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 hot spot beneath the moving continent. The hot spot is now located beneath Yellowstone National Park and will presumably 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 each mountain range. The faults cut deep and the stretched crust in the area has become thinner, bringing the mantle closer to the surface The deep faults and thin crust may be why basalt flows have erupted from fissures in the Basin and Range region even though there are no volcanoes. Basalt forms from mafic magma, which comes from molten mantle rocks.
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.
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.
The Columbia River Basalts
A great volume of basalt stacked in layers like pancakes, one on top of another, forms the Columbia Plateau. These are uncommonly thick, far-reaching basalt flows. Some 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 Columbia River Basalts (referred to by geologists as "the CRBs") began erupting 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 or 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 made of a form of quartz.
There has been discussion and debate among geologists as to what could have caused so much basaltic lava to erupt across eastern Washington during the Miocene epoch. One hypothesis is that it was set off by a meteorite impact. The meteorite hypothesis is a testable hypothesis. Large meteorite impacts produce a wide variety of evidence in the geologic record, from "shocked" quartz grains to widespread drops of solidified molten material from the impact site, and other items of evidence too numerous to detail here. 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 CRBs, so the meteorite hypothesis can be ruled out.
The hot spot hypothesis is most favored by geologists for what caused the CRBs. This relates the CRBs to the same hot spot in the mantle that caused Yellowstone National Park to acquire its volcanic character and created a hot spot track across the Snake River Plain. The hot spot would have been near (though somewhat south of) the source area of the CRBs at the time most of the basalt erupted.
Rocks and Minerals
Rocks are the foundation not only of the mantle and crust of the earth, but also of the science of geology. Most of what we know about the geology and geologic history of the Pacific Northwest comes from studying rocks. 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 in the rocks. Nevertheless, it all starts with rocks.
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 hot spot, 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; plate tectonics; volcanic arc; shield volcano; basalt; rhyolite; obsidian; felsic; tuff; caldera
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Lecture #3
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 10/14/01