Welcome to Week 4 of Pacific Northwest Geology. The topics of this week's lecture are:

• Plate Tectonics
  • Theory of Plate Tectonics
  • Plate Boundaries-Where the Action Is
  • Plate Tectonics in the Pacific Northwest
  • Leading Edge
  • Accretionary Complex
  • Forearc Basin
  • Volcanic Arc
  • The Backarc
• The Rock Cycle
  • Magma
  • Igneous Rocks
  • Sediments
  • Sedimentary Rocks
  • Metamorphism and Metamorphic Rocks
• Web Links
• Glossary Terms

Related Basics Pages: Plate Tectonics; Rock Cycle; Rocks and Minerals
Related Focus Page: #6--Plate Tectonics of the Pacific Northwest

Plate Tectonics

The theory of plate tectonics, which came together in the 1960s and revolutionized the science of geology, has solved many of the geologic conundrums of the Pacific Northwest. Although there are still many unanswered geologic questions about the region, plate tectonics has shed light on most of the essential aspects of Pacific Northwest geology. Geological features of the Northwest explained by plate tectonic theory include the existence of the Coast Ranges and Cascade Range, the volcanic activity of the Cascade Range, the major earthquakes that occur west of the Cascades, and how pieces of crust from faraway places came to be part of the local landscape. (We will explore this aspect of Pacific Northwest geology, the exotic pieces of the crust, in greater detail next week.)

Theory of Plate Tectonics

The theory of plate tectonics states that the outer, rigid layer of the earth, the lithosphere, consists of about 12 large plates along with a few smaller ones, and each plate is moving, driven by heat coming out of the interior of the earth. The lithosphere averages about 100 km (60 miles) in thickness. The layer in the mantle beneath the lithosphere is called the asthenosphere. Unlike the strong, rigid lithosphere, the asthenosphere is a layer of weak rock, on the verge of melting. Rock in the asthenosphere can flow sort of like putty or soft plastic. Circulation of the soft asthenosphere, as it brings heat out of the deeper interior of the earth, drives the tectonic plates of the lithosphere in their slow movements across the face of the earth.

Plate Boundaries-Where the Action Is

Where plates meet up with each other, they either diverge (move away from each other), converge (move toward each other), or transform (slide side-by-side in opposite directions, moving neither toward nor away from each other). Profound geologic processes take place at these plate boundaries, including:

• the growth of oceanic crust
• the growth of continental crust
• most of the world's volcanoes
• most of the world's earthquakes
• all of the world's major mountain ranges

Plate Tectonics in the Pacific Northwest

The most important thing to know about the plate tectonics of the Pacific Northwest is that the corridor from the Pacific Coast to the Cascade Mountains is a subduction zone, the Cascadia Subduction Zone. In this convergent plate boundary the Juan de Fuca plate, with its carapace of oceanic crust, is subducting beneath the edge of the North American plate, which carries the North American continent.

Leading Edge

According to plate tectonic theory and in accord with a wide range of geologic evidence, the moving Juan de Fuca Pate bends and begins entering the Cascadia Subduction Zone just off the coast of the Pacific Northwest. The leading edge of the Cascadia Subduction Zone is unusual in not having an oceanic trench. All other subduction zones have a deep trench at their leading edge. Two possible explanations for the apparent lack of a trench are: (1) the Columbia River and other coastal rivers have discharged so much sediment offshore that the trench has been filled in, and (2) the Juan de Fuca plate is converging on North America at a relatively slow rate, which may form a trench that is shallower than usual in the first place.

The coastal portion of the Cascadia Subduction Zone, between the leading edge of the continent and the Cascade Mountains, is where the most powerful earthquakes in the Pacific Northwest occur. These are subduction earthquakes. They result from the Juan de Fuca plate forcing its way deeper into the mantle. The most recent large earthquake that occurred in the subducting plate was the Nisqually earthquake in February of 2001, which knocked down parts of buildings in Olympia and Seattle.

Even larger earthquakes disrupt the coast of the Pacific Northwest every 200 to 600 years, according to the geologic record. The last one occurred on the coast 300 years ago. This is relatviley new knowledge, uncovered by geologists researching sedimentary deposits along the coast since 1985, and it has caused government planners to revise emergency preparedness plans and building codes in the area to be better prepared for the next big earthquake.

Accretionary Complex

The coast ranges, including the Olympic Mountains and Willapa Hills in Washington, the mountains of Vancouver Island in British Columbia, and the Oregon coast ranges, are an accretionary complex--pieces of oceanic crust that have been shoved up onto the leading edge of the North American continent. There are no volcanoes in the Coast Ranges. The Coast Ranges have major faults along which contorted pieces of oceanic crust have been brought up onto the continent by plate convergence.

Forearc Basin

In a subduction zone the forearc basin is the low region that lies between the accretionary complex and the volcanic arc. In the Cascadia Subduction Zone in Washington State the forearc basin is the Puget Sound Lowland. The Puget Sound Lowland lies between the accretionary complex of the Olympic Mountains and the volcanic arc of the Cascade Range.

In Oregon, the forearc basin continues south in the guise of the Willamette Valley.

Volcanic Arc

The Cascade Range is the volcanic arc of the Cascadia Subduction Zone. As with all volcanic arcs associated with subduction zones, it is a chain of composite cone volcanoes (also called stratovolcanoes). It is also a zone where the crust is compressed and thickened by the pressures of plate convergence, and the deeper parts of the crust have been intruded by magmas and metamorphosed by all the heat and tectonic pressure.

The North Cascades in Washington State have undergone so much uplift and erosion that deep crustal rocks, including plutonic rocks and high-grade metamorphic rocks, are now exposed at the surface across much of the range. The active composite cones of Mt. Baker and Glacier Peak cover relatively small spots in the North Cascades.

In contrast, in the South Cascades in Washington State, and in all of the Cascade Range in Oregon and Northern California, most of the rocks at the surface are volcanic. In those parts of the Cascade Range, most of the plutonic igneous rocks and regional metamorphic rock are still buried beneath the volcanic cover.

The Backarc

The Columbia Plateau in Washington and the Central Oregon Plateau are in the backarc of the Cascadia Subduction Zone, behind the volcanic arc. It is common in subduction zones to have some volcanic activity in the backarc, along with minor rifting of the crust.

It has been proposed that backarc rifting and volcanism is what caused the Columbia River Basalts to form. However, it is not common for such voluminous basalt eruptions to occur in a backarc setting, and more recently the favored hypothesis for the origin of the Columbia River Basalts is the hot spot hypothesis. By this hypothesis, the same hot spot that underlies Yellowstone National Park was responsible for the Columbia River Basalts and may have set off much of the volcanism in eastern Oregon as well.

The Rock Cycle

James Hutton first laid out the rock cycle concept in detail in the late 1700s. A key idea of the rock cycle is that all geological materials can be, and are, transformed into other geological materials through the systematic processes of the geological earth.

Magma

In the rock cycle, molten rock-magma-is the starting point. Rocks can be caused to melt and form magma by several means. One is by raising the temperature high enough. If mafic magma, which is the hottest type of magma and originates by melting of the upper mantle, rises and intrudes the continental crust, it may raise the temperature of the rock in the crust enough to cause it to melt. If the mafic magma from the mantle mixes with the felsic magma that forms by melting of the crust, intermediate magma may result. This may be how most andesite and granodiorite forms.

Hot rocks in the mantle (and all rocks in the mantle are hot) can be caused to melt by the addition of water. A subducting plate with oceanic crust probably releases a lot of water at depth in the mantle. This water may then cause the mantle rocks to melt, forming mafic magma that rises up into the crust above. This may be why all subduction zones have chains of volcanoes above the location where the subducting plate reaches a depth in the mantle of about 50 miles, which may be the depth at which the plate releases most of its water.

A third way that hot rocks in the mantle can be made to melt is to lower the pressure. Beneath divergent plate boundaries the hot rocks of the asthenosphere are rising up. As the flowing asthenosphere rises to shallower depths it encounters lower pressure. This causes the mantle rocks in the rising asthenosphere to melt and send mafic magma up. The mafic magma erupts on the ocean floor at divergent plate boundaries to form pillow basalt.

Igneous Rocks

Igneous rock is solidified magma. If the magma solidifies at depth within the crust it forms plutonic rock. If it erupts to the surface of the earth and solidifies there it forms volcanic rock.

Sediments

All rocks at the earth's surface are altered by changes in temperature, reactions with oxygen and other gases, reaction with water in all states (solid, liquid, and gas), biological activities such as plant roots and bacteria in the soil, the physical force of flowing water, and the force of gravity. These chemical reactions and physical forces cause rocks to weather--to break down physically and chemically. The weathered materials are eroded and removed as sediment. The sediments are deposited, usually in layers in a basin at some distance from where they were first weathered and eroded.

Some of the chemical elements in rocks and minerals dissolve in water. Elsewhere, these dissolved elements may crystallize and separate back out of the water, forming chemical sediments such as calcium carbonate on the ocean floor, or salt on the floor of an evaporating lake. Some chemical sediment forms through a biological intermediary, such as silica being used by diatoms to form their tiny "shells." The accumulated remains of dead diatoms then form a layer of chemical sediment made of silica.

Another example of a chemical sediment with biological intermediation is carbonate sediment, sediment that turns into limestone. Most carbonate sediment, which turns into the minerals calcite and dolomite, forms from ocean-floor accumumulation of the hard parts of organisms. Carbonate sediments are commonly microscopic bits and pieces of dead organisms, but some are larger -- some limestones are actually lithified coral reefs.

Some sediments are weathered, fractured, and eroded pieces of solid rocks and minerals. These are the clastic sediments. Sand, gravel, boulders, silt, and fine mud made essentially of clay, are all clastic sediments. Of the common minerals that occur in rocks, quartz is the most resistant to chemical and physical weathering, so clastic sediments subjected to long-term weathering processes will tend to become rich in quartz. That is why many beach sands are rich in quartz.

Clay is a mineral that is stable in wet conditions at the earth's surface, and it forms by weathering and chemical alteration of other common minerals such as the feldspars. As a result, clay is a very common type of clastic sediment.

Sedimentary Rocks

When sediment is buried beneath more sediment, eventually it is likely to be compacted by the increased pressure, cemented together by the pressure and heat, and lithified into sedimentary rock.

Metamorphism and Metamorphic Rocks

A rock inside the earth that is subjected to a large enough change in conditions, such as higher temperature, higher pressure, or hot fluids percolating through the rock, will change into a new set of minerals that are stable in the new conditions. This recrystallization into new minerals is called metamorphism and changes the rock into a new type of rock, a metamorphic rock.

Most metamorphism in the earth's crust takes place at two plate tectonic locations. The first plate tectonic location where much metamorphism takes place is divergent plate boundaries on the ocean floor, also known as the mid-ocean spreading ridges. There, seawater percolates down into the crust, is heated by the magma rising from the mantle toward the spreading ridge, and is circulated back up through the crust by the rising heat. This heat and hot water in the rocks cause the rocks to metamorphose.

The second plate tectonic location where much metamorphism occurs is at subduction zones, including the accretionary complexes and volcanic arcs associated with subduction zones. In the subducting plate and at the base of the accretionary complex rocks are quickly shoved deep into the earth, subjecting them to relatively high pressure but not very high temperatures. As a result, metamorphic rocks at the leading edge of the subduction zone and in the accretionary complex tend to have minerals stable at relatively high pressures and low temperatures in the earth.

In the volcanic arc of a subduction zone, the rocks deep in the crust are metamorphosed by heat from all the magma rising into the crust beneath the volcanoes, along with the pressure from the compressive forces of converging plates. As a result, metamorphic rocks that form in the volcanic arc tend to have minerals stable at higher temperatures along with higher pressures.

Web Links

For an illustrated primer on the theory of plate tectonics, see the USGS Web site called This Dynamic Earth
http://pubs.usgs.gov/publications/text/dynamic.html

For diagrams and maps of the plate tectonic situation in the Pacific Northwest, look at the following Web pages:
http://www.geophys.washington.edu/SEIS/PNSN/INFO_GENERAL/eqhazards.html (UW Geophysics Department)
http://www.pgc.nrcan.gc.ca/seismo/eqinfo/eq-westcan.htm (Geological Survey of Canada)
http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/map_plate_tectonics_cascades.html (USGS)

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Glossary terms that appear on this page: plate tectonics; lithosphere; asthenosphere; subduction zone; accretionary complex; forearc basin; volcanic arc; composite cone; magma; plutonic rock; metamorphic rock; backarc; basalt; hot spot; mafic; felsic; intermediate; andesite; granodiorite; pillow basalt; igneous rock; volcanic rock; chemical sediment; clastic sediment; sedimentary rock; mineral