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, revolutionized the science of geology and profoundly influenced our understanding of the geologic history 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, such as 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, which is called the lithosphere, consists of about 12 large plates and a few smaller plates. 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 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, helps drive 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 on its back.

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 in the world have a deep trench at their leading edge. Several factors have been proposed to explain the apparent lack of a trench off the Pacific Northwest coast. Each factor may play a role. One factor may be that the Columbia River and other coastal rivers have deposited so much sediment offshore that the trench has been filled in. Another notable factor about the Juan de Fuca plate is its slow rate of convergence with North America, which may form a trench that is shallower than other subduction zones.

The coastal portion of the Cascadia subduction zone, between the leading edge of the continent and the Cascade Mountains, is the location of the most powerful earthquakes in the Pacific Northwest. These are subduction earthquakes. They result from the Juan de Fuca plate forcing its way deeper into the mantle. The most recent major 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 300 to 600 years, according to the geologic record. The last one occurred on the coast 300 years ago, in 1700 AD, abruptly shifting the elevation of parts of the coast by up to several m (on the order of 10 ft) and creating a great tsunami (a great surge of ocean water) that inundated coastlines. The occurrence of the great Cascadia subduction earthquake in 1700 is relatively new geological 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 and, along the coast, a possible subsequent tsunami.

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 plastered against the leading edge of the North American continent. There are no volcanoes in the Coast Ranges. The Coast Ranges have major faults along which pieces of oceanic crust have been thrust into 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. The Willamette Valley is the southern extension of the forearc basin in Oregon.

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 stress of plate convergence, and the deeper parts of the crust have been intruded by magmas and metamorphosed due to the added heat and 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 most of the rocks at the surface are volcanic, just as they are in all of the Cascade Range in Oregon and Northern California. In the southern parts of the Cascade Range, most of the plutonic igneous rocks and regional metamorphic rock are presumably 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. Subduction zones commonly have some volcanic activity in the backarc, along with minor rifting of the crust.

Some geologists have proposed that backarc rifting and volcanism is what caused the Columbia River Basalt to form. However, the Columbia River Basalt is one of the largest outpourings of basalt eruptions known to exist on earth, in terms of volume and area. Such wide, thick volumes of basalt are known as flood basalt. Flood basalt is not thought to to occur as a result of plate-teconics-driven backarck volcanism. The hotspot hypothesis for the origin of the Columbia River Basalts has come to be supported by most geologists. By this hypothesis, the same hotspot that underlies Yellowstone National Park now was responsible for the Columbia River Basalts and related volcanism in eastern Oregon in the Miocene epoch.

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 geological processes of the earth.

Magma

In the rock cycle, molten rock, magma, is the starting point. Rocks can be caused to melt and produce 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. When mafic magma from the mantle mixes with felsic magma that forms by melting of the crust intermediate magma is the result. Intermediate magma is the predominant magma type in volcanic arcs at subduction zones like the Cascade Range.

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 releases a lot of water at depth in the mantle. This water then causes the mantle rocks to melt, forming mafic magma that rises up into the crust above. This could be why all subduction zones have chains of volcanoes above the location where the subducting plate reaches a depth in the mantle of about 100 km (60 miles), as that 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. Magma is less dense than solid rock, so a sufficient drop in pressure on hot rock at depth in the earth, which previously had been constrained from melting by the high pressure inside the earth, will enable the rocks to start melting. Beneath divergent plate boundaries hot rock of the asthenosphere rises. As the flowing rock of the asthenosphere rises to shallower depths it encounters lower pressure. This causes the rock of the rising asthenosphere to melt. The resulting mafic magma rises, cools, and solidifies into igneous rock, forming new oceanic crust at divergent plate boundaries.

Igneous Rocks

Igneous rock is solidified magma. If the magma solidifies at depth within the crust it forms plutonic rock such as granite or granodiorite. If it erupts to the surface of the earth and solidifies there it forms volcanic rock. Some volcanic rocks, such as andesite and basalt, form from solidified lava flows. Other volcanic rocks form from solidified pyroclastic material from explosive eruptions, such as volcanic ash which solidifies into a volcanic rock known as tuff.

Sediments

All rocks at the earth's surface are altered, or "broken down," by changes in temperature, chemical reactions with air and water, biological activities such as plant roots and bacteria in the rocks and soil, and the force of gravity. These chemical reactions and physical forces cause rocks to weather, to break down physically and chemically. The result is sediment. Erosion and transport removes sediment and deposits it, usually in layers and usually at some distance from where it was first weathered and eroded. Two classes of sediments arise from these processes: clastic and chemical.

Clastic sediments are weathered, fractured, and eroded pieces of solid rocks and minerals. 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, which is why 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.

Some of the chemical elements in rocks and minerals dissolve in water, leading to chemical sediments. Chemical sediments form when these dissolved elements crystallize and separate back out of the water, forming sediments such as calcium carbonate on the ocean floor or a layer of salt on the floor of an evaporating lake.

Some chemical sediment forms through a biological intermediary, such as siliceous sediments made of the tiny "shells" of diatoms, and carbonate sediments consisting of the accumulated hard parts of organisms made of the carbonate minerals calcite or dolomite.

Sedimentary Rocks

When sediment is buried deep enough beneath more sediment, it will be compacted by the increased pressure, cemented together by the pressure and heat, and lithified into sedimentary rock. Clastic sedimentary rock is lithified clastic sediment. Chemical sedimentary rock is lithified chemical sediment.

Metamorphism and Metamorphic Rocks

Any type of rock - igneous, sedimentary, or metamorphic - that is subjected to a large enough change in conditions will recrystallize. It grows a new set of minerals that are stable in the changed conditions. Metamorphic recrystallization is driven by higher temperature, higher pressure, or hot fluids percolating through the rock. The recrystallization is called metamorphism and transforms the rock into a metamorphic rock. A rock does not melt for metamorphism to occur.

Rocks adjacent to an igneous intrusion inside the crust are metamorphosed by the heat from the magma without a significant change in pressure. This is called contact metamorphism.

Rocks buried, stressed, and heated as a result of tectonic processes acting across a region will experience increased temperature, increased pressure, and directed stress - stress that pushes or pulls more in one direction than another. This is called regional metamorphism. Regional metamorphic rocks develop distinctive types of metamorphic layering due to the directed stress that acts on them as they recrystallize. Regional metamorphic rock, including gneiss and schist, is characteristic of the cores of mountain ranges and the "basement" level of the continent crust (along with plutonic igneous rock such as granite or granodiorite).

Most metamorphism in the earth's crust takes place at two plate tectonic locations - divergent plate boundaries and convergent plate boundaries. At divergent plate boundaries on the ocean floor, 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.

Convergent plate boundaries, particularly subduction zones, are major sites of regional and contact metamorphism. In the subducting plate and at the base of an 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 in the accretionary complex of a subduction zone 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 website called This Dynamic Earth
http://pubs.usgs.gov/publications/text/dynamic.html

For a diagram and map of the plate tectonic situation in the Pacific Northwest, look at the following web page:
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; hotspot; mafic; felsic; intermediate; igneous rock; volcanic rock; clastic sediment; mineral; chemical sediment; diatom ; carbonate mineral; lithify; sedimentary rock;

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