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Plateau
Geology of the Pacific Northwest

Basics -- Plate Tectonics

Introduction

The theory of plate tectonics revolutionized geology in the 1960s. By 1970, college geology majors were taught a set of ideas unheard of by most geology students prior to 1960. The foundation for the new way of comprehending earth processes is the understanding that the outer layer of the earth is the lithosphere rather than simply the crust.

The Layers of the Earth

The earth is layered in terms of chemical composition as follows:

  1. The outer layer is the crust. Continental crust is thick (25-70 km thick), low in density, and has an intermediate average composition; oceanic crust is thin (typically 5-10 km thick), higher in density, and has a mafic average composition.
  2. The mantle consists of dense, ultramafic rock.
  3. The core consists of a mixture of iron and nickel.

The earth is also layered in terms of physical behavior - how rigid a layer is, how soft it is. Of the physically defined layers of the earth, it is the lithosphere and the asthenosphere that are most important for plate tectonics.

  1. The lithosphere is the outer physical layer of the earth. It is a rigid layer consisting of the tectonic plates. The lithosphere averages about 100 km (60 miles) thick. The lithosphere is thickest - up to about 200 km (120 miles) thick - beneath the old interiors of continents.
  2. The asthenosphere is the layer beneath the lithosphere. The asthenosphere is between 500 and 650 km (300 and 400 miles) thick. The most important characteristic of the asthenosphere is that is the weakest layer of the solid mantle. Although the asthenosphere is solid, it is a "soft" solid, which flows at a geological rate.

The lithosphere consists of the tectonic plates, and the softness and flow of the asthenosphere helps enable and empower the movements of the plates.

To understand plate tectonics, the different ways of classifying the layers of the earth - by composition and by physical behavior - must be kept in mind.

The Theory of Plate Tectonics

Plate tectonic theory allowed geologists to understand the connections between the world's volcanic arcs and deep earthquake zones; exotic terranes and thrust fault zones; and transform faults and shallow earthquake zones. Now that we realize these plate tectonic connections, we can explain the origins of the volcanic arcs, earthquakes, and exotic terranes.. Plate tectonics also allows us to account for the origins of the oceanic crust and the continents.

According to plate tectonic theory, the lithosphere is divided into rigid plates that interact with one another at their boundaries. Earthquakes, faulting, and folding take place at these boundaries. Voluminous igneous intrusions and frequent volcanic eruptions occur at two of the major types of plate boundaries. In sum, most (though not all) of the earthquakes and volcanic eruptions that take place in the world happen in association with plate boundaries. Much of the action in geology that you read about in the news - devastating earthquakes which in some cases set off giant ocean waves and volcanic eruptions that bury countrysides, divert airplane flights, and cloud the atmosphere with aerosols - occur because tectonic plates interact with each other along their boundaries.

The boundaries of several tectonic plates are close to the Pacific Northwest. The motion of these plates, and how the plates interact along their boundaries, underlies the major geological themes of the region, including the uplift of the Coast Ranges, the formation of the Puget-Willamette Lowland, and the volcanism of the Cascade Range. Plate boundary processes also explain how most of the land of Washington and Oregon has come to be part of North America in the last 200 million years. Since the Pacific Northwest became an active plate boundary zone about 200 million years ago, the North American continent has grown considerably in the region. Most of Oregon and approximately 2/3 of Washington state have come into existence as part of North America during this time.

Plate Boundaries

There are three general types of plate boundaries:

  1. divergent plate boundaries, where two plates move away from each other
  2. transform plate boundaries, where two plates move horizontally side-by-side in opposite directions
  3. convergent plate boundaries, where two plates move toward each other and either collide with each other or one plate bends down and goes beneath the other

(Follow this link to a Basics Table that summarizes plate boundary information, including the map symbol for each type of boundary.)

Divergent Plate Boundaries

Most of the world's divergent plate boundaries are on the ocean floor, in the form of mid-ocean spreading ridges. At divergent boundaries, the two plates are continually moving apart, heading in opposite directions away from each other. The divergence causes normal faults and rift valleys (grabens) to form there as a result of the tension in the crust. In other words, in response to getting pulled apart by tectonic forces, the crust cracks apart and sections of it drop down into rift valleys.

At a divergent plate boundary, the spreading crust forms channels through which magma rises from the mantle. Some of the magma erupts on the ocean floor and builds up piles of pillow basalt. Some of it solidifies within the cracks, beneath the surface of the crust, forming igneous dikes. Some of it solidifies as intrusions of gabbro deeper in the crust. At the places where the magma pools within the crust, olivine and other dense minerals settle into layers at the bottom of the pools and form layered mafic and ultramafic igneous rocks.

All these eruptions and intrusions solidify and become new oceanic crust, which moves away from the mid-ocean spreading ridge and makes way for yet more magma to rise and continue the process. Creation of oceanic crust is part of a continual process that occurs at divergent plate boundaries on the ocean floor. The new oceanic crust is part of a moving tectonic plate. It continues to move as part of the ocean floor and will eventually collect layers of sediment settling out from the water above.

Transform Plate Boundaries

Transform plate boundaries are strike-slip faults that separate tectonic plates which are moving parallel to each other but in opposite directions. Tectonic plates average about 100 km in thickness. As the two plates slide next to each other, trying to move in opposite directions, friction creates stress between them. As a result, transform plate boundaries are zones of frequent earthquakes.

Most transform plate boundaries are on the ocean floor, in the oceanic crust, connecting segments of mid-ocean spreading ridges. However, in a few places transform plate boundaries cut through continental crust. The most famous example is the San Andreas Fault in California, which is a transform plate boundary between the North American Plate and the Pacific Plate.

Convergent Plate Boundaries

Two plates move toward each other at convergent plate boundaries. Subduction is a process that occurs at convergent plate boundaries. The Pacific Northwest coast is at a convergent plate boundary with subduction.

The subduction zone of the Pacific Northwest, which is called the Cascadia subduction zone, currently extends from the Pacific coast inland to the Cascades volcanic arc and runs north-south from California to British Columbia. Some of the effects of subduction and plate interactions along the coast reach all the way across the Rocky Mountains to the edge of the Great Plains.

Depending on the type of earth's crust that composes the upper part of each plate, there are three types of convergent plate boundaries: continent-continent, ocean-ocean, and ocean-continent.

Continent-Continent Convergent Plate Boundaries

Continental crust is too low in density to go down into the mantle. Continent-continent convergent plate boundaries are not zones of subduction. Instead, the two continents collide with each other, folding, thrust faulting, and building upward into a high, wide mountain range. The Himalayas in south central Asia are an example of a continent-continent convergent plate boundary.

Although large earthquakes occur in association with continent-continent convergent plate boundaries, there are no volcanoes. Mountain ranges such as the Himalayas do not have volcanoes because there is no oceanic plate subducting beneath them.

Ocean-Ocean Convergent Plate Boundaries

At ocean-ocean convergent plate boundaries, as the two plates with oceanic crust converge, one goes down beneath the other and into the mantle. This zone where a plate with oceanic crust on it is diving back down into the mantle, beneath the edge of an adjacent plate, is called a subduction zone.

The outer edge of a subduction zone is an oceanic trench, which forms where the subducting plate bends and pushes downward into the earth. Oceanic trenches at ocean-ocean subduction zones are the deepest places in the ocean. Island arcs, which are composite cone volcanoes arrayed in the form of an island chain, are also associated with ocean-ocean convergent plate boundaries. The Aleutian Islands of Alaska are an example of an island arc. The island arc, the chain of volcanoes, forms where the subducting plate reaches a depth of about 100 km (60 miles) in the earth.

Ocean-Continent Convergent Plate Boundaries

At an ocean-continent convergent plate boundary, the plate that carries oceanic crust subducts into the mantle beneath the edge of a continent. Ocean-continent convergent plate boundaries are similar to ocean-ocean subduction zones, but the much thicker continental crust leads to a greater range of geological features, including a volcanic arc and an accretionary complex. Just as with ocean-ocean subduction zones, the volcanic arc forms where the subducting plate reaches a depth of about 100 km, which is usually in the range of 100-200 km inland from the trench where subduction begins.

Table of Convergent Plate Boundaries

Type Examples Landforms Geologic Processes
ocean-ocean
  • Aleutian Islands
  • West Indies
  • Mariana Islands
  • oceanic trench
  • island arc (chain of composite cone volcanic islands in the ocean)
  • subduction
  • major deep earthquakes
  • shallow earthquakes
  • volcanism
  • igneous intrusion
ocean-continent
  • Andean subduction zone
  • Cascadia subduction zone
  • oceanic trench
  • accretionary complex
  • forearc basin
  • volcanic arc (chain of composite cones on continent)
  • subduction
  • major deep earthquakes
  • shallow earthquakes
  • volcanism
  • igneous intrusion
  • terrane accretion
  • orogeny
continent-continent
  • Himalayas
  • Alps
  • broad, high mountain range (no volcanoes)
  • metamorphism deep in thickened crust
  • thrust faulting
  • shallow earthquakes
  • folding
  • mountain building

Subduction Zones

Because the convergent plate boundary along the Northwest coast is a subduction zone, we need to examine the parts of a subduction zone in a little more detail.

subduction zone diagram

The Oceanic Trench

Most subduction zones start at an oceanic trench, where the subducting plate begins the process of bending and pushing downward. The apparent lack of an oceanic trench off the Northwest coast is an anomaly. To some extent, there may be a trench that has been filled in with the abundant sediments dumped onto the continental shelf by the Columbia River and other rivers that drain to the Pacific Coast.

Deep Earthquakes (Subduction Earthquakes)

Another characteristic of subduction zones is that they have powerful earthquakes that occur within the subducting plate, as it forces its way down into the mantle. The most powerful earthquakes on earth are these earthquakes in subducting plates. The stress of the subduction process also causes shallower earthquakes to take place in the continental crust of the overlying plate.

The Accretionary Complex

At most ocean-continent subduction zones, the leading edge of the continent is the site of an accretionary complex, also called an accretionary prism or accretionary wedge. An accretionary complex is an elevated zone built up of pieces of oceanic crust or lithosphere that were accreted from the subducting plate onto the edge of the continent along reverse faults. Accretionary complexes tend to build up high enough to form coastal mountain ranges. However, unlike the main volcanic arc mountain range, accretionary complex coast ranges are not volcanic.

The Forearc Basin

Between the accretionary mountain range and the volcanic arc is the forearc basin, a low area into which rivers drain and which may contain an arm of the ocean.

The Volcanic Arc

All subduction zones have, at some distance in from the leading edge of the upper plate, arcs or chains of composite cone volcanoes. The subducting plate, as it goes down deep into the mantle, releases water. This changes the chemistry of the already hot rocks in the mantle and causes them to melt, forming magma. The magma is less dense than the solid rocks around it, so it rises, culminating in volcanic eruptions and the build-up of volcanoes at the earth's surface.

In addition to the volcanoes in a volcanic arc at an ocean-continent subduction zone the stress of plate convergence compresses the continental crust there, causing it to thicken through a combination of folds and thrust faults. Igneous intrusions and volcanic eruptions also thicken the crust there. Deep within the crust, the igneous intrusions solidify into batholiths of rocks such as granite, and the pre-existing rocks that are intruded by the batholiths are regionally metamorphosed into new rocks. The result is a high mountain range with granitic and metamorphic rock at its core, folded and faulted sedimentary and volcanic rocks outside the zones that have been exposed by deep uplfit and erosion, and a chain of composite cone volcanoes distributed along the crest of the range.

Terrane Accretion

A large tectonic plate, such as the Pacific Plate, carries more than oceanic crust. It also carries island arcs and oceanic plateaus, which are zones of unusually thick oceanic crust. Large island complexes such as the islands of Japan, which were built by the assemblage of several island arcs, also ride on tectonic plates. Other plate passengers include ocean islands such as the Hawaiian Islands, which build from volcanic eruptions that emanate from mantle hotspots.

As the oceanic plate carrying these larger pieces of crust comes into an ocean-continent subduction zone, the island arcs, oceanic plateaus, island complexes, and oceanic islands will not go down the subduction zone. Instead, they will be plastered to the edge of the continent, becoming accreted terranes. Examples of all these types of crust, swept in and accreted to North America by a subducting oceanic plate, can be found in the Pacific Northwest.

In addition, the subduction process also can accrete pieces of normal oceanic crust onto continents. Sometimes the faulting caused by accretion of large blocks of rock cuts down into the lithosphere and thrusts mantle rock along with crustal rock from the ocean floor, causing the emplacement of an ophiolite - a piece of oceanic lithosphere that includes part of the mantle - onto a continent.

Web Links

For an illustrated review of the basics of plate tectonics, go to the online primer from the US Geological Survey at http://pubs.usgs.gov/publications/text/dynamic.html

For a review of plate tectonics that includes some more about how the theory was developed, and how continents have moved across the face of the globe during the course of earth history, go to the University of California Museum of Paleontology site at http://www.ucmp.berkeley.edu/geology/tectonics.html

Glossary terms that appear on this page: plate tectonics; volcanic arc; exotic terrane; thrust fault; transform fault; oceanic crust; lithosphere; crust; intrusion; divergent plate boundary; transform plate boundary; convergent plate boundary; mid-ocean spreading ridge; graben; tension; tectonic; magma; mantle; pillow basalt; dike; gabbro; mafic; ultramafic; strike-slip fault; stress; subduction; oceanic trench; island arc; composite cone; orogeny; continental crust; accretionary complex; reverse fault; forearc basin; compression; batholith; regional metamorphism; metamorphic rock; sedimentary rock; volcanic rock; ocean island; hotspot


Geology of the Pacific Northwest
Basics--Plate Tectonics
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 7/14/13