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Reading: Basics of Plate Tectonics

INTRODUCTION

The theory of plate tectonics revolutionized geology in the 1960s. By 1970, college geology majors were taught a set of ideas that were 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 (25–50 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 or mechanical behavior. In those terms, the outer layer of the earth is the lithosphere, which is a rigid layer that is broken up into tectonic plates and averages about 100 km (60 miles) thick. The layer beneath the lithosphere is the weak, soft asthenosphere, which is roughly 300 to 400 km thick. To understand plate tectonics, the different ways of classifying the layers of the earth-by composition and by mechanical behavior-must be kept in mind.

THE THEORY OF PLATE TECTONICS

Plate tectonic theory allowed geologists to understand the origins of and the relationships between: the world’s volcanic arcs and deep earthquake zones; exotic terranes and thrust fault zones; and transform faults and shallow earthquake zones. Plate tectonics also enabled geologists to explain 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, faults, and folds 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 gets peoples’ attention—volcanic eruptions, devastating earthquakes—happens because of how plates interact with each other along their boundaries.

The Pacific Northwest lies near the boundaries of several tectonic plates. The influence of these plates and their boundary interactions 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. Prior to the addition of continent the area west of Idaho was an ocean basin.

PALEOMAGNETISM

The earth, as you know, has a magnetic field. Some types of rock, when they originate, record the magnetism of the earth at the time the rock formed. This happens because magnetic minerals in the rock orient themselves, like little compasses, in the direction of the earth’s magnetic field, then are locked in place as the rock is lithified. After the rock has formed, as long as it does not get heated up to nearly its melting point—as long as it does not get above what is called the Curie point, the temperature at which the magnetism in the minerals is destroyed—it will retain that record of the earth’s magnetism at the time the rock formed. This provides the basis of paleomagnetism, the study of the magnetic record of the earth preserved in the rocks. Paelomagnetism has been a key for unlocking much of our modern knowledge of the geology of earth, especially the theory of plate tectonics.

The Magnetic North Pole on earth today is in far northern Canada, only approximately in the direction of the Geographic North Pole. The Geographic North Pole, which is also called the True North Pole, is the northern end of earth’s axis of rotation. The Geographic or True North Pole is not the same thing as the Magnetic North Pole and should not be mistaken for it. The Geographic North Pole is is at 90° N latitude, at the very top of the earth. The Magnetic North Pole is currently at about 85° latitude, but it wanders up to a degree every few years.

The magnetism of earth is created by convection and other movements of earth’s outer core. The outer core consists of hot, molten metal, mostly iron and nickel, in which many of the electrons move about freely among the atoms. The daily spinning around of this sea of molten metal and its virtually free electrons, combined with the convection of the molten core, creates the magnetic field of the earth. Unlike the magnetism of a bar magnet, the earth’s magnetic field is not very stable. The location of the Magnetic North Pole wanders many miles every year. Even more amazing, every several hundred thousand to several million years, the magnetism produced by earth’s core becomes so unstable, or “tangled up,” that it ends up reversing its north and south magnetic poles. There is no regular timing to when the earth’s magnetic field reverses itself. The last magnetic reversal was 780,000 years ago.

By the way, earth’s magnetic field is not very powerful in terms of its direct effects on living things. If it shut down right now, you would not feel anything. If you have played with bar magnets or touched old-fashioned televisions with cathode-ray tubes—the common type of TV and computer monitor before flat screens—while they were turned on, then you have been exposed to much more powerful magnetism than the earth’s magnetism, yet you did not feel that much stronger magnetism, either. As far as the geological record indicates, no species have gone extinct at any of the times when earth’s magnetic field reversed itself.

These reversals of the magnetic field of earth, and how long ago they took place, have been detected and measured from many rocks in the earth that preserve a record of the earth’s magnetism, including igneous rocks and sedimentary rocks from the floor of the ocean. Basalt flows are the strongest recorders of the earth’s magnetism, but other types of igneous rock and certain types of sedimentary rock also record earth’s magnetic field as they form. The ocean floor consists largely of basalt flows, which provide a strong enough record of earth’s paleomagnetism that it can be measured from a ship passing above. The ocean floor contains many layers of sediment and sedimentary rock that are relatively easy to recover from drill cores, which also recorded the magnetic field when they originated. Igneous and sedimentary rocks from the continents have also been used, along with fossil records, to study earth’s magnetic past.

Together, these various ways of studying paleomagnetism have verified and refined our knowledge of how the magnetic poles have wandered in the past, and when each reversal of earth’s magnetic field has taken place. This has allowed us to construct a timeline of magnetic reversals over the course of earth’s history. The accuracy and precision of the magnetic reversal timeline becomes much weaker for geologic ages more than about 200 Ma (millions of years ago). This is because nearly all older ocean floor has been subducted (recycled) back into the earth, and the paleomagnetic record from rocks on the continents is spottier—less continuous—than the paleomagnetic record derived from measurements of oceanic crust.

The paleomagnetism of rocks on the floor of the ocean was the key to unlocking the theory of sea floor spreading, an essential component of the theory of plate tectonics. As oceanic crust forms at the divergent plate boundaries and spreads away from there, it acts like a magnetic tape recorder, spreading a record of earth’s magnetism across the ocean floor. The magnetic reversals recorded in rocks on the ocean floor are still called magnetic anomalies because, when they were first discovered in the 1950s, neither sea floor spreading nor the fact that earth’s magnetic field has often reversed itself was yet realized. By knowing how long ago a particular magnetic reversal occurred, along with the distance of that magnetic isochron to the ridge where it originated, you can determine the rate at which the plate has been spreading away from the ridge. This is done by dividing the distance from the isochron to the ridge, by the number of years that have passed since that magnetic reversal occurred. In addition, the direction in which a plate has been moving can also be determined by analyzing the map patterns of its magnetic anomalies on the ocean floor.

Similarly, the paleomagnetism of rocks on the continents has been the key to unlocking continental drift, another building block of the theory of plate tectonics. The paleomagnetism of rocks on the continents is used to reconstruct the motions of continents across the face of the earth.

Paleomagnetism has also been one of the keys to unlocking the origins of many accreted terranes. If a part of a continent is suspected of being an accreted terrane, and it contains rocks with measurable paleomagnetism, the paleomagnetism may determine if the rocks did indeed originate far from their present-day location, on a tectonic plate separate from the continent, only to be moved in and accreted to the continent later. See also, the Basics page on Exotic Terranes.

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

Divergent Plate Boundaries

Most of the world’s divergent plate boundaries are on the ocean floor, in the form of mid-ocean spreading ridge. 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 igneousdikes. Some of it solidifies as gabbro intrusions 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 descending 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, there is much friction and 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 that separates the North American Plate from the Pacific Plate.

Convergent Plate Boundaries

Convergent plate boundaries are where two plates move toward each other. Subduction is a process that occurs at convergent plate boundaries. The western part of the Pacific Northwest is at a convergent plate boundary, and the effects of subduction have reached all the way across the Rocky Mountains to the edge of the Great Plains.

Depending on the type of 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 and stay there. Continent-continent convergent plate boundaries are not zones of subduction in the normal sense. 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 is diving back down into the mantle, beneath the edge of the 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 as it enters the subduction process. 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.

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 the 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 that forms above the region in the crust at which the subducting plate reaches a depth of 65 to 80 miles beneath the surface and an accretionary complex.

Convergent Plate Boundaries
Type Examples Landforms Geologic Processes

 


ocean-ocean

  • Aleutian Islands
  • West Indies
  • Mariana Islands
  • oceanic trench
  • island arc (chain of composite cone 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)
  • 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.

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 major 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 [GLOSS] of the overlying plate.

The Accretionary Complex

At 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 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 upward, culminating in volcanic eruptions at the earth’s surface.

The volcanic arc at an ocean-continent subduction zone is not only a chain of volcanoes. The stress of plate convergence compresses the 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 around its margins, 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 hot spots.

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.

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