The Dynamic Earth: A Geologic Primer

Geologic processes that operate on a global scale have profoundly affected the Pacific Northwest. By understanding just a few of these processes, you’ll be equipped to tackle Washington’s geologic history.

We place special emphasis on the basic processes of plate tectonics responsible for the evolution of Washington and the Pacific Northwest.  Finally, we introduce the geologic time scale – a chronology of Earth and life history.

The beautiful blue planet.  The Earth is a giant heat

engine constantly working to expel its inner heat into

the cold universe that surrounds us.  (Image:  NASA)

Internal Structure of the Earth

In composition, the Earth is made of three nested spheres.  At the center of the Earth, a body of intensely hot iron and nickel forms the core.  A less dense and much larger middle sphere is made of materials rich in iron, magnesium and calcium comprises the mantle.  The outermost layer is a thin veneer of lighter rocks called the crust.  The crust beneath the oceans is composed of heavy, dark volcanic rocks such as basalt.  Continental crust is composed mostly of lighter rocks rich in silica such as granite. 

The Earth’s interior also has distinct physical layers without regard to composition.  The outer layer is comprised of a rigid sphere called the lithosphere (“rocky sphere”).  The lithosphere includes the upper, solid part of the mantle and all of the crust.  The lithosphere is floating above a zone known as the asthenosphere (“weak sphere”). The asthenosphere behaves as a hot, ductile plastic.

To fully understand how the Earth changes, it is important to appreciate the basic distinction between crust and lithosphere.

The crust (both continental and oceanic) is the thin layer of distinctive chemical composition overlying the mantle. Very different processes form oceanic crust and continental crust.  Oceanic crust is much thinner (typically 4 – 5 miles thick) and heavier than continental crust, which can range from 21 to nearly 30 miles thick.

 Internal structure of the Earth showing both compositional (core, mantle and crust) and physical layers (lithosphere and asthenosphere). Image: USGS

The lithosphere, in contrast, is the rigid outer layer of the Earth.  It differs from the underlying asthenosphere by its physical and mechanical properties rather than its chemical composition.  The lithosphere responds essentially as a rigid shell whereas the asthenosphere behaves as a very thick fluid.  The diagram above illustrates both the chemical and physical layers within the Earth.

Earth on a Slow Boil: Transferring Heat by Convection

Nature is always uncomfortable with any unequal distribution of heat.  It constantly works to equalize any temperature difference in the most efficient way possible.  The Earth has a basic obligation to the laws of physics to transfer its internal heat into the cold universe that surrounds us. 

Because the interior of the earth is hot and under great pressure, it transfers much of its internal heat by a phenomenon called “convection.”   Convection is the process by which hot materials rise, move laterally, cool, and then descend in a cycle. 

You can see convection operating in a pan of boiling water.  When water reaches a rolling boil, hot water rises in the center, cools as it spreads across the surface, then descends along the outside of the pan to complete the cycle.  These cycles are convection cells. 

Within the Earth, irregular convection cells within the mantle transfer heat from the core to the surface of the planet.  This mechanism is the driving force behind both heat transfer and the global processes of plate tectonics.

Irregular convection cells transfer heat from the Earth’s core to the surface.  Convection is the driving heat engine that powers the motion of the Earth’s great tectonic plates. Plate tectonics is the Earth’s way of expelling heat to space, thus fulfilling its obligation to the Second Law of Thermodynamics.

Plate Tectonics: Geology’s Unifying Theory

Tectonics is concerned with movement in the Earth and the forces that produce movement. Plate tectonics is the theory that the Earth's lithosphere (outer rigid shell) is broken into a dozen or so rigid "plates” that float on the hot, ductile mantle like slabs of ice on a pond. Much of the Earth’s ancient history is the result of plates rifting into pieces to form new ocean basins and converging back together to form mountains and giant continents.

The Great Tectonic Plates

The rigid lithospheric plates differ in size, direction of motion, and in the type of crustal rocks included in the plate.  Some plates (such as the Pacific Plate) are completely covered by oceans and are made of oceanic crust.   Other plates (such as the North American Plate) carry continents and adjoining pieces of the ocean floor.

The tectonic plates move ponderously about the Earth’s surface about as fast as your hair grows.  Convection cells operating within the mantle power their motion.  The entire jigsaw puzzle of plates interconnects at a global scale, and no single plate can move without affecting its neighbors.  As we’ll see, the activity of one plate can profoundly change the behavior of other plates on opposite sides of the Earth.   

The map below shows the global distribution of lithospheric plates.  Notice how the North American plate includes both continental rocks and oceanic rocks of the Atlantic Ocean.  The Pacific Plate, in contrast, carries only oceanic crust.

 Closer to home, notice the Juan de Fuca Plate offshore of Washington and the Cocos Plate adjacent to Mexico and Central America.  As we’ll see, the Juan de Fuca and Cocos plates are the last remnants of a once huge oceanic plate (called the Farallon Plate) largely destroyed beneath the North American continent.  The Farallon Plate and its descendants played a major role in the geologic evolution of Washington and the Pacific Northwest.

Distribution of the major lithospheric plates of the Earth.  These plates move ponderously about the globe, powered by convection in the underlying asthenosphere.  (Image:  USGS)

Plate Boundaries

                  

There are three types of boundaries between the tectonic plates:

1) Divergent boundaries – mid-ocean spreading ridges that generate new oceanic crust.  Because the plates are pulling apart, these are “extensional” boundaries.

2) Convergent boundaries -- where lithosphere one lithospheric plate dives under another in a process called subduction.  These are “compressional” boundaries. 

3) Transform boundaries -- where plates slide horizontally past each other along giant faults.  California’s San Andreas fault is the best-known transform plate boundary.

Mid-Ocean Spreading Ridges

Most plates diverge (move away from each other) at mid-ocean spreading ridges.  The mid-ocean ridges are undersea mountains more than 12,000 feet high and 1,200 miles wide.  Molten rock moving upward by convection reaches the surface along these ridges and releases much of the Earth’s internal heat.  Near the surface, the oceanic lithosphere is mostly basalt, a black, fine-grained volcanic rock.

As molten rock rises to the surface, the entire oceanic lithosphere moves away from the spreading center.  In this way, new ocean floor constantly forms and slides away from either side of the ridge as solid plates of the lithosphere.  This process is responsible for the phenomenon known as “seafloor spreading.”

Hot Spots

When convection rises as a single plume rather than along a linear spreading ridge, the result is a “hot spot.”  On the surface, hot spots erupt as volcanoes of dark, basaltic-type rocks.  The source of the molten rock is probably at the core-mantle boundary. Hot spot volcanoes often form long chains that result from the relative motion of the lithosphere plate over the hot-spot source

The Hawaiian Islands are our best-known modern examples of hot spots derived from mantle plumes.  Another well-known example is the hot spot beneath Yellowstone National Park today.  As we’ll see, hot spots have played a major role in creating some of Washington’s volcanic regions.

Mid-ocean ridges forming divergent plate boundaries.

The Atlantic Ocean is growing at the expense of the

Pacific Ocean, where oceanic plates are being

destroyed by subduction.  (Image: Marie Tharp)

Hot spots are single plumes of molten rock ascending

from the mantle into the overriding lithosphere.  Such

plumes have built the Hawaiian Islands and the volcanic

 features of Yellowstone National Park.  They have also

had an enormous impact on the geologic history of 

Washington (Image: USGS)

Subduction Zones: The Ultimate Fate of Oceanic Crust

A subduction zone is a boundary where two tectonic plates collide and, because of differences in density, one dives beneath the other.  Plates carrying oceanic rocks move across the face of the planet until they run into an obstruction, typically a continent. Continents are made of lighter rocks such as granite.  When the two plates collide, the heavier oceanic rocks sink downward (subducted) underneath the lighter continent.  Where the ocean plate dives underneath the continent, a deep trench forms adjacent to the edge of the continent.  Trenches within subduction zones are the deepest depressions of the Earth.

Continental Arcs

When the floor of an oceanic plate subducts beneath a continent, high pressures, high temperatures and high water concentrations combine to melt some of the lighter minerals in the descending plate.  These molten rocks form bulbous bodies (like in a lava lamp) of silica-rich magma called “plutons,” named after Pluto, the Greek god of the underworld.  Many plutons never reach the surface.  Rather, they cool and crystallize into bodies of granite deep beneath the Earth’s surface.  Some plutons, however, make their way to the surface and erupt as a chain of volcanoes rising along the edge of the continent.  Geologists call the arc-shaped belt of plutons and volcanoes a “continental arc.”  The modern Cascade volcanoes are a classic modern example of a continental arc.

Cross-section through an active subduction zone adjacent to a continent. Note the continental volcanic arc rising parallel to the subduction zone.  (Image: USGS)

When two plates carrying oceanic crust collide, the winner is usually the younger of the two because plates grow colder and denser with age.  The older, denser plate subducts beneath the younger plate and begins melting at depth. 

Molten rock from this subduction rises to the surface to erupt as an arc-shaped chain of volcanic islands.  These chains are “island arcs.”  The volcanic islands of Japan are a modern example of an active island arc system.  As we’ll see, such ancient island arcs played a major role in Washington’s geologic history.

Cross-section through an oceanic subduction zone.  The

older lithospheric plate dives beneath the younger, lighter

plate.  Partial melting of the descending plate produces

hot plutons of molten rock moving toward the surface.  When they reach the surface, they erupt to form a chain of volcanic islands.  (Image: USGS)

The illustration below shows how all three types of plate boundaries (convergent, divergent and transform) operate as part of a vast global system.  New oceanic lithosphere forms at divergent plate boundaries such as mid-ocean spreading centers and continental rift zones.  The lithosphere and oceanic crust descend along subduction zones located either at continental boundaries or in convergence with other oceanic crust.  In both cases, a volcanic arc at the surface reflects the partial melting of the subducting slab at depth and the upward movement of magma bodies (plutons).  The creation and destruction of oceanic lithosphere occur at about the same rate on a global scale.

As the continents have moved through time, they have repeatedly collided to form “supercontinents.”   Most of the rocks that make up continents are insulators -- they are reluctant to transfer thermal energy.   Eventually, heat builds up beneath the continent.  The continental crust swells, stretches, and finally ruptures.  New ocean floor begins to build within the rupture zones.  Fragments of the supercontinent spread as the ocean plate grows along a new seafloor spreading center.

 Because the Earth is a sphere, the moving continental fragments inevitably reassemble about every 500 million years.  As we will see, the creation and destruction of giant continents has played a major role in the geologic history of Washington and the Pacific Northwest.

The modern Atlantic Ocean is spreading at the expense of the Pacific.  As North America moves westward, the Pacific Ocean basin is getting smaller along subduction zones (convergent plate boundaries) under North and South America, and Japan, as western North America and Asia get closer together. Sometime in the future the Pacific ocean will close completely and Asia and North America will collide to form yet another supercontinent.

Before Pangaea, yet another supercontinent called Rodinia existed between 1.2 billion and 750 million years ago. It also formed from the accumulation of isolated continents, only to fragment 350 million years after its formation. There is growing evidence that even older supercontinents predated Rodinia early in Earth’s history.

 This supercontinent cycle – the assembly, rupture, breakup, spreading and reassembly  -- has left an indelible record on the geology of the Pacific Northwest.  We will consider the impact of the supercontinent cycle as we explore Washington’s geologic history.

Geologists think of time very differently from most people.  The Earth is nearly 4.7 billion years old.  Washington’s geologic history goes well beyond a billion years.   The most recent plate tectonic cycle that built the Pacific Northwest began 200 million years ago.  Clearly, we need a different type of calendar to order geologic events that occurred so long ago.

The geologic time scale has evolved over the last 200 years as geologists began to order events in Earth history.  The geologic time scale developed in the 1800’s based on the history of life preserved in fossils.  Each division of the time scale marked significant changes in the fossil record, such as the extinction of certain life forms and the appearance of new ones. 

Since the 1950’s, the modern science of geochronology has used the decay rates of radioactive isotopes to put absolute ages on the geologic time scale.  The time scale is a working document, often amended in detail as our ability to date rocks improves.

As we discuss Washington’s geologic history, we will make frequent reference to the periods of the geologic time scale.  This scale will help you keep the chronology of our ancient history in order.

 

The modern geologic time scale widely used in North America.  (Image: Geological Society of America)