Plate Tectonics

Plate tectonics is the scientific understanding of how large, contiguous blocks of the Earth’s outermost layers—the crust and the uppermost mantle—move and interact with one another. Plate tectonics effectively explains the distribution of Earth’s oceans and landmasses, natural resources, and natural hazards. Moreover, it provides a framework for understanding how these features evolve over timescales ranging from seconds to billions of years, and how to relate modern observations across the entirety of the Earth sciences.

Large, rigid blocks of lithosphere called plates float atop a more-deformable layer in the Earth called the asthenosphere. Plate motion is driven by a combination of currents in the asthenosphere and pushing and pulling forces at plate boundaries.

There are three primary types of plate boundaries that are defined by the relative movements of the plates, and each boundary type produces earthquakes.

Boundaries where the plates pull apart are called divergent boundaries, which form distinctive rifts (e.g., East Africa) and ridges (e.g., the Mid-Atlantic Ridge). New crust is formed at these boundaries, which provides some pushing forces that move plates apart on either side of the boundary. Earthquakes produced by divergent boundaries tend to be less energetic compared to the other two boundary types.

Boundaries where plates come together are called convergent boundaries, producing the highest (the Himalayan Mountains) and deepest (the Mariana Trench) features on the Earth’s surface. This is occurring in the Pacific Northwest’s Cascadia Subduction Zone (Cascadia Subduction Zone Megathrust) where old ocean lithosphere dives beneath the North American Plate, beginning the recycling stage of its journey in the rock cycle, sinking and melting back into the mantle. This sinking provides pulling forces on the yet-to-be-subducted plate. As subducted plates melt, they help to produce magmas that fuel volcanic systems like the Cascades Volcanic Arc (Volcanoes). These boundaries can produce the largest known earthquakes.

Boundaries where plates slide past one another are called transform boundaries. One of the most famous faults in the USA, the San Andreas Fault, is one such plate boundary. These boundaries create more subtle topography, controlled by small amounts of convergent or divergent motion that can vary along the plate boundary, resulting in strings of basins and ridge lines. These boundaries can produce strong, shallow earthquakes.

The relative motions of plates defining these boundaries are nicely animated in this video from the EarthScope Consortium:

Plate tectonics is a relatively young scientific theory, with most of its evidence being realized starting in the 1950s and 1960s following advances in observational and computing technologies. It was first posed as the hypothesis of “continental drift”, most notably by Alfred Wegener in 1912, using evidence including similarities of rock formations and fossils across ocean basins, the presence of fossils in polar regions with strong similarities to modern tropical life, and the very shape of coastlines appearing like puzzle pieces separated by oceans.

The advances in the 1950s and 1960s were driven by substantial contributions from the United States military and mineral exploration companies. As the US Navy scanned for magnetic signatures of submarines during and after World War II, they discovered repeating patterns of changing magnetic polarity. Continued efforts by the U.S. Navy and scientists revealed that these reversals formed bar-code-like patterns paralleling undersea ridges and coastlines. When combined with rock ages, these patterns provide records of plate motions over tens of millions of years.

To monitor nuclear tests, global seismic networks were established to detect, locate, and estimate the size of nuclear blasts. These same networks, data, and techniques allowed global monitoring of earthquakes, illuminating linear features along mountain ranges, ocean troughs, and other conspicuous topographic features that define the edges of tectonic plates and features within those plates.

In the search for new reservoirs and deposits, mineral exploration companies drove advances in geophysical imaging methods, and due to the ever-growing size of the data they needed to analyze, they also drove development of faster and more powerful computers. To better predict the presence and richness of reservoirs, they also drove advances in scientific understanding across the Earth sciences ranging from the co-evolution of life and landscapes, to the chemical genesis of ore bodies, to the deformation caused by the interaction of tectonic plates. This work is highly interdisciplinary and depends on broad collaborations across industry, government, and research organizations.

As earth scientists continue to grow our knowledge of the shape, behavior, and history of our planet, the underpinning framework of plate tectonics theory remains largely unchanged. The consistency of the principles of plate tectonics in connecting vast amounts of diverse observational evidence is why plate tectonics theory is universally accepted by scientists throughout the world.

The landscapes and natural hazards of the Pacific Northwest are driven by the movement of three tectonic plates: the Pacific, the North American, and the Juan de Fuca. All these plates are moving in different directions at different speeds. In the Pacific Northwest, the North American Plate is moving west-southwest at a rate of ~2.3 cm per year (roughly how fast your fingernails grow), largely in response to pushing forces from the Mid-Atlantic Ridge (a divergent boundary) thousands of kilometers away. Along the U.S. Pacific coast, the Pacific Plate is moving to the northwest at speeds ranging from 7–11 cm per year in response to a combination of pulling forces from subduction in Cascadia and Alaska, mantle flow, and pushing forces from spreading ridges in the equatorial Pacific.

The Juan de Fuca Plate is a remnant of a much larger plate, the Farallon Plate, that subducted beneath the Americas, resulting in the San Andreas Fault system as the new contact between the North American and Pacific Plates as the Juan de Fuca was severed from other remnants now subducting beneath South and Central America. The Juan de Fuca Plate’s modern motion is to the east-northeast at a rate of 4 cm per year driven by spreading at the Juan de Fuca ridge and subduction beneath Vancouver Island, all of Washington and Oregon, and Northern California down to Cape Mendocino. The 1,000 km long interface between the Juan de Fuca and North American plates constitutes the Cascadia Subduction Zone, and motion on this boundary is not steady. As these plates converge, they build up strain on the plate interface until it abruptly slips many meters at once, producing megathrust earthquakes. It takes a continuous rupture over most of the Cascadia Subduction Zone (Cascadia Subduction Zone Megathrust), with slips exceeding 10 m, to generate the magnitude 9+ earthquakes that occur every 550 years on average.

The interactions between these three plates also create strains within the North American Plate that fuel crustal earthquakes in the Pacific Northwest. Although major tectonic plates are broadly thought of as homogeneous masses, continental plates like North Americal are actually composed of lots of smaller blocks that became stuck together over billions of years. Northward convergence between the Pacific Plate and North American Plate on parts of the San Andreas Fault passes strains through the Sierra Nevada block and into block underlying the Oregon Coastal Range, causing it to rotate as it subsequently passes strains into the Puget Lowland of Washington. Using precise GPS observations, we can observe the subtle shifts and rotations of blocks comprising the North American Plate as shown below, as well as the relative motions of plates used to estimate the plate speeds quoted above.

To the north, rigid blocks underlying British Columbia act like a buttress, so the less-rigid rocks of the Puget Lowland compress like an accordion. Some of the uplift and down warping of the crust is visible in the topography of Washington, but thick sediments hide much of this pattern. Using small changes in the strength of gravity sensed with delicate geophysical tools, we can “see” through the sediment to fully uncover this accordion structure. The abrupt changes in gravitational strength correspond to fault-bounded structures that correspond to much of the seismicity regularly reported by the Pacific Northwest Seismic Network. We know from the geologic record that these faults can produce magnitude 7 earthquakes.

As the oceanic crust of the Farallon/Juan de Fuca plate subducts, it deforms and changes under the mounting heat and pressure from the mantle enveloping it. Strains from these changes can produce deep earthquakes like the widely felt Nisqually earthquake in 2001. The heat and pressure also squeeze fluids from the subducting plate, which act as a catalyst for magma production – notice how the Cascades Volcanic Arc matches the extent of the subduction zone.