Subduction Zone

Brent Mccoy
• Monday, 17 January, 2022
• 14 min read

The plates are pieces of crust that slowly move across the planet's surface over millions of years. Where two tectonic plates meet at a subductionzone, one bends and slides underneath the other, curving down into the mantle.

zone crust oceanic continental plate subduction boundary trench ocean earth formed lithosphere earthquake arc than geology island convergent type beneath
(Source: gotbooks.miracosta.edu)


(Sometimes, oceanic crust may grow so old and that dense that it collapses and spontaneously forms a subductionzone, scientists think.) “We can see very clear pictures of how the plates move, mostly due to GPS data,” said Vasily Tito, director of National Oceanic and Atmospheric Administration's Center for Tsunami Research in Seattle, Washington.

Subduction zones occur all around the edge of the Pacific Ocean, offshore of Washington, Canada, Alaska, Russia, Japan and Indonesia. Called the “Ring of Fire,” these subduction zones are responsible for the world's biggest earthquakes, the most terrible tsunamis and some of the worst volcanic eruptions.

“ Subduction zones are huge boundaries, so they generate very large earthquakes,” Tito told Live Science. Seismic waves from these temblors and tremors help scientists “see” inside the Earth, similar to a medical CT scan.

The quakes reveal that the sinking slab tends to bend at an angle between 25 and 45 degrees from Earth's surface, though some are flatter or steeper than this. Subduction zones are usually along coastlines, so tsunamis will always be generated close to where people live, Tito said.

When subductionzone earthquakes hit, Earth's crust flexes and snaps like a freed spring. For earthquakes larger than a magnitude 7.5, this can cause a tsunami, a giant sea wave, by suddenly moving the seafloor.

subduction zone oceanic continental things
(Source: www.slideshare.net)

Whatever their cause, the tsunami threat from subduction zones is monitored by government agencies such as NOAA in countries around the Pacific Ocean. Tsunamis may strike in minutes for coastal areas near an earthquake, or hours later, after the waves travel across the sea.

These fluids, such as seawater and carbon dioxide, rise into the upper plate and can partially melt the overlying crust, forming magma. Inland of each subductionzone is a chain of spouting volcanoes called a volcanic arc, such as Alaska's Aleutian Islands.

Where they collide and one plate is thrust beneath another (a subductionzone), the most powerful earthquakes, tsunamis, volcanic eruptions, and landslides occur. At shallow depths, less than about 25 kilometers (16 miles), the interface between the plates may become stuck, or “locked,” and stresses build along these giant “mega thrust” faults.

Eventually stresses exceed the fault’s strength, and it breaks free, releasing the stored energy as seismic (shaking) waves in an earthquake. This can happen either when earthquake faults move vertically just below the surface, or when submarine landslides transport large masses.

In the hundreds of years between mega thrust earthquakes, the squeezing motions cause the upper plate to bulge and uplift just above and inboard of the locked region, over thousands of square kilometers. Schematic diagram of a subductionzone, showing the location of the outer rise and tension stresses within the subducting plate.

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(Source: www.slideshare.net)

Onshore, high rates of rainfall on the seaward side of the mountain chains created by the squeezing of the plates makes landslides more probable. A subductionzone is a region of the Earth’s crust where tectonic plates meet.

Tectonic plates are massive pieces of the Earth’s crust that interact with each other. That is how the surface of the earth makes way for the crust created over time at other plate boundaries.

Subduction zones have key characteristics that help geologist and seismologist identify them. The next is volcanic activity as a plate is subducted the pressure and heat turns it into magma.

These pockets of magma find paths to the surface and create volcanoes. One common theory is that subduction was initiated by major impacts by asteroids or comets early in Earth’s history.

This makes a lot of sense due to the geologic evidence of large impacts scattered around the world. Understanding how subduction zones work is important because it helps scientist to identify areas of high volcanic and seismic activity.

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(Source: commons.wikimedia.org)

Subduction is a process in geology where one tectonic plates slides underneath another one and merges into the Earth’s mantle. The earthquakes that result due to the plates grinding against each other often cause magma to spill out through the trench in submarine volcanoes.

Various formations such as mountain ranges, islands, and trenches are caused by subduction and the volcanoes and earthquakes it triggers. Instead, the less dense material slips into a trench behind the denser oceanic crust where it gets stuck.

The pressure continues to build until the trench flips over and the less dense plate slips underneath the one with the continent. The spreading pushes the plate slowly toward the subductionzone until the whole thing disappears.

Shaded, raised relief map of the United States, highlighting National Park Service sites in modern and ancient Subduction Zones. The rising water melts rock in its path, forming a volcanic arc on the overriding plate.

Images above modified from “Oregon's Island in the Sky: Geology Road Guide to Mary's Peak, by Robert J. Lillie, Wells Creek Publishers, 75 pp., 2017, www.amazon.com/dp/1540611965. Sites in the Sierra Nevada Mountains reveal the eroded roots of an ancient volcanic range that formed when the subductionzone extended much farther south.

subduction zone sediments seismogenic zones diagram science calcareous schematic structure eurekalert weak earthquake caption multimedia pub
(Source: www.eurekalert.org)

And very large parks in the Southern Alaska SubductionZone display magnificent mountains, rock layers and active volcanoes that tell the story ongoing plate convergence along the North Pacific Coast. The Coastal Ranges are forming as material from the ocean is scraped off the top of the subducting Juan de Fuca Plate.

The light-colored granite rocks are the cooled remnants of magma chambers that fed ancient volcanoes when the subductionzone extended through California and into Mexico. The rock layers are lifting out of the sea as the Pacific Plate subducts beneath southern Alaska.

National Park Service sites in the Coastal Ranges of Washington, Oregon and northern California contain rugged mountains of rocks that were manufactured in the ocean, then scraped off the plate and lifted out of the sea. They contain explosive volcanoes formed as fluids rise from the top of the subducting plate and generate magma as they melt their way to the surface.

And as time ticks on, the region awaits sudden release of energy locked between the converging plates as a devastating earthquake. The Cascadia SubductionZone, extending from northern California through western Oregon and Washington to southern British Columbia, is a type of convergent plate boundary.

Two parallel mountain ranges have been forming as a result of the Juan de Fuca Plate subducting beneath the edge of North America. Near the western edge of the continent, the Juan de Fuca Plate plunges downward and some layers of hard crust and ocean sediments are scraped off the top and squeezed upward as the Olympic Mountains and other coastal ranges.

chile map seamounts earthquake subducting coastline showing cantner quake blocked et al earthmagazine prevented locations april magazine
(Source: www.earthmagazine.org)

The low region between the two mountain ranges is the Puget Sound area of Washington and the Willamette Valley in Oregon. The Cascadia SubductionZone is the Region where the Juan De Fuca and North American Plates Converge The Coast Ranges, including the Olympic Mountains, are made of oceanic sediments and hard rocks that were caught in the vise between the converging plates, uplifted, and added to the edge of the continent.

Puget Sound and the Willamette Valley are areas near sea level between the coastal and volcanic mountain ranges. Modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893.

The Coastal Mountain Ranges, including the Olympic Mountains in northwest Washington and the Coast Range in southwest Washington, western Oregon and northwest California, form as sedimentary and volcanic layers are scraped off the top of the subducting oceanic plate and added to the edge of the continent. 150 miles inland, the top of the subducting plate reaches depths where it’s hot enough to generate fluids, forming volcanoes in the Cascades.

The Cascadia SubductionZone has two parallel mountain ranges separated by a low region near sea level. Modified from “Oregon's Island in the Sky: Geology Road Guide to Mary's Peak, by Robert J. Lillie, Wells Creek Publishers, 75 pp., 2017, www.amazon.com/dp/1540611965.

Storms coming in from the Pacific Ocean drop most of their moisture on the Coast Range and Cascades, leaving eastern Oregon and Washington high and dry. The physical landscape was an important component of the practical and spiritual aspects of Native Americans’ lives.

seattle earthquake cascadia earthquakes ways 1900
(Source: news.nationalgeographic.com)

The largest (locked zone earthquakes) occur where the Juan de Fuca and North American plates are stuck together, as they have been for the past three centuries. When the plates suddenly let go, a massive earthquake will shake the entire Pacific Northwest, a series of tsunami waves will pound the Coast, and landslides will make it difficult to reach some of those in need.

Visits to coastal NPS sites in the Pacific Northwest can help us appreciate the landscapes that develop over time because of these large quakes, and learn how we are preparing our homes, communities, and infrastructure for the next “Big One.” The Coast Range (including the Olympic Mountains) consists of sedimentary rock layers and hard crust scraped off the ocean floor where the Juan de Fuca Plate begins to dive downward.

The grinding action also produces devastating earthquakes, including some that result in giant tsunami waves. Forming the Coast Range Olympic National Park, Washington Tilted layers of thick sandstone (pink) and thin shale (dark) along the coast at Olympic National Park reveal the enormous forces that lifted and deformed the oceanic layers as the Juan de Fuca and North American plates converged.

In her version of the Oreo® cookie demonstration, the creamy filling is the layers of sediment and basalt on the ocean floor. Modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893.

Olympic National Park, Washington The same forces that shift the land so dramatically also gradually build spectacular landscapes like the coastlines of the Pacific Northwest. Pillow lava form on the ocean floor where erupting magma encounters cold seawater.

fault cascadia zone pacific northwest subduction oregon earthquake coast rising along earth north map found portland seismic mantle under pieces
(Source: around.uoregon.edu)

When caught between the converging Juan de Fuca and North American plates, the pillows and other ocean layers can be lifted upward to more than a mile above sea level. Layers of sand and mud, deposited on the ocean floor and later turned into the sedimentary rocks sandstone and shale, are seen above the pillows.

Dramatic examples of these layers in tilted and contorted forms can be found in Olympic National Park and elsewhere along the coast and in the mountains. A visit to Olympic National Park is an opportunity to witness features of an accretion wedge as they develop.

But if the lava spills out from beneath the ocean, it encounters cold water and hardens quickly as a pile of pillows. As they were caught in the vise between the converging Juan de Fuca and North American plates, some layers were so contorted that they are now in vertical orientations along the coast.

Formation of Pillow Basalt Layers of pillow basalt exposed at NPS sites in the Coastal Ranges are evidence that lava flows that formed in the Pacific Ocean were later scraped off the subducting plate and lifted upward as part of the accretion wedge. As fluid basalt lava flows into the ocean it forms a pile of globular structures known as pillows.

Garibaldi in southern British Columbia, through Washington and Oregon, to Lassen Peak in northern California. The volcanoes are forming above the region where the top of the subducting Juan de Fuca Plate reaches about 50 miles (80 kilometers) depth.

tsunami earthquake zealand massive inevitable nz newshub lifetime experts hikurangi
(Source: www.newshub.co.nz)

The rising hot water causes overlying rock to melt, generating magma that at times erupts out on the surface as lava flows and other materials forming Mt. The initial magma formed as mantle rock melts beneath a subductionzone has low silica (basalt) composition.

As the basalt magma rises up through the thick continental crust of North America, it melts some of that rock, too. The resulting magmas can therefore have a variety of compositions, ranging from the original (low-silica) basalt, to (intermediate-silica) despite, all the way up to (high-silica) cryolite.

Their high-silica magmas produce not only despite and cryolite lava flows, but also lots of other volcanic materials such as ash and pumice. When mixed with water from glaciers, Snowbelt, and streams, these materials can make very dense, fast-moving volcanic mudflows (known sometimes by the Indonesian term “Lamar”).

And at times, lower-silica components of their magma chambers erupt, producing basalt lava flows as well as cinders and volcanic bombs. National Park Service sites in the Cascades reveal colorful landscapes that result from a variety of volcanic processes.

Rainier is a steep-sided composite volcano composed of numerous despite lava flows and volcanic mudflows and covered by glaciers. Bum pass Hell has hot springs, mud pots and fumaroles, the largest collection of hydrothermal features in the continental United States outside Yellowstone.

philippines map tectonic features showing zones subduction philippine zone pmb faults salient simplified mobile belt figure including major archipelago negros
(Source: www.researchgate.net)

Volcanoes in national parks in the Cascades have eruptive “personalities” that reflect the complex magma systems associated with subduction zones. The lake partially fills a collapsed crater (“caldera”) that formed when a 12,000-foot (3,700-meter) composite volcano, Mt.

A series of explosions and ash clouds from the peak between 1914 and 1921 represents the last large volcanic eruption within the lower 48 states prior to Washington’s Mt. During the initial stages of its climactic eruption, the upper part of the magma chamber poured out ash, pumice, and cryolite lava flows.

Later eruptions of low-silica magma partially filled the caldera with basalt lava flows and cinder cones, including what is now Wizard Island. Over a few centuries rain and Snowbelt partially filled the hole with the waters of Crater Lake.

Illustrations above modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893. A glance at a map of the western United States might suggest that the Sierra Nevada are a southward continuation of the Cascade Volcanoes.

West Coast Tectonic Evolution Granite rocks found in national parks in the Sierra Nevada are the cooled and eroded remnants of magma chambers that fed ancient subductionzone volcanoes in California. Subduction of the Carillon Plate beneath the entire West Coast created a line of volcanoes from Alaska to Central America.

tambora mount volcano eruption indonesia 1815 year volcanoes summer without volcanic deadliest climate today crater history change famous after gunung
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The Sierra Nevada are the eroded remnants of the volcanic arc developed when the Carillon Plate subducted beneath the continent. Massive areas of granite from the cooled magma chambers that fed the volcanoes form portions of the Sierra Nevada Mountains, including Yosemite National Park.

The Coast Range (accretion wedge), Great Valley (forearm basin), and Sierra Nevada (volcanic arc) still reflect the subductionzone topography. Modified from “The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172.

Modified from “The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172. The three West Coast states are especially meaningful because they have geologic features that, when the quarters are stacked, represent the surface and subsurface of the chain of volcanoes that has been developing over the past 200 million years.

Rainier National Park is an active composite volcano rising more than 14,000 feet above sea level. Half Dome in Yosemite National Park is made of granite that solidified from magma tens of millions of years ago, when a subductionzone extended along the entire west coast.

The overlying volcanoes have since eroded away, exposing a vast expanse of the ancient magma chambers in the Sierra Nevada. Modified from “Beauty and the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest”, by Robert J. Lillie, Wells Creek Publishers, 92 pp., www.amazon.com/dp.1512211893.

subduction geography grade canada zones oceanic ppt powerpoint presentation
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Mazama, erupted and collapsed 7,700 years ago, forming the large cavity (caldera) that now holds Crater Lake. Five park sites in southern Alaska showcase landscapes formed by the subduction of the northward-moving Pacific Plate beneath North America.

One of them, Kenai Fjords National Park, lies within the accretion wedge of uplifting oceanic sedimentary strata and hard crust. Another, Wrangell-St. Elias National Park, is a vast region that extends across the accretion wedge and volcanic arc, and is built of accreted terraces.

National Park Service Sites in Alaska SubductionZone Letters in ovals refer to NPS sites that lie on the accretion wedge and volcanic arc formed by northward subduction of the Pacific Plate beneath southern Alaska. Elias and Denali national parks, are also included in the “Accreted Terraces” part of these web pages.

Plate convergence that built Alaska outward as a series of accreted terraces during the past 200 million years continues today. Kenai Fjords has oceanic sedimentary layers that have been metamorphosed, uplifted, and deformed as part of the modern accretion wedge.

Aniakchak, Katmai and Lake Clark are part of the active volcanic arc built on older accreted terraces. Wrangell-St Elias spans older accreted terraces as well as active volcanoes and uplifting accretion wedge material.

subduction zones
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Kenai Fjords National Park lies within a coastal mountain range (accretion wedge) formed as the Pacific Plate subducts beneath southern Alaska. Sandstone and shale layers at Kenai Fjords are commonly metamorphosed and were so deformed during subduction and uplift that they are vertical in places.

The park is right above the focus of the Great Alaska Earthquake of 1964, one of the two largest ever recorded (the other, in 1961, occurred along the subductionzone off western South America). Effects of the 1964 earthquake are quite spectacular in the park; the coastline dropped so much that in places it lies submerged beneath about 8 feet (2.5 meters) of water.

Accretion Wedge in Southern Alaska Volcanic and metamorphic rocks exposed in Kenai Fjords National Park reveal the incredible forces that occur at an ocean/continent subductionzone. Kenai Fjords National Park Sea lions resting on layers of pillow basalt that formed beneath the ocean and were uplifted as part of the accretion wedge.

National Park Service Sites in the Southern Alaska SubductionZone reveal a variety of active volcanic features above the zone where the top of the Pacific Plate reaches sufficient depth where temperature and pressure are high enough to dehydrate rocks and form magma. Katmai National Park and Preserve lies where the world’s largest volcanic event of the 20th Century occurred.

The vent was the source for most of the ash and pyroclastic flows expelled during the 1912 eruptions, and has since filled with a lava dome known as Novarupta. The park is a complex amalgamation of blocks of continental and oceanic crust that have slammed into North America.

tsunami inundation oregon map zone tillamook subduction north cascadia county tim plate coos bay maps bend till dogami source
(Source: www.oregongeology.org)

Poking through the mass of some of the highest mountains in North America are very young volcanoes formed by the ongoing Pacific Plate subduction. Lake Clark National Park and Preserve contains an active composite volcano with a profile and recent history reminiscent of Mt.

Redoubt, on the eastern side of the park, had four explosive eruptions that resulted in large volumes of ash and mudflows. After a terrifying free-fall that lasted 8 minutes, the pilots finally managed to restart the engines at only 6,000 feet (2,000 meters).

Produced under a Cooperative Agreement for earth science education between the National Park Service's Geologic Resources Division and the American Geosciences Institute.

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