what sequence of plate tectonic events is causing these volcanoes to form

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The Cascadia Subduction Zone and Southern Alaska are the sites of ongoing subduction every bit the Pacific and Juan de Fuca plates slide below the N American Plate. Some parks in the Sierra Nevada Mountains reveal igneous magma chamber rocks that represent the eroded remnants of an ancient subduction zone, when volcanoes like to those constitute in the modern Pour Mountains extended southward all the manner through California.

Shaded, raised relief map of Us, highlighting National Park Service sites in modern and ancient Subduction Zones. Messages refer to parks listed below.

Modified from "Parks and Plates: The Geology of our National Parks, Monuments and Seashores," by Robert J. Lillie, New York, W. Westward. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172.

Subduction Zones—Agile

Subduction Zones—Aboriginal

Introduction

Subduction zones grade where a plate with thinner (less-buoyant) oceanic crust descends beneath a plate with thicker (more-buoyant) continental crust. 2 parallel mountain ranges usually develop above such a subduction zone – a coastal range consisting of sedimentary strata and hard stone lifted out of the body of water (accretionary wedge), and a volcanic range farther inland (volcanic arc). Ancient magma chamber rocks can be exposed if subduction stops and the volcanoes erode abroad.

Buoyancy

Where tectonic plates converge, the one with dumbo, sparse oceanic crust subducts below the one with thick, more buoyant continental crust.

Subduction Zone—Two Parallel Mount Ranges

• An accretionary wedge forms between the converging plates as textile is scraped off the subducting plate.
• A forearc basin develops in the low surface area between the two mountain ranges.
• Farther inland, the subducting plate reaches depths where it "sweats" hot water. The rising water melts rock in its path, forming a volcanic arc on the overrriding plate.
• Almost their boundary, the plates tin can lock together for centuries, and then all of a sudden let become as a giant earthquake. If the seafloor rises or falls, giant sea waves (a tsunami) can form.

Cascadia Subduction Zone

• The Coast Range and Cascades are the two parallel mountain ranges that form the Cascadia Subduction Zone in the Pacific Northwest.
• The forearc basin is the Willamette Valley in Oregon and Puget Sound in Washington.

Images above modified from "Oregon's Island in the Heaven: Geology Road Guide to Marys Top, by Robert J. Lillie, Wells Creek Publishers, 75 pp., 2017, world wide web.amazon.com/dp/1540611965.

Many National Park Service sites are found in agile and ancient subduction zones. Visitors can witness mountains as they are forming, and sometimes experience the accompanying earthquake and volcanic activeness. Parks in the Cascadia Subduction Zone dramatically display the two distinct mountain ranges – the Coast Range just above where the Juan de Fuca Plate begins to subduct, and the volcanic Cascade Range farther inland, where the peak of the plate is deeper. Sites in the Sierra Nevada Mountains reveal the eroded roots of an ancient volcanic range that formed when the subduction zone extended much farther south. And very large parks in the Southern Alaska Subduction Zone display magnificent mountains, rock layers and active volcanoes that tell the story ongoing plate convergence forth the N Pacific Coast.

Redwood National and Country Parks, California

The Coastal Ranges are forming as material from the bounding main is scraped off the tiptop of the subducting Juan de Fuca Plate.

NPS photo.

Mountain Rainier National Park, Washington

^{Mount Rainier is a 14,000 foot (4,300 meter) volcano in the Cascade Range developed in a higher place the place where the subducting Juan de Fuca Plate reaches sufficient depth to release hot fluids into the overriding North American Plate.}

Photograph courtesy of Robert J. Lillie.

Yosemite National Park, California

The lite-colored granite rocks are the cooled remnants of magma chambers that fed ancient volcanoes when the subduction zone extended through California and into Mexico.

Photo courtesy of Robert J. Lillie.

Kenai Fjords National Park, Alaska

The stone layers are lifting out of the sea as the Pacific Plate subducts beneath southern Alaska.

Photo courtesy of Robert J. Lillie.

Cascadia Subduction Zone

The Pacific Northwest is an exciting place to detect geologic processes in action. 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 body of water. But parks in the Cascade Mountains – within the same subduction zone – are dramatically different. They contain explosive volcanoes formed as fluids rise from the superlative of the subducting plate and generate magma as they melt their way to the surface. And as fourth dimension ticks on, the region awaits sudden release of energy locked between the converging plates as a devastating earthquake.

The Cascadia Subduction Zone, extending from northern California through western Oregon and Washington to southern British Columbia, is a type of convergent plate purlieus. 2 parallel mountain ranges accept been forming every bit a event of the Juan de Fuca Plate subducting below the border of North America. Virtually the western edge of the continent, the Juan de Fuca Plate plunges downward and some of the layers of difficult chaff and ocean sediments are scraped off the top and squeezed upward as the Olympic Mountains and other coastal ranges. Farther east the Juan de Fuca Plate descends deeper and deeper. The rocks on top of the plate are metamorphosed due to the smashing temperatures and pressures at those depths. A by-product of the metamorphism is the release of hot fluids, especially water. The h2o rises and melts rock in its path. Some of the resulting magma makes it all the mode to the surface and forms Mount Rainier, Mount Hood, Mount Shasta and other spectacular volcanoes of the Cascade Range. The low region between the two mountain ranges is the Puget Sound surface area of Washington and the Willamette Valley in Oregon.

The Cascadia Subduction Zone is the Region where the Juan De Fuca and Northward American Plates Converge

• The Coast Ranges, including the Olympic Mountains, are made of oceanic sediments and hard rocks that were defenseless 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 littoral and volcanic mountain ranges.
• The Cascades form in a higher place the line where the subducting plate extends to depths where it heats up, dehydrates and causes magma to class. The line of active volcanoes, from Mt. Garibaldi to Lassen Peak, coincides with the north-to-s extent of the subducting plate purlieus.

Modified from "Dazzler from the Fauna: Plate Tectonics and the Landscapes of the Pacific Northwest," by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, world wide web.amazon.com/dp/1512211893.

Topography of the Cascadia Subduction Zone

• The Coastal Mountain Ranges, including the Olympic Mountains in northwest Washington and the Coast Range in southwest Washington, western Oregon and northwest California, form every bit sedimentary and volcanic layers are scraped off the top of the subducting oceanic plate and added to the edge of the continent.
• The depression region between the two parallel mountain ranges is the Puget Audio area in Washington, the Willamette Valley in Oregon and the Great Valley in northern California.
• 150 miles inland, the top of the subducting plate reaches depths where it'southward hot enough to generate fluids, forming volcanoes in the Cascades.

^{The Cascadia Subduction Zone has two parallel mountain ranges separated by a depression region near bounding main level.}

Modified from "Oregon's Island in the Heaven: Geology Road Guide to Marys Peak, by Robert J. Lillie, Wells Creek Publishers, 75 pp., 2017, www.amazon.com/dp/1540611965.

The ii parallel mountain ranges influence many of the physical and cultural aspects of the Pacific Northwest. 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. This affects not only plant and fauna communities, but also man habitation and state use. The concrete mural was an of import component of the practical and spiritual aspects of Native Americans' lives. Later on, most of the settlers coming on the Oregon Trail continued by central Oregon and Washington to the lush farmland of the Willamette Valley and Puget Sound, the low-lying region betwixt the two rising mount ranges.

Cascadia Subduction Zone: Topography, Earthquakes and Volcanic Eruptions

Subduction of the Juan de Fuca Plate results in the germination of the Coastal Ranges and Cascade Volcanoes, likewise every bit a diverseness of earthquakes, in the Pacific Northwest. Olympic and Mt. Rainier national parks showcase the contrasting landscapes of the two parallel mountain ranges.

Modified from "Oregon's Island in the Heaven: Geology Road Guide to Marys Elevation, by Robert J. Lillie, Wells Creek Publishers, 75 pp., 2017, world wide web.amazon.com/dp/1540611965.

Subduction in the Pacific Northwest creates incredible beauty in the class of coastlines, mountains, and valleys—iconic features of the many National Park Service sites in the region. This beauty is part of the reason that residents and visitors accept learned to tolerate, and develop ways to mitigate, the effects of earthquakes, tsunamis and volcanic eruptions—beasts that are consequences of the aforementioned tectonic forces that create the region's beauty. Individual earthquakes can showtime or elevator the land a fraction of an inch or a few inches (centimeters) at a time. That may not seem like much, but when tens of thousands of earthquakes occur over a few million years, coastal ranges course. Too, individual volcanic eruptions may add together merely a sparse layer to the surface. But thousands of eruptions over less than a million years have congenital Mount Rainier and other Cascade peaks to elevations of more than 10,000 feet (three,000 meters)!

A variety of earthquakes shake the Pacific Northwest due to plate-tectonic action. 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 convulsion will shake the unabridged Pacific Northwest, a series of tsunami waves volition pound the Coast, and landslides will make it difficult to reach some of those in demand. These mega-earthquakes occur every 200 to 600 years or so, and the last one was in the year 1700. Visits to coastal NPS sites in the Pacific Northwest can assistance united states appreciate the landscapes that develop over time because of these large quakes, and learn how nosotros are preparing our homes, communities, and infrastructure for the adjacent "Big One."

Coast Range

The Coast Range (including the Olympic Mountains) consists of sedimentary rock layers and hard crust scraped off the bounding main flooring where the Juan de Fuca Plate begins to dive down. The grinding action too produces devastating earthquakes, including some that result in giant seismic sea wave waves.

Forming the Coast Range

Olympic National Park, Washington
Tilted layers of thick sandstone (pink) and sparse shale (dark) forth the declension 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.

^{Photograph courtesy of Robert J. Lillie.}

Ranger Jen'southward Oreo Demo
Jennifer Natoli was a seasonal ranger at Redwood National and State Parks in California. In her version of the Oreo® cookie demonstration, the creamy filling is the layers of sediment and basalt on the bounding main flooring. Every bit the Juan de Fuca Plate (lower cookie) subducts beneath the North American Plate (upper cookie), the layers are scraped off the ocean floor and pile upwardly equally the Coast Range.

^{Modified from "Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest," past Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893.}

Earthquakes Build Dramatic Coastal Landscapes

Visitors to Olympic National Park, Oregon Caves National Monument, or Redwood National and Land Parks can look downward and imagine the peak of the Juan de Fuca Plate about 10 miles (xvi kilometers) beneath their feet. This is the zone where the 2 plates have been locked together since the last nifty earthquake occurred in the year 1700. A sudden unlocking could produce an earthquake every bit as big every bit the 1 that occurred in Alaska in 1964, when a similar subduction zone boundary snap occurred.

Copalis River, Washington
Trees forth the Washington coast were killed by table salt-water invasion when the land suddenly dropped during the terminal great Cascadia Subduction Zone earthquake in the year 1700.

^{USGS photo.}

Olympic National Park, Washington
The same forces that shift the country so dramatically also gradually build spectacular landscapes like the coastlines of the Pacific Northwest.

^{Photograph courtesy of Robert J. Lillie.}

Redwood National and State Parks, California

^{NPS photo.}

Some geological processes act so slowly that we may recollect of the Earth as stone-solid and unmoving. But given enough time, the Coast Ranges elevator up distances nosotros can mensurate, while at the same time wind, pelting, and the pounding of waves wear the mural back downward. Pillow lavas form on the ocean flooring where erupting magma encounters cold sea water. When caught betwixt the converging Juan de Fuca and North American plates, the pillows and other ocean layers tin be lifted upward to more than a mile above body of water 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 forth the coast and in the mountains. This contortion attests to the great forces that squeezed and lifted the layers from the sea.

A visit to Olympic National Park is an opportunity to witness features of an accretionary wedge equally they develop. Igneous, sedimentary, and metamorphic rocks are all establish in abundance. Nigh of the igneous rocks were manufactured in the ocean and are basalt. The spectacular pillow lavas seen on the road upwardly Hurricane Ridge betoken that lava formed on the sea flooring and was afterwards uplifted. Basalt lava is thin and runny and will flow for miles and miles. But if the lava spills out from beneath the ocean, information technology encounters common cold h2o and hardens quickly equally a pile of pillows. Well-nigh of the sedimentary rocks formed from layers of sand and mud deposited in the ocean. As they were caught in the vise between the converging Juan de Fuca and North American plates, some of the layers were so contorted that they are now in vertical orientations forth the coast. Other rocks seen in the high mountains at the center of the park were pushed more deeply into the Earth, where increasing temperature and pressure changed them into metamorphic rocks known as quartzite and slate.

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 Sea were afterward scraped off the subducting plate and lifted upwards as part of the accretionary wedge.

Pillow basalt layers formed more than than a mile (1.half-dozen kilometers) beneath the surface of the ocean are at present almost a mile above bounding main-level on Hurricane Ridge Road in Olympic National Park, Washington.

^{Photo courtesy of Robert J. Lillie.}

As fluid basalt lava flows into the bounding main information technology forms a pile of globular structures known as pillows.

^{Modified from "Earth: Portrait of a Planet", by S. Marshak, 2001, W. West. Norton & Comp., New York.}

Cascade Volcanoes

The Cascade Volcanoes extend in a northward-due south line from Mt. Garibaldi in southern British Columbia, through Washington and Oregon, to Lassen Acme in northern California. The volcanoes are forming above the region where the top of the subducting Juan de Fuca Plate reaches almost 50 miles (80 kilometers) depth. Increased temperature and pressure at that depth cause the rocks to metamorphose and dehydrate ("sweat"). The ascension 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. Rainier in Washington, Mt. Hood in Oregon, Mt. Shasta in California, and other Cascade Volcanoes.

The initial magma formed every bit pall stone melts below a subduction zone has depression silica (basalt) composition. Every bit the basalt magma rises up through the thick continental crust of Northward America, it melts some of that stone, too. In doing so the magma is enriched in silica, because high-silica minerals commonly are the first ones to cook. The resulting magmas can therefore have a variety of compositions, ranging from the original (depression-silica) basalt, to (intermediate-silica) andesite, all the manner up to (loftier-silica) rhyolite. Solids and gases are also part of the mix.

Magmas loftier in silica are thick and gummy. Gases may be trapped in them under high pressure. Thus subduction-zone volcanoes—including those in the Cascades—can be explosive (retrieve of the release of steam in a boiling pot of thick oatmeal). Their high-silica magmas produce not only andesite and rhyolite lava flows, but also lots of other volcanic materials such as ash and pumice. When mixed with water from glaciers, snowmelt, and streams, these materials can make very dense, fast-moving volcanic mudflows (known sometimes by the Indonesian term "lahar"). And at times, lower-silica components of their magma chambers erupt, producing basalt lava flows as well as cinders and volcanic bombs.

NPS Volcanic Sites in the Cascades

National Park Service sites in the Cascades reveal colorful landscapes that result from a varity of volcanic processes.

Mt. Rainier National Park, Washington

Mt. Rainier is a steep-sided blended volcano composed of numerous andesite lava flows and volcanic mudflows and covered by glaciers.

^{Photo courtesy of Robert J. Lillie.}

Crater Lake National Park, Oregon

Crater Lake lies inside a caldera formed as Mt. Mazama violently exploded and complanate 7,700 years agone.

Photo courtesy of Robert J. Lillie.

Lassen Volcanic National Park, California

Bumpass Hell has hot springs, mudpots and fumaroles, the largest drove of hydrothermal features in the continental Usa outside Yellowstone.

^{Photo courtesy of Robert J. Lillie.}

Volcanoes in national parks in the Cascades have eruptive "personalities" that reverberate the complex magma systems associated with subduction zones. Mountain Rainier National Park showcases a xiv,411-foot (4,393-meter) composite volcano. The mountain is mainly andesite lava flows and volcanic mudflows covered past numerous glaciers. Crater Lake National Park has the 9th deepest lake in the world. The lake partially fills a collapsed crater ("caldera") that formed when a 12,000-pes (iii,700-meter) composite volcano, Mt. Mazama, violently erupted 7,700 years ago. Lassen Peak in Lassen Volcanic National Park is an affiliation of rhyolite lava domes. A series of explosions and ash clouds from the pinnacle betwixt 1914 and 1921 represents the last big volcanic eruption inside the lower 48 states prior to Washington's Mt. St. Helens in 1980. Hydrothermal features on the flanks of Lassen Top include numerous hot springs, mudpots and fumaroles that propose at that place is nevertheless hot magma beneath the mountain.

Formation of Crater Lake

Crater Lake partially fills the caldera of a volcano that erupted and complanate in on Itself 7,700 years ago.

Mt. Mazama was a composite volcano up to 12,000 anxiety (3,700 meters) in elevation. Its magma chamber was filled with heavy (low-silica) magma in its lower office and lighter (high-silica) magma above. Paul Rockwood painting, NPS.

During the initial stages of its climactic eruption, the upper function of the magma sleeping accommodation poured out ash, pumice, and rhyolite lava flows. Paul Rockwood painting, NPS.

So much magma erupted that the upper function of the mount collapsed into the void, forming the caldera. Paul Rockwood painting, NPS.

Later eruptions of low-silica magma partially filled the caldera with basalt lava flows and cinder cones, including what is now Wizard Island. (The photo on the right is actually Aniakchak Volcano in Aniakchak National Monument and Preserve, a simiar complanate volcano in Alaska). NPS photo.

The coating of volcanic materials sealed the bottom of the caldera. Over a few centuries pelting and snowmelt partially filled the pigsty with the waters of Crater Lake. At 1,943 feet (592 meters), Crater Lake is the deepest lake in the United states of america and the 9th deepest in the globe. Photograph courtesy of Robert J. Lillie.

^{Illustrations to a higher place 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.}

Ancient Subduction in the Sierra Nevada

A glance at a map of the western United states of america might advise that the Sierra Nevada are a s continuation of the Cascade Volcanoes. In the by, that was truthful. Well-nigh 200 million years ago a large tectonic plate (called the Farallon Plate) started to subduct beneath the western edge of North America. The subduction formed a line of volcanoes stretching all the fashion from Alaska to Fundamental America. Beginning about 30 million years ago, then much of the Farallon Plate was consumed by subduction that the Pacific and North American plates were in contact, forming the San Andreas transform plate purlieus in western California. Over time, the San Andreas boundary has grown longer every bit the Farallon Plate split into ii dissever plates—the Juan de Fuca Plate on the n, and the Cocos Plate on the south.

West Coast Tectonic Evolution

Granite rocks institute in national parks in the Sierra Nevada are the cooled and eroded remnants of magma chambers that fed aboriginal subduction zone volcanoes in California.

xl Million Years Ago
Forty million years ago, a large tectonic plate, known every bit the Farallon Plate, was betwixt the Pacific and North American plates. Subduction of the Farallon Plate beneath the entire West Declension created a line of volcanoes from Alaska to Fundamental America.

xx One thousand thousand Years Ago
Equally the mid-bounding main ridge separating the Farallon and Pacific Plates entered the subduction zone, the Farallon Plate separated into the Juan de Fuca and Cocos Plates. A transform plate boundary developed where the Pacific Plate was in contact with the North American Plate and the volcanism ceased in central California. Farther east, the continent began to rift apart in the Bowl and Range Province.

Today
The Cascades are the mod volcanic arc developing where the Juan de Fuca Plate subducts beneath the Due north American Plate. The Sierra Nevada are the eroded remnants of the volcanic arc adult when the Farallon Plate subducted beneath the continent. The San Andreas Fault and Queen Charlotte Error are transform plate boundaries developing where the Pacific Plate moves north past the North American Plate.

^{Illustrations above modified from "Earth: Portrait of a Planet", by S. Marshak, 2001, W. W. Norton & Comp., New York.}

In fundamental and southern California the ancient volcanoes accept largely eroded away. Massive areas of granite from the cooled magma chambers that fed the volcanoes course portions of the Sierra Nevada Mountains, including Yosemite National Park.

Cascade - Sierra Volcanic Connectedness

The Sierra Nevada are a remnant of volcanoes that extended south when the ancient Farallon Plate pigeon beneath the edge of North America. The Coast Range (accretionary wedge), Great Valley (forearc bowl), and Sierra Nevada (volcanic arc) even so reflect the subduction zone topography. Rocks in Yosemite, Kings Canyon, Sequoia, and Joshua Tree national parks contain exumed magma chamber stone that formed beneath the ancient volcanoes. National Park Service sites are shown in red. Four-letter codes indicate the aboriginal volcanic arc parks listed about the tiptop of this page.

^{Modified from "The Geology of our National Parks, Monuments and Seashores," by Robert J. Lillie, New York, West. Westward. Norton and Company, 298 pp., 2005, world wide web.amazon.com/dp/0134905172.}

Ancient Subduction Zone

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.

Country Quarters of W Coast National Parks Represent Modern and Aboriginal Volcanic Landscapes

State quarters can be used to demonstrate the current and ancient volcanic landscapes of the Pacific Northwest and California. From 1999 to 2008, the United States Mint issued 25-cent coins representing each of the l states. These quarters featured iconic images that have special meaning to each state. Not surprisingly, many of the engravings draw landscapes of national parks. The three West Coast states are particularly meaningful because they have geologic features that, when the quarters are stacked, correspond the surface and subsurface of the concatenation of volcanoes that has been developing over the past 200 million years. This tin be demonstrated past stacking the three quarters with California on the lesser, overlain by Oregon and so Washington.

Washington

Left image
Mount Rainier.
Credit: Photo Courtesy of Robert J. Lillie.

Right image
Feature labels. (Click on arrows and slide left and right to come across labels.)

Mt. Rainier in Mt. Rainier National Park is an active composite volcano rising more than 14,000 feet above sea level.

Oregon

Left prototype
Feature labels. (Click on arrows and slide left and right to encounter labels.)

Right image
Crater Lake.
Credit: Photo courtesy of Robert J. Lillie.

Crater Lake in Crater Lake National Park partially fills the large low formed when a composite volcano erupted and collapsed in on itself 7,700 years ago.

California

Left image
Yosemite Valley and One-half Dome.
Credit: Photo Courtesy of Robert J. Lillie.

Right prototype
Feature labels. (Click on arrows and slide left and right to run across labels.)

Half Dome in Yosemite National Park is made of granite that solidified from magma tens of millions of years agone, when a subduction zone extended along the entire west coast. The overlying volcanoes have since eroded away, exposing a vast surface area 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.}

Stack the quarters with Washington on top, Oregon in the heart and California on the bottom. Looking first at Washington, one sees Mt. Rainier, a composite volcano ascension to over xiv,000 feet to a higher place sea level in Mt. Rainier National Park. Imagine if Mt. Rainier were to suddenly and violently erupt—so much that its magma chamber empties and the volcanic superlative collapses in on itself. Remove the Washington quarter and y'all can see what the landscape might expect like, in the form of Crater Lake in Crater Lake National Park in Oregon. An ancient volcano, Mt. Mazama, erupted and collapsed 7,700 years ago, forming the large cavity (caldera) that now holds Crater Lake. Now suppose the subduction processes that form the volcanoes were to stop, shutting off the magma supply that feeds the volcanoes. With continued uplift and erosion, most of the volcanic material would erode. Remove the Oregon quarter to reveal cooled magma chamber rocks beneath. This is what has happened in the Sierra Nevada of key and southern California, as represented by the granite-type rocks of Half Dome within Yosemite National Park shown on the California state quarter.

Southern Alaska Subduction Zone

Five park sites in southern Alaska showcase landscapes formed past the subduction of the northward-moving Pacific Plate beneath North America. One of them, Kenai Fjords National Park, lies within the accretionary wedge of uplifting oceanic sedimentary strata and hard crust. Three others, Aniakchak National Monument, Katmai National Park, and Lake Clark National Park, are further inland, along the active volcanic arc. Another, Wrangell-St. Elias National Park, is a vast region that extends beyond the accretionary wedge and volcanic arc, and is built of accreted terranes.

National Park Service Sites in Alaska Subduction Zone

Messages in ovals refer to NPS sites that lie on the accretionary wedge and volcanic arc formed by due north subduction of the Pacific Plate beneath southern Alaska. Those sites, as well as Wrangell–St. Elias and Denali national parks, are also included in the "Accreted Terranes" part of these web pages. Sites in northern Alaska are in the Brooks Range, discussed in the "Collisional Mountain Range" web pages. B–B' is the line of cross section in the adjacent illustration.

Modified from "Parks and Plates: The Geology of our National Parks, Monuments and Seashores," by Robert J. Lillie, New York, West. Due west. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172.

Southern Alaska Subduction Zone

Plate convergence that built Alaska outward every bit a series of accreted terranes during the by 200 one thousand thousand years continues today. Kenai Fjords has oceanic sedimentary layers that have been metamorphosed, uplifted, and plain-featured as part of the modern accretionary wedge. Aniakchak, Katmai and Lake Clark are part of the active volcanic arc built on older accreted terranes. Wrangell-St Elias spans older accreted terranes equally well equally active volcanoes and uplifting accretionary wedge fabric.

Modified from "Parks and Plates: The Geology of our National Parks, Monuments and Seashores," by Robert J. Lillie, New York, West. W. Norton and Visitor, 298 pp., 2005, www.amazon.com/dp/0134905172.

Accretionary Wedge

Kenai Fjords National Park lies within a coastal mountain range (accretionary wedge) formed as the Pacific Plate subducts beneath southern Alaska. Pillow basalts attest to the oceanic origins of the stone layers, equally they formed from lava flows that cooled on the sea floor. 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 to a higher place the focus of the Great Alaska Earthquake of 1964, one of the two largest always recorded (the other, in 1961, occurred forth the subduction zone off western Southward America). Effects of the 1964 convulsion are quite spectacular in the park; the coastline dropped and then much that in places it lies submerged beneath virtually 8 feet (2.five meters) of water.

Accretionary Wedge in Southern Alaska

Volcanic and metamorphic rocks exposed in Kenai Fjords National Park reveal the incredible forces that occur at an bounding main/continent subduction zone.

Kenai Fjords National Park
Layers of sandstone and shale deposited on the top of the Pacific Plate were after metamorphosed to quartzite and slate every bit the plate subducted beneath southern Alaska, and so shoved upward as part of the accretionary wedge.

^{Photo courtesy of Robert J. Lillie.}

Kenai Fjords National Park
Sea lions resting on layers of pillow basalt that formed beneath the ocean and were uplifted as part of the accretionary wedge.

^{Photo courtesy of Robert J. Lillie.}

Volcanic Arc

More than xl volcanoes have erupted in Alaska since Russians kickoff arrived in the mid 1700s – a great many of the volcanoes are in areas that are at present National Park Service sites. They are role of the volcanic arc that extends northeastward from the Aleutian Islands, across the Alaska Peninsula and Aniakchak National Monument, Katmai and Lake Clark national parks, then bends eastward to Wrangell-St. Elias National Park.

Volcanic Arc Features in Southern Alaska

National Park Service Sites in the Southern Alaska Subduction Zone 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 loftier plenty to dehydrate rocks and form magma.

Active Volcanoes

Lake Clark National Park
Redoubt Volcano

^{USGS photo.}

Wrangell–St. Elias National Park
Mt. Wrangell (left) and Mt. Drum (right).

^{USGS photo.}

Collapse Calderas

Aniakchak National Monument

^{NPS photo.}

Katmai National Park

^{USGS photo.}

Lava Domes

Katmai National Park
Novarupta Lava Dome

^{USGS photo.}

Lake Clark National Park

^{USGS photo.}

Aniakchak National Monument and Preserve is on the Alaska Peninsula. The eruption of Aniakchak Volcano nigh 3,500 years ago rivals the eruption and plummet of Mt. Mazama that formed Crater Lake. The original composite volcano was about seven,000 anxiety (2,000 meters) high, merely lost about 2,500 feet (750 meters) of its height. The 6-mile (10-kilometer) wide caldera is about two,000 feet (600 meters) deep. Like Mt. Mazama, Aniakchak spewed lava across its caldera flooring for centuries after its collapse. The volcano is still agile, having erupted in 1931. Only dissimilar Mt. Mazama, Aniakchak no longer holds a big lake because the caldera wall is breached.

Katmai National Park and Preserve lies where the world'due south largest volcanic event of the 20th Century occurred. In 1912 the Katmai region erupted an estimated seven cubic miles (xxx cubic kilometers) of ash and pyroclastic cloth, roughly 50 times the corporeality that came out of Washington'south Mt. St. Helens in 1980. The plummet caldera from the Katmai eruption is nearly 3 miles (5 kilometers) across and 2,000 feet (600 meters) deep. The caldera has partially filled with water – an approximately half-scale version of the meliorate-known Crater Lake in Oregon. Katmai volcano did collapse, just different Mt. Mazama seven,700 years earlier, the fabric expelled from its magma bedchamber did not spew forth out of the meridian of the volcano. Rather, it flowed clandestine to a vent 6 miles (x kilometers) abroad! 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.

At 20,625 foursquare miles (53,396 foursquare kilometers), Wrangell – St. Elias National Park and Preserve is the largest U. Due south. national park – vi times the size of Yellowstone. The park is a circuitous affiliation of blocks of continental and oceanic crust that have slammed into Northward America. Poking through the mass of some of the highest mountains in North America are very young volcanoes formed by the ongoing Pacific Plate subduction. Amidst them are Mt. Wrangell, a 14,163 foot (4,317 meter) volcano that last erupted during the early part of the 20th Century.

Lake Clark National Park and Preserve contains an active composite volcano with a profile and recent history reminiscent of Mt. St. Helens. In 1989 and 1990, Mt. Redoubt, on the eastern side of the park, had four explosive eruptions that resulted in large volumes of ash and mudflows. And, every bit at Mt. St. Helens, a lava dome afterward grew within the breached crater near the mount's summit.

Volcano Monitoring in National Parks

The eruption of Mt. Redoubt, an active volcano in Lake Clark National Park, illustrates how a volcano can pose an unexpected adventure in our modernistic age – and why monitoring volcanic activeness is so important (see photo below). On December. 15, 1989 a jet with 244 passengers and crew was en road from Amsterdam to Anchorage. Without warning the plane flew right through the eruption deject at 25,000 feet (7,500 meters) altitude. Silica-rich ash melted and coated the hot engine turbines with glass, causing all iv of them to close down. After a terrifying free-fall that lasted eight minutes, the pilots finally managed to restart the engines at only 6,000 feet (2,000 meters). Fortunately, no one was injured. But the incident highlighted the importance of monitoring volcanic action and making the information immediately available to the airline industry and other segments of the public.

Lake Clark National Park, Mt. Redoubt.

USGS photo.

Figures Used

Site Index & Credits

Plate Tectonics and Our National Parks (2020)

• Text and Illustrations past Robert J. Lillie, Emeritus Professor of Geosciences, Oregon State University [E-post]

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

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Source: https://www.nps.gov/subjects/geology/plate-tectonics-subduction-zones.htm

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