Mount Saint Helens - New World Encyclopedia

à 27 avril

Mount St. Helens is the most infamous and deadly. New methods of probing inside the mountain show surprising ways that molten rock moves from the For the previous two months the volcano had been spitting out ash and steam, and the Edwardses were among dozens of observers on surrounding...Mount St. Helens is an active stratovolcano in Skamania County, Washington, in the Pacific Northwest region of the United States. It is located 96 miles (154 km) south of the city of Seattle and 53 miles (85 km) northeast of Portland, Oregon. The mountain is part of the Cascade Volcanoes and the...Glacier Peak and Mount St. Helens are the only volcanoes in Washington State that have generated very large explosive eruptions in the past 15,000 years. The Cascade province is actually made up of two volcanic regions, the older, broader, and deeply eroded Western Cascades and the dominating...Ratings 80% (10) 8 out of 10 people found this document helpful. This preview shows page 14 - 19 out of 33 pages. 29) Mount St. Helens and the young, active stratovolcanoes built on a continental margin above a sinking slab of oceanic lithosphere B)a row of young, active, shield volcanoes built as...Mount St. Helens is an active stratovolcano located in Washington state. Similarly to other volcanoes in the Cascade Range, Mount St. Helens is a large eruptive cone consisting of lava rock interlayered with ash, pumice , and other deposits.

Mount St. Helens | volcano, stratovolcano

Other eruptions during the last 2,500 yr produced dacite and andesite pyroclastic flows and lahars, and around the base of the volcano and partly filled valleys leading away from Mount St. Helens. The first stratigraphic evidence of the existence of Mount St. Helens consists of voluminous dacitic in the Cascade Range were at or near their maximum extents, and the products of eruptions generally...BarrettArcher BarrettArcher. Answer: Mount st. helens and the cascade volcanoes present in Skamania County, Washington. These volcanoes are quiet young and active strato volcanoes. They are mainly built on the continental margin that are marked above the sinking slab of the oceanic...Mount St. Helens is geologically young compared with the other major Cascade volcanoes. It formed only within the past 40,000 years, and the pre-1980 summit cone began rising about 2,200 years ago.[4] The volcano is considered the most active in the Cascades within the Holocene epoch (the...Mount St. Helens was Washington's fifth-highest peak. Towering above Spirit Lake, this beautiful volcanic cone rose over 5,000 feet above its base. Formed within the last 40,000 years, Mount St. Helens is geologically young compared to the other major volcanoes of the Cascade Range and is...

Cascade Volcanoes | Mt. Rainer

just at Mount St. Helens, but for the Cascade Range volcanoes. Forest Service rangers express the signiﬁcance of Mount. St. Helens as a master teacher to a whole new generation of. eruption of Mount St. Helens, scientists, ofﬁcials, and the. public have a new appreciation for potential effects of.Mount St. Helens is primarily an explosive dacite volcano with a complex magmatic system. The volcano was formed during four eruptive stages beginning about 275,000 years ago and has been the most active volcano in the Cascade Range during the Holocene. Prior to about 12,800 years ago...Mount St. Helens is a volcano in the Cascade Mountains, in the area called the Mt. St. Helens National Volcanic Monument. The joint project by scientists at Rice University, the University of Washington, the University of Texas at El Paso and others involves placing 3,500 seismic sensors...Mount St. Helens (known as Lawetlat'la to the Indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington...Further, Mount St. Helens is like other volcanoes in the Cascade Range. problem as choice A and B. Even though like normally signifies correctly formed transition in the GMAT, in this case the formation of the comparison is secondary to the issue and the sentence does not make grammatical sense.

A volcano is an opening in the surface of a planet (or moon) that allows hot material to escape from an area of magma storage below the surface, commonly called a magma chamber. The location where the magma erupts is called the volcanic vent. When the magma breaches the surface of Earth's crust, the volcano erupts, and the erupted magma is referred to as lava. Over time, the erupted materials pile up to form the tall, broad slopes of the volcano. The magma or lava is itself a combination of molten rock, crystals, and gases, and it is this composition that influences the style of eruption and the type of volcano. Eruptions can be explosive, sending hot mixtures of ash, gas, and rock high into the sky. Eruptions can also be calmer, spurting out steam or sending minor amounts of lava down the slope.

But where does the magma come from? Magma is formed in the mantle of the Earth, the layer of the earth just below the crust. The earth has three main layers. If you think of the Earth like an egg, the egg white inside is like the mantle, and the egg yolk is like the core. The mantle is made of solid rock that is very hot, and it actually moves very slowly. The rock in the mantle cycles up and down in a circular motion called convection. This is similar to the way a pot of boiling water heats up on your stove, and this is also a process that regularly occurs within Earth's atmosphere and oceans.

The shell of the egg is like Earth's crust, which is broken into pieces like a jigsaw puzzle. These pieces of crust along with a small portion of the mantle below them are relatively rigid and brittle, and are referred to as tectonic plates. Tectonic plates float on the hot mantle beneath. (The brittle plates are known to geologists as lithosphere, and the more malleable/ductile mantle below is known as the asthenosphere). Once mantle rock melts, the magma has to move through the mantle and through the crust to erupt at a volcano. There are three ways the magma can make it to the surface:

Subduction zones, mid-ocean ridges, and hot spots. Image modified from Nasa SpacePlace.

1) Subduction zones—When tectonic plates move towards each each other at convergent plate boundaries, one of the tectonic plates is pushed under the other, and dives into the mantle. This process is called subduction. The 'diving' tectonic plate is subjected to very high temperatures and pressures as it is forced downward, releasing water from the plate that melts the surrounding mantle and makes magma. Molten magma is less dense than the rest of the mantle (think of what happens when you mix oil and water; the oil always pushes to be on top of the water because the oil is less dense) so it rises to the surface and forms a volcano. Many volcanoes in Washington were built in this way.

2) Mid-Ocean Ridges—When tectonic plates move in opposite directions away from each other at divergent plate boundaries, an opening forms at the surface. This opening releases pressure on the mantle below, allowing it to melt and form magma. Once again, the magma is less dense than its surroundings so it rises to fill the space. This typically happens in oceanic crust underwater and forms long ridges of underwater volcanoes.

3) Hot Spots—The third way that volcanoes can form is from a hot spot inside the Earth. Scientists are still figuring out exactly why hot spots happen where they do, but the basic idea is that a portion of molten magma rises and pushes its way through the middle of a tectonic plate to reach the surface. Yellowstone and the Hawaiian islands are two famous examples of hot spot volcanoes.

Are all volcanoes alike? While many people think of a volcano as a cone-shaped mountain that spits red hot lava and has a plume of ash, there are in fact multiple types of volcanoes. Many of these types of volcanoes (or remnants of them) can be found in Washington state.

The shape, size, and lifespan of a volcano depends on its location (under the ocean, at a convergent plate boundary, a hot spot, etc.) and the chemistry and composition of the magma that erupts from it. In particular, the amount of water vapor (dihydrogen monoxide, a.k.a. H2O) and other gases and the amount of silica (silicon dioxide, SO2) greatly influence the type of magma and the type of eruption. Generally, magmas that contain more silica will be stickier (more viscous), and more likely to flow slowly and erupt explosively. Magma is also influenced by its location; magmas that push through continental versus oceanic crust will mix in with different types of rocks on their journey to the surface. The chemistry of the magma determines whether the volcano erupts either explosively or non-explosively, and the style of eruption also affects the overall shape of the volcano.

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Cinder Cones

Cinder cones are steep, cone-shaped hills made up of cooled, air-filled lava called cinder or scoria that was ejected from a single vent. They usually erupt lavas ranging from basalt to andesite. Cinder cones are commonly found near shield volcanoes, stratovolcanoes, or calderas, and are the most common type of volcano on Earth. They are relatively small, generally no taller than a few hundred feet. Some cinder cones only erupt once while others may erupt many times, and eruptions can last days or even decades! Parícutin cinder volcano in Mexico grew from an eruption that lasted from 1943 to 1952.

Layers and eruptive products of a typical cinder cone volcano. Click the image to enlarge it.

When there is a high gas content in the magma, more explosive eruptions occur, ejecting jagged, fragmented pieces of rock. Gases trapped within magma stored underground will expand when the magma migrates toward the surface for an eruption. As the gases expand, they exert pressure that can cause the magma to break into tiny pieces and explode. During the eruption, these tiny rock fragments build up around the mouth of the volcano and construct a cone over long periods of time. Cinder cone volcanoes are formed entirely of rock fragments and rocks with holes (these holes are known as vesicles). Cinder cones can also produce fire fountains, which are vertical fountains of lava. In addition, lava flows may also flow out from the base of the cone.

Composite Volcanoes/Stratovolcanoes

Composite volcanoes or stratovolcanoes are typically some of the world's most beautiful and beloved mountains. Many of the well-known volcanoes in Washington, including Mount Baker, Glacier Peak, Mount Rainier, Mount Adams, and Mount St. Helens, are stratovolcanoes. These beautiful mountains are what most people think of when they picture a volcano—steep-sided, symmetrical cones that typically have a crater at the summit. Stratovolcanoes can be very tall, many are more than 14,000 feet. They are commonly found at convergent plate boundaries, such as along the edge of the Pacific Ocean within the Ring of Fire.

Stratovolcanoes are built from alternating layers of volcanic ash, lava flows, debris flows, and cinder. These volcanoes form when less fluid, stickier lavas erupt, especially andesite and dacite. These lavas usually erupt after accumulating over long periods of time in a magma chamber within continental crust. As the magma sits underground, it cools partially and crystallizes. If new magma comes into the chamber, it can re-heat the older magma. It can also heat and pull in rocks from the more solid boundary of the magma chamber. This process of new magma entering, re-heating, solidifying, and incorporating surrounding rocks is what makes stratovolcano eruptions increasingly silicic and compositionally diverse.

Anatomy and eruptive activity for a typical stratovolcano. Click the image to enlarge it.

These volcanoes can also be deadly when they erupt due to the gases trapped inside. The biggest hazard for people living near stratovolcanoes is not from lava, which moves slowly down the volcano, but from lahars (fast-moving volcanic mudflows) or pyroclastic flows (fast-moving mixtures of sharp particles and hot gases) that can barrel down the slopes of the volcano at incredible speeds (up to 120 miles per hour!) destroying everything in their path.

Shield Volcanoes

Shield volcanoes include some of the largest volcanoes in the world. They are called shield volcanoes because when you look at them from afar they resemble a warrior's shield. Mauna Loa, a shield volcano on the big island of Hawaii, is one of the largest mountains on earth. It reaches more than 55,000 feet from its base and is approximately 70 miles across. However, some shield volcanoes in the Pacific Northwest are smaller than stratovolcanoes.

Anatomy and eruptive activity for a typical shield volcano. Click the image to enlarge it.

Shield volcanoes have shallow slopes and are made of layer upon layer of cooled lava that flowed down the slopes in all directions from a central summit vent, or group of vents. Lava can also erupt from fractures or fissures along the edges of shield volcanoes, or from non-central vents. Shield volcanoes usually erupt low-viscosity basalt that flows easily down the flanks of the volcano. Though slow-moving, usually averaging only 1 mile per hour, they can move up to 6 miles per hour on steep slopes and they cannot easily be diverted. Near communities, slow-moving lava flows can obstruct roads and burn through homes.

Mid-Ocean Ridge Volcanoes

Though you might not normally think about them as volcanoes, mid-ocean ridges are the largest, most extensive volcanic features on earth. They erupt large amounts of lava—in fact, more than any other type of volcano! Mid-ocean ridges are tectonic plate boundaries where pieces of oceanic crust are being pulled apart to make way for new oceanic crust. As the plates move away from each other, the space between them opens up, reducing pressure on the underlying mantle material. This allows the mantle to melt and magma rises up between the plates.

Diagram of a mid-ocean ridge volcano. Modified from Press and others (2003). Click the image to enlarge it.

As the magma rises from the mantle to the surface, it repeatedly melts and cools, leaving behind a distinctive sequence of rock types. The lowest layer includes peridotite (formed of the bright green mineral olivine) which settles to the bottom of the magma pile. Next are layers of gabbro, an intrusive igneous rock that has the same composition as basalt but cools and solidifies underground. Above that are dikes or magma pathways, essentially cooled columns of basalt. The topmost rock layer consists of pillow basalts, lavas that erupt on the surface of the ocean and make contact with ocean water, causing them to expand into pillowy shapes. These layers comprise the oceanic crust, upon which deep-sea sediments are deposited. The newly made oceanic crust spreads outward and moves away from the ridge, eventually being recycled back into the mantle at a subduction zone.

Fissure Volcanoes

Prolonged eruptions from cracks in the Earth's surface, known as fissure eruptions, can produce very thick lava layers. The Columbia River Basalt Group (CRB) that covers southern Washington and large portions of Idaho and Oregon erupted from multiple fissures over thousands to millions of years. These lavas are a classic example of flood basalt volcanism.

Flood basalts form around the world, and the CRBs, though impressive, are not close to being the largest. In fact, the CRBs are the smallest flood basalt province on Earth, but they are widely studied because they are the youngest and best preserved example of flood basalts worldwide. Other notable flood basalt provinces are in India, known as the Deccan Traps (about 60 million years old), and in Russia and Kazakhstan, known as the Siberian Traps (about 250 million years old). The Siberian Traps are the largest flood basalts known. They cover an area the size of Western Europe and are more than 1 km thick!

Diagram of a fissure eruption/flood basalt in action. Click the image to enlarge it.

The reason these flood basalt provinces erupt so much magma has been a long-standing research question for geologists. The leading hypothesis is that these eruptions are produced from 'mantle plumes', large upwellings of mantle material (think of a lava lamp) that rise up through the crust. When the 'head' of the plume (the broad top of the upwelling) reaches the base of the lithosphere, it generates a lot of magma. This magma then rises through the crust to erupt in these 'hotspots' as flood basalts.

These massive eruptions release large quantities of greenhouse gases, producing a poisonous atmosphere and raising temperatures on Earth. Many researchers think these events may have been partly or wholly responsible for mass extinctions. The Deccan Traps, for example, erupted around the same time that the dinosaurs went extinct, commonly known as the 'K-T' or 'P-T' extinction. The Siberian Traps may be related to the end-Permian mass extinction, during which 96 percent of marine species and 70 percent of land-based species went extinct. This was the most devastating extinction to have occurred in Earth's history.

Caldera Volcanoes

Calderas form when a volcano erupts so much material that its magma chamber empties significantly. When this happens, there is a vacant cavity in the subsurface where the magma used to be. No longer supported, the rocks on top of the empty chamber collapse inward, forming a large collapse caldera.

Diagram of a caldera volcano. Click the image to enlarge it.

Maar Volcanoes

Maars are volcanic craters that form when magma interacts with groundwater and generates explosions. These types of eruptions often happen only once, or they may happen multiple times within quick succession. When magma infiltrates cracks in the Earth and encounters groundwater, the hot magma causes the water to flash to steam. The heat and energy associated with this meeting produces a big explosion, ejecting steam, nearby rocks, and magma fragments, and leaving behind a crater. Over time, these craters often fill with water, making lakes.

Diagram of an erupting maar volcano. Modified from T. L. Thornberry-Ehrlich in the National Park Service. Click the image to enlarge it.

There are many types of lavas. Some volcanoes are capable of producing multiple types of these lavas while others tend to produce the same lavas time after time. Lavas span many compositions from basalt to andesite to dacite and to rhyolite, as well as compositions inbetween. Read the sections below to learn more about the characteristics of different lavas.

Basalt

Basalt is mafic: it's dark in color, often dark gray to black, because it contains between 45 to 53 percent silica. It has a relatively low silica content relative to other lavas, meaning it can flow with ease (it has low viscosity). Because of this flowability, erupted basalt can reach distances greater than 20 km. Basalt is also primitive magma, meaning that is produced simply by the melting of rock in the mantle. Basalt contains the minerals olivine, pyroxene, and plagioclase. Basalt rock is the primary composition of the oceanic crust, of ocean island shield volcanoes like Hawaii, and of flood basalts and large igneous provinces. When it erupts, its temperature is the hottest of all lavas between 1100 and 1250 degrees Celsius. Basalt is the extrusive form of gabbro.

Basalt from Drumheller Channels, Adams County, WA. Photo by Dan Coe, WGS/DNR.

Andesite

Andesite is intermediate, often gray to light black, but contains more silica than basalt: 52 to 63 percent. Andesite rock contains the minerals plagioclase, pyroxene, and some hornblende. Andesites commonly erupt from stratovolcanoes, producing explosive eruptions. The lava flows from Mount Rainier have historically been andesite. Andesite erupts at temperatures between 900 and 1100 degrees Celsius. Andesite is the extrusive form of diorite.

Andesite from Jumbo Dome volcano near Healy, Alaska. Photo by C.E. Cameron, Alaska Volcano Observatory/U.S. Geological Survey.

Dacite

Dacite is intermediate: it's lighter in color, usually light gray, and contains 62 to 69 percent silica. Dacite cotains the minerals plagioclase and quartz with some hornblende, biotite, and pyroxene. Dacite lavas are viscous and tend to form explosive eruptions and lava domes. Dacite is often found in volcanoes near continental subduction zones, such as Mount St. Helens. Dacite is usually produced through mixing of different magmas or when a magma incorporates rock fragments from the continental crust (crustal assimilation). Dacites erupt at temperature between 800 and 1000 degrees Celsius. The intrusive equivalent of dacite is granodiorite.

Dacite from the lava dome inside the crater of Mount St. Helens volcano. Photo by Dan Coe, WGS/DNR.

Rhyolite

Rhyolite is felsic: light gray or tan to pinkish in color, and contains 69 to 80 percent silica. Rhyolite is composed of the minerals quartz, plagioclase, and sanidine, with some hornblende and biotite. The crystals in rhyolite are usually very small and hard to see (aphanitic). Rhyolite is usually erupted explosively as ash or pumice, and is associated with calderas. Some rhyolite lavas were produced by the eruption of Mount Mazama, which was the predecessor to Crater Lake in Oregon. Rhyolite is the extrusive form of granite.

Rhyolite from Novarupta volcano in Alaska. Photo by Cyrus Read, Alaska Volcano Observatory/U.S. Geolgoical Survey.