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VOLCANO FEATURES AND TYPES

VOLCANO FEATURES AND TYPES

There are several different types of volcanoes based on their shape, eruption style, magmatic composition, and other aspects. The figure shows the main features of a typical stratovolcano: 1) magma chamber, 2) upper layers of lithosphere, 3) the conduit or narrow pipe through which the lava erupts, 4) the base or edge of the volcano, 5) a sill of magma between layers of the volcano, 6) a diapir or feeder tube to the sill, 7) layers of tephra (ash) from previous eruptions, 8 & 9) layers of lava erupting from the vent and flowing down the sides of the volcano, 10) the crater at the top of the volcano, 11) layers of lava and tephra on (12), a parasitic cone [A parasitic cone is a small volcano located on the flank of a larger volcano such as Shastina on Mount Shasta. Kilauea sitting on the flank of Mauna Loa is not considered a parasitic cone because it has its separate magma chamber, 13) the vents of the parasite and the main volcano, 14) the rim of the crater, 15) clouds of ash blown into the sky by the eruption; this settles back onto the volcano and surrounding land.

The most massive craters are called caldera, such as the Crater Lake Caldera in Oregon. Many volcanic features are produced by viscosity, a fundamental property of lava. Viscosity is the resistance to flowing by a fluid. Low viscosity magma flows easily more like syrup, the basaltic volcanism that occurs in Hawaii on shield volcanoes. High viscosity means a sticky magma, typically felsic or intermediate, that flows slowly, similar to toothpaste.

SHIELD VOLCANOES

The largest volcano is a shield volcano and is characterized by broad, low-angle flanks, a small vent or groups of vents at the top, and basaltic magma. The name “shield” comes from the side view resembling a medieval warrior’s shield. They are typically associated with hotspots, midocean ridges, or continental rifts where upper mantle material rises, and build up slowly from many low-viscosity basaltic lava flows that can travel long distances, hence making the low-angle flanks. Because the magma is basaltic and low viscosity, the eruption style is not explosive but rather effusive, meaning that volcanic eruptions are small, localized, and predictable. Therefore, this eruption style is not typically much of a hazard.

Mauna Loa (info) and the more active Kilauea (info) in Hawaii are good examples of vents on a shield volcano. The eruption of Kilauea from fissures in Hawaii in 2018, while not explosive, produced viscous lavas that did considerable damage to roads and structures. Shield volcanoes are also found in Iceland, the Galapagos Islands, Northern California, Oregon, and the East African Rift (USGS, 2011).

The most substantial volcanic edifice in the Solar System is Olympus Mons on Mars, a shield cone as large as the state of Arizona indicating little if any plate tectonic activity on Mars as the volcano erupted over the same hotspot for millions of years.

Basaltic magma can form several rock types and unique landforms. Based on magma temperature, composition, and content of dissolved gases and water vapor, there are two main types of basaltic volcanic rocks with Hawaiian names – pahoehoe and aa. Pahoehoe is a basaltic magma that flows smoothly into a “ropey” appearance. In contrast, aa (sometimes spelled a’a or ʻaʻā and pronounced “ah-ah”) has a crumbly blocky appearance [17]. (Peterson and Tilling 1980). Felsic silica-rich lavas also form aa flows.

In basaltic lava flows, the low viscosity lava can smoothly flow, and it tends to harden on the outside but continue to flow internally within a tube. Once the interior flowing lava subsides, the tube may be left as an empty lava tube. Lava tubes famously make caves (with or without collapsed roofs) in Hawaii, Northern California, the Columbia River Basalt Plateau of Washington and Oregon, El Malpais National Monument in New Mexico, and Craters of the Moon National Monument in Idaho. Fissures, cracks that originate from shield-style eruptions, are also common. Magmas from fissures are typically very fluid and mafic. The volcanic activity itself causes some fissures, and some can be influenced by tectonics, such as the common fissures parallel to the divergent boundary in Iceland. See above for fissure flows from Kilauea in 2018.

Since basalt flows are thick accumulations of lava with a homogeneous composition that flows quickly when the lava begins to cool it can contract into columns with a hexagonal cross-section called columnar jointing. This feature is common in basaltic lava flows but can be found in more felsic lavas and tuffs as well.

COMPOSITE VOLCANOES

Composite volcanoes, also called stratovolcanos, has steep flanks, a symmetrical cone shape, a distinct crater, and rises prominently above the surrounding landscape. The figure at the beginning of this section shows a stratovolcano. Examples include Mount Rainier in the Cascade Range in Washington and Mount Fuji in Japan. Stratovolcanoes can have magma with felsic to mafic composition. However, felsic to intermediate magmas are most common. The term “composite” refers to the alternating layers of pyroclastic materials (like ash) and lava flows. The viscous nature of the intermediate and felsic magmas in subduction zones results in steep flanks and explosive eruption styles. Stratovolcanoes are made of alternating lava flows and ash.

LAVA DOMES

Lava domes are a relatively small accumulation of silica-rich volcanic rocks, such as rhyolite and obsidian, that is too viscous to flow, and therefore, pile high close to the vent. The domes often form within the collapsed crater of a stratovolcano near the vent and grow by expansion from within. As it grows, its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. An excellent example of a lava dome is inside of a collapsed stratovolcano crater is Mount Saint Helens. Examples of a stand-alone lava dome are Chaiten in Chile and the Mammoth Mountain in California.

CALDERAS

Calderas are usually large, steep-walled, basin-shaped depressions formed by the collapse of a volcanic edifice into an emptying magma chamber. Calderas are generally very large with a diameter up to 15 miles. Although the word caldera only refers to the vent, many use calderas as a volcano type, typically formed by high-viscosity felsic volcanism with high volatile content. Crater Lake, Yellowstone, and Long Valley Caldera are good examples. At Crater Lake National Park in Oregon, about 6,800 years ago Mount Mazama was a composite volcano that erupted in a large explosive blast ejecting massive amounts of volcanic ash. The eruption rapidly drained the underlying magma chamber causing the top to collapse into it forming a significant depression that later filled with water. Today a resurgent dome is found rising through the lake as a cinder cone, called Wizard Island.

The Yellowstone caldera erupted three times in the recent past, at 2.1, 1.3, and 0.64 million years ago. Each eruption created large rhyolite flows and pyroclastic clouds of ash that solidified into tuff. These extra large eruptions rapidly emptied the magma chamber causing the roof to collapse and form a caldera. Three calderas are still preserved from these eruptions, and most of the roads and hotels of Yellowstone National Park are located within the caldera. Two resurgent domes are located within the last caldera.

Yellowstone volcanism started as a hot spot under the North American lithosphere about 17-million years ago near the Oregon/Nevada border. As the North American plate slid southwestward over the stationary hotspot, surface volcanism followed and helped form Idaho’s Snake River Plain, eventually arriving at its current location in northwestern Wyoming. As the plate moved to the southwest over the stationary hotspot, it left a track of past volcanic activities.

The Long Valley Caldera near Mammoth California is a massive explosive volcano that erupted 760,000 years ago and dumped a significant amount of ash throughout the United States, similar to the Yellowstone eruptions. This ash formed the large Bishop Tuff deposit. Like the Yellowstone caldera, the Long Valley Caldera contains the town of Mammoth Lakes, a major ski resort, an airport, and a major highway. Further, there is a resurgent dome in the middle and active hot springs.

CINDER CONES

Cinder cones are small volcanoes with steep sides, made of cinders and volcanic bombs ejected from a pronounced central vent. Typically, they come from mafic lavas that have high volatile content. Cinders form when hot lava is ejected into the air, cooling and solidifying before they reach the flank of the volcano. The largest cinders are called volcanic bombs. Cinder cones form in short-lived eruption events that are relatively common in the western United States.

A relatively recent and striking example of a short-lived cinder cone is the 1943 eruption near the village of Parícutin, Mexico. The cinder cone started with an explosive eruption shooting cinders out of a vent in the middle of a farmer’s field. Quickly, volcanism continued building the cone to a height of over 300 feet in a week and 1,200 feet in the first eight months. After the initial explosive gases and cinders were released, growing the cone, basaltic lava poured out around the base of the cone. This order of events is typical for cinder cones: first violent eruption, then the formation of cone and crater, followed by a low-viscosity lava flow from the base (the cone of cinders is not strong enough to support a column of lava rising to the top of the crater). The Parícutin cinder cone was built over nine years and covered about 100-square miles with ashes and destroyed the town of San Juan.

FLOOD BASALTS

A rare volcanic eruption type, unobserved in modern times, is the flood basalt. Flood basalts are some of the largest and lowest viscosity types of eruptions known. They are not known from any eruption in human history, so the exact mechanisms of eruption are still up for debate. Some famous examples include the Columbia River Flood Basalts in Washington, Oregon, and Idaho, the Deccan Traps, which cover about 1/3 of the country of India, and the Siberian Traps, which may have been involved in the Earth’s largest mass extinction at the end of the Permian.

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Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Kategoria: Moje artykuły | Dodał: kolo (2019-04-04)
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