Stratovolcanoes have relatively steep sides and are more cone-shaped than shield volcanoes. They are formed from viscous, sticky lava that does not flow easily. The lava therefore builds up around the vent forming a volcano with steep sides. Stratovolcanoes are more likely to produce explosive eruptions due to gas building up in the viscous magma. Andesite named after the Andes Mountains , is perhaps the most common rock type of stratovolcanoes, but stratovolcanoes also erupt a wide range of different rocks in different tectonic settings.
Stratovolcanoes, also known as composite cone volcanoes, erupt viscous lava that forms a steep-sided, triangular-shaped structure. Its last known eruption was in The low afternoon sun emphasises the conical shape of the volcano. As viscous lava is not very fluid, it cannot flow away from the vent easily when it is extruded. Instead it piles up on top of the vent forming a large, dome-shaped mass of material. Magma is stored beneath a volcano in a magma chamber.
When a very large, explosive eruption occurs that empties the magma chamber, the roof of the magma chamber can collapse to form a depression or bowl with very steep walls on the surface. These are calderas and can be tens of miles across. Calderas can also be formed during an eruption that removes the summit of a single stratovolcano. Caldera-forming eruptions can remove massive portions of a single stratovolcano.
The top can literally be blown off! This satellite photo shows the summit caldera of the volcano, which is 6 km in diameter and 1. In rare cases they can even form away from volcanic features. It all depends on the magmatic attributes of the area around them. There are domes on Mount St. Helens which are located on its flanks. These types of domes will eventually help rebuild the mountain. There are different types of composition that a lava dome can have and those include anything from rhyolite to basalt.
It is important to know however that most of the time the lava that will form a dome will have a higher content of silica Si That composition is what makes the lava more viscous.
While a dome can take one of many different types of textures, usually they tend to be blocky in nature. A lava dome can take many different shapes the first one of which is the Tortas kind.
The reason for the name is the similarity that the shape has to a cake or otherwise known as a torta. This is a very common type of formation but it does happen more often in South America near the Andes Mountains. The Peleean type of lava dome is probably the steepest of all domes. Typically they are similar to Tortas with the difference that they have smoother upper surfaces. As the cone is stripped away, the hardened magma filling the conduit the volcanic plug and fissures the dikes becomes exposed, and it too is slowly reduced by erosion.
Finally, all that remains is the plug and dike complex projecting above the land surface--a telltale remnant of the vanished volcano. Shishaldin Volcano, an imposing composite cone, towers 9, feet above sea level in the Aleutian Islands, Alaska. A n interesting variation of a composite volcano can be seen at Crater Lake in Oregon. From what geologists can interpret of its past, a high volcano--called Mount Mazama- probably similar in appearance to present-day Mount Rainier was once located at this spot.
Following a series of tremendous explosions about 6, years ago, the volcano lost its top. Enormous volumes of volcanic ash and dust were expelled and swept down the slopes as ash flows and avalanches. These large-volume explosions rapidly drained the lava beneath the mountain and weakened the upper part. The top then collapsed to form a large depression, which later filled with water and is now completely occupied by beautiful Crater Lake.
A last gasp of eruptions produced a small cinder cone, which rises above the water surface as Wizard Island near the rim of the lake. Depressions such as Crater Lake, formed by collapse of volcanoes, are known as calderas.
They are usually large, steep-walled, basin-shaped depressions formed by the collapse of a large area over, and around, a volcanic vent or vents. Calderas range in form and size from roughly circular depressions 1 to 15 miles in diameter to huge elongated depressions as much as 60 miles long. Crater Lake, Oregon; Wizard Island, a cinder cone, rises above the lake surface. The Evolution of a Composite Volcano A. Magma, rising upward through a conduit, erupts at the Earth's surface to form a volcanic cone.
Lava flows spread over the surrounding area. As volcanic activity continues, perhaps over spans of hundreds of years, the cone is built to a great height and lava flows form an extensive plateau around its base. During this period, streams enlarge and deepend their valleys. When volcanic activity ceases, erosion starts to destroy the cone. After thousands of years, the great cone is stripped away to expose the hardened "volcanic plug" in the conduit.
During this period of inactivity, streams broaden their valleys and dissect the lava plateau to form isolated lava-capped mesas. Continued erosion removes all traces of the cone and the land is worn down to a surface of low relief.
0コメント