What type of cone does mt st helens

These are the remnants of pyroclastic flows superheated avalanches of gas, ash and pieces of rock that carved deep channels down the slopes and onto the relatively flat areas near the base of the mountain. The partially-filled Spirit Lake can be seen just to the northeast of the crater blue-black on the image , and the where most of the energy was directed during the blast is the gray area immediately to the northwest of the crater.

However, on other parts of the mountain, the rejuvenation process is obvious. Ash deposits have supplied minerals which have accelerated vegetation growth various shades of green. Though far from what it looked like 20 years ago, Mount St Helens is actively recovering. Image of the Day Land. Nearly three decades after the catastrophic eruption of Mount St. Helens, the impact on the forest in the blast zone is still obvious in this astronaut photograph.

South of the mountain, lush green forests cover the landscape, while north of the mountain, vegetation remains sparse.

Image of the Day Land Life. EO Explorer. Helens Rebirth. Mount St Helens remains a potent threat and, like many subduction zone volcanoes, has a long history of large eruptions. Although it is now very well monitored, the most recent eruption which started in September , began with very little warning, and it is fair to say that this volcano remains rather unpredictable. Devastation Potential — Although this is a large volcano with a history of damaging eruptions, it is still sufficiently far from major towns and cities that the potential for damage from future activity is not as high as it might otherwise be.

Helens Science and Learning Centre. Helens page. There is also a wealth of footage of the eruption, and its aftermath, on the web. Some of the most spectacular of these have been gathered together in on the Eruptions blog by Erik Klemetti. It is also worth seeing an excellent film from the United States Forestry Service that documents the build up to the eruption, the eruption itself and its impacts.

Castle Creek time marked the start of eruptions that built the modern volcano. It is interesting to note that the change in eruptive behavior from that of the preceding 35,plus years did not follow a long period of dormancy like several that occurred during Mount St.

Helens' earlier history. The dormant interval that followed Castle Creek time apparently lasted about yr. During the next 1, yr, the only eruptions recorded at Mount St. Helens are those associated with the formation of Sugar Bowl, a dome of hypersthene-homblende dacite at the north base of the volcano.

During extrusion of the dome, a directed blast carried rock fragments laterally northeastward in a sector at least 50 degrees wide and to a distance of at least 10 km. The resulting deposits are as much as 50 cm thick and consist of ash , lapilli , and breadcrusted blocks of dacite from the dome, fragments of charcoal, and stringers of material eroded from the underlying soil.

A single fragment of charcoal from within the deposit has a radiocarbon age of about 1, yr, whereas a sample of wood charred and buried by the deposit has an age of about 1, yr Hoblitt and others, , p.

We provisionally assign an age of about 1, yr to the blast deposit; the older date may have been obtained from a fragment of a mature tree that was overridden by the blast. A pyroclastic flow deposit of breadcrusted blocks, as well as prismatically jointed blocks of dacite of the same composition as the dome, was found on the north slope of Mount St.

Helens downslope from Sugar Bowl; this pyroclastic flow may have occurred at the time of the lateral blast. Three lahars containing breadcrusted blocks of similar dacite were formerly exposed in the North Fork Toutle River valley west of Spirit Lake.

These lahars may have been caused by melting of snow by the lateral blast or by the pyroclastic flow. East Dome, a small dome of hypersthene-homblende dacite at the east base of the volcano, may have been formed at about the same time as the Sugar Bowl dome.

East Dome is overlain by tephra of the Kalama period but not of the Castle Creek period, and could have been formed any time between the Castle Creek and Kalama eruptive periods, a time span of about 1, yr.

Most of the rocks visible at the surface of the volcano before eruptions began in were formed during the Kalama eruptive period. Although the range in radiocarbon dates and ages of trees on deposits of Kalama age suggest that the eruptive period lasted from nearly to yr ago, all the events described here probably occurred during a shorter time span, perhaps less than a century.

The Kalama eruptive period began with the explosive eruption of a large volume of dacite pumice layer Wn which forms the basal part of tephra set W. Layer Wn was deposited northeastward from the volcano across northeastern Washington and into Canada Smith and others, , p. At about the same time, pyroclastic flows of pumiceous and lithic dacite moved down the southwest flank of the volcano. The relative timing of these events is poorly known because most of the air-fall tephra was carried eastward and northeastward, whereas the pyroclastic flows have been found only on the southwest flank of Mount St.

A short time later, scoriaceous tephra of andesitic composition was erupted. In addition, andesite lava flows extended down the west, south, and east slopes of the volcano, and andesite pyroclastic flows moved down the north, west, and south flanks. These eruptions of andesite were followed by the extrusion of the dacite dome that formed the summit of the volcano before the May 18, , eruption. Avalanches of hot debris from the dome spilled down over the upper parts of the preceding lava flows, and some of this hot debris partly filled channels between levees of the andesite lava flows on the south side of the volcano Hoblitt and others, , p.

Late in this eruptive period, a pyroclastic flow of pumiceous dacite moved northwestward from the volcano down the Castle Creek valley and covered lahars of summit-dome debris. Charcoal from the pyroclastic-flow deposit has a radiocarbon age of about yr Hoblitt and others, , p. The Kalama eruptive period was characterized by frequent volcanism of considerable variety; rock types being erupted alternated from dacite to andesite and back to dacite, and the volcano grew to its pre size and shape.

The eruptive period was followed by a dormant interval of about yr. The Goat Rocks eruptive period began about A. This pumice was carried northeast-ward across Washington to northern Idaho Okazaki and others, , p. Many minor explosive eruptions of the Goat Rocks period were observed by explorers, traders, and settlers from the 's to the mid's. The last eruption of the Goat Rocks eruptive period was in , when "volumes of dense smoke and fire" were noted Frank Balch, quoted in Majors, , p.

A recent study of old records has suggested that minor eruptions of Mount St. Helens also occurred in , , and Majors, , p. The published descriptions of these events suggest that they were small-scale steam explosions, and none produced deposits that were recognized in our studies. One of the most interesting features of Mount St. Helens' history is the change in eruptive behavior that occurred about 2, yr ago. Eruptions of dacite had characterized the volcano for more than 35, yr.

Then, with virtually no interruption in eruptive activity, andesite and basalt began to alternate with dacite, and not always in the same order. The chemical composition of eruptive products changed gradually during some episodes and abruptly during others. Thus, basalt followed dacite and dacite succeeded basalt; andesite followed dacite of considerably different SiO2 content, and vice versa.

Some of these changes in composition of eruptive products are not adequately explained as results of eruption of cyclic sequences of compositionally different magmas derived from successively deeper levels in a larger magma body that differentiated at shallow depth, as proposed by Hopson and Hopson and Melson An alternative explanation that fits the stratigraphic record better, suggested by R.

Wilcox oral commun. Explosive eruptions of volumes on the order of 0. Helens during some eruptive periods in the past. This record suggests that a similar sequence could occur during the present period of activity and could result in one or more explosive magmatic eruptions of similar or larger volume than the eruption of May If the lengths of the last two eruptive periods are a valid guide to the future, we might expect intermittent eruptive activity to continue for several decades.

Eruptive History References. Alley, N. Carithers, Ward. Crandell, D. Helens, Washington: U. Geological Survey Bulletin A, 23 p. Helens volcano, Washington: U. Geological Survey Bulletin C, 26 p.

Geological Survey Professional Paper D, p. Helens volcano; recent and future behavior: Science, v. Fulton, R.