How can shield volcanoes be described

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Volcanic hazards - pyroclastic currents, glowing clouds and lahars

As fascinating as the natural spectacle of a volcanic eruption is, it also contains a great potential for danger. The dangers do not only come from the volcano itself. Earthquakes and tidal waves (tsunamis) triggered by them can occur in connection with volcanic eruptions and cause devastating damage even at a great distance from the volcano. In extreme cases, climatic changes can have global effects and lead to crop failures and famine. In addition to these catastrophes, it is often the small, everyday dangers that harbor a high potential for accidents. Burns or shortness of breath from gas leaks or scalds from hot springs are the order of the day. There are an unusually high number of injuries at Ol Doinyo Lengai in Tanzania. The black and relatively cold lava encourages carelessness!

You can hardly protect yourself against the great volcanic dangers. Reliable information and spatial distance can only help to avoid this. But how big does it have to be? The example of the Mount St. Helens eruption in 1980 shows that even 10 km can be too close! At that time, a geologist was killed in a directional explosion who was on a ridge 10 km away. He was the victim of a pyroclastic flow, arguably the greatest danger in a volcano. The prominent volcanologist couple Katja and Maurice Krafft also died in a pyroclastic flow on the Unzen volcano in 1991.
You can protect yourself against minor injuries to a limited extent. The standard equipment of a volcanologist definitely includes a helmet, gas mask, protective goggles, leather gloves and alpine boots. Nomex clothing or an aluminum-coated heat protection suit protect against heat radiation. Flammable textiles such as fleece and other synthetic fibers should not be worn near lava flows.



Pyroclastic currents (also called PF's from "pyroclasic flow") arise on dome-forming volcanoes when parts of the lava dome break off. Then a lot of gas is released and a super-hot gas cushion forms on which a mixture of various lava rocks, ashes and glowing lava races down the slope. Inside the stream, temperatures between 300 and 800 degrees Celsius prevail. They can reach speeds of up to 400 km / h and cover distances of up to 60 kilometers (and more). The insidious thing about pyroclastic flows is that they are almost noiseless. They are almost imperceptible in fog or in the dark. In addition, their size is unpredictable. Mostly they find their way through valleys, but like a hovercraft they can overcome larger obstacles and terrain formations. In connection with the pyroclastic currents, the terms glow avalanche and glow cloud are often used. They actually describe the same phenomenon, but differ in how they originate:
Glowing avalanches (base surges) arise preferentially in Plinian eruptions through the collapse of the eruption cloud. They contain more finer material than pyroclastic currents.
Glowing clouds (nuée ardente) form in a sideways explosion.

Slag throwing activity explosive eruptions, on the other hand, take place in a tighter radius around the crater. Most of the time, large boulders and volcanic bombs only fly several hundred meters. At most, you have to reckon with a bombardment of lava rocks a few kilometers away. The size of the pyroclastics can vary widely. There are tons of bombs a few meters in diameter.

Ash rain and the accompanying Eruption cloud let fine particles rain down on large areas. During strong eruptions, the ash clouds darken the sunlight so that day becomes night. Public life is paralyzed. Large amounts of ash collect on streets and roofs. House roofs can collapse under their load, especially in connection with rainfall. In Pompeii this happened after just a few hours. If the ashes are inhaled there is a risk of suffocation. The ashes mix with the moisture in the lungs to form a concrete-like mass. Even light ash rain is very uncomfortable. Somewhat comparable to being constantly sprinkled with sand, which is also irritating to the eyes! The eruption clouds also endanger air traffic.

Acid rain poses a particularly serious danger to plants. These die off after a short time, which leads to crop failures. Acid rain is irritating to the respiratory tract and eyes.

Explosions endanger not only by flying lava and causing the eruption clouds to rise, but also cause a pressure wave. In extreme cases, the explosion bang generated in this way can still be heard a few hundred or thousand kilometers away. At Mount St. Helens, the blast knocked over trees like matches.

Gas eruptions can also cause disasters. Volcanic gases can accumulate in crater lakes. Initially, these gases are dissolved in the water, but an eruption or an earthquake can suddenly release them and float down to the valley as a gas cloud. In 1986, 1,700 people died in a gas eruption on Lake Nyos in Cameroon.

Lava flows mostly flow slowly, so you can comfortably get to safety on foot. Real estate is, of course, at risk once lava flows actually reach inhabited areas. Houses are often destroyed and people lose their apartments. But there are also cases where thin lava can be very fast. In Hawaii, or on Pt. Fournaise speeds of 50 km / h have already been measured. At Lengai, hornitos can collapse in which a lava lake is boiling. This then empties in a real torrent at 500 degrees hot lava!

Streams of mud (Lahars) hold great potential for danger. In 1985, the city of Armero in Colombia was destroyed. It is 72 kilometers from the Nevado del Ruiz volcano. 28,000 people died at that time. The mud stream formed during an explosive eruption that melted parts of the glacier at the top of the volcano. Heavy rains or leaking crater lakes can also trigger lahars. The rainwater mixes with the loose products on the volcanic slope and rushes down into the valley in a mud stream.

Debris and debris avalanches can form during an eruption or arise independently of eruptions. The largest landslide in history occurred on Mount St. Helens in 1980. In the initial stage of the eruption, a volcanic flank and its summit sheared off and plunged as a gigantic debris avalanche into Spirit Lake, 8 km away. By releasing the pressure, a directed explosion and a cloud of embers then took place.

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