Lahar

A lahar is a mud and debris flow emanating from a volcano. The word comes from the Javanese. Here, eruptive material from some of meter-sized blocks mixed with unconsolidated sediments and water. Depending on the terrain slope lahars up to 100 km / h, flowing over 100 km and inundate large areas by gravity, velocity. They can be triggered by a volcanic eruption, but also created entirely independently. Depending on the nature of their creation can lahars up to 100 ° C be hot.

Definition

The definition is not entirely consistent in the literature. The Encyclopedia of Volcanoes limited the term to mud flows in a broader sense, the sediment have a share of between 20 % and 90 % and out of a volcano. This corresponds in sedimentological terms a debris flow ( sediment share about 50-60 % to about 90%) and a hyper- concentrated stream (sediment fraction between 20 % and 50-60 %). Mud flows with lower sediment transport sediment concentrations, such as rivers, either in suspension or as ground transportation. The Encyclopedia of Volcanoes also recommends to limit the term lahar on the process and not apply to the deposit, although the term has since been extended also in the literature in part to the deposits ( " fossil lahars ").

Origin and Cause

Lahars can occur during or shortly after a volcanic eruption. But they can also be completely triggered independently of a volcanic eruption.

Prerequisite for the formation of lahars are:

A large water reservoir in the form of snow, ice or lakes
Large amounts of unconsolidated material, such as pyroclastic flow or fall deposits, glacial deposits, soils and colluvium
Steep slopes and some relief of the delivery area
A trigger that produces for a lahar the right mix (see above)

Typical tripping mechanisms:

Volcanic activity above the snow line. Snow and ice are rapidly melted (see also glacier skiing) and mix with unconsolidated sediments. The result may be very large lahars.
Heavy rains that hit volcanic unconsolidated sediment. Heavy rains are frequent as a result of eruption thunderstorms companion of volcanism, when much water vapor is released from the red-hot molten melt, which then combines with atmospheric water and condensed to the finest ash particles. Mostly this only smaller but numerous lahars are triggered.
As a result of volcanic activity can lakes that were previously dammed by volcanic deposits, by the rupture of natural dams suddenly flow out into the plain. This mechanism also can produce very large lahars.
Collapse -induced lahars. Due to the instability of volcanic masses may Locker slopes or smaller parts of a volcano building collapse. If the sediment pore water enough, it can liquefy the sediment. Shallow intrusions of magma in a volcanic edifices here are the most common causes. Mostly, however, only minor lahars are triggered.

Erosion

Lahars can act on its way down the valley strongly erosive, especially in volcanic Locker masses. This is washed down slopes and hillsides erode, also can secondarily smaller landslides are triggered. In general, erosion is the higher, the more water containing the sludge flows. Decisive for the erosion, however, is that in a short time to drain huge amounts of water or a mud / water mixture. A tidal wave eroded more than a continuous drainage.

Change of lahars when flowing

Flood flows, ie flows of water with less than 20% sediment leadership emanating from volcanoes can be changed by inclusion of sediment also to lahars. This slows the flow rate, but is rather rare. Lahars can also by the further uptake of water " diluted " and thus "normal" will flood flows. Mostly, however, take on lahars sediment and water and change to deposit in their flow behavior is not essential. When draining "collect" lahars literally everything on what is on their way, trees, debris and even human beings.

Natural disasters by lahars

While the direct vicinity of volcanoes that is most at risk of outbreaks, usually remains without colonization, the wider area of the volcanoes is often densely populated because of the fertile soils. The threat of the volcanoes is estimated there to be low. Lahars therefore have a considerable potential for destruction, especially by their wide flow range.

On 24 December 1953, New Zealand's heaviest railway accident when a lahar swept away a railway bridge at Tangiwai occurred. Shortly after the collapse of the overnight train Wellington - Auckland reached the spot and rushed into the sludge stream. 151 people lost their lives, many of the victims were never found.
A lahar during the eruption of Mount St. Helens in the south of the State of Washington, USA May 18, 1980 contributed to the extent of the devastating disaster.
On 13 November 1985 a lahar of the Nevado del Ruiz volcano in Colombia caused the second largest number of deaths by a volcanic eruption in the 20th century. The up to 5 m high mudflow reached the 47 km distant city of Armero about two and a half hours after the eruption and cost two -thirds of the 28,700 inhabitants life.
In the southern Andes of Chile a lahar of the Villarrica volcano grub ( 2847 m above sea level. NN ) December 29, 1971 At a distance of 14 km a flow channel, which is 128 m wide and 8 m deep at its upper edge.
In March 2007, as in 1953 spilled a lahar from Ruapehu volcano, New Zealand. Human lives were not this time to complain, as the authorities had taken along the river Whangaehu timely action.

Laharablagerungen

The deposits of lahars differ in the sedimentary structures indistinguishable from those of a debris flow and a hyper- concentrated flow. There arise depending on the starting composition Debrite, diamictites, conglomerates and breccias Para. They differ from "normal" debris and mud flows basically only by the volcanogenic material of which they consist mainly. There are transitions to "normal" mud flows.

The sudden cover with mud masses also many creatures can be fossilized in certain circumstances. There may be a fossil deposit arise.

Fossil Laharablagerungen

About 5600 years ago was built on Mount Rainier in Washington state a huge lahar. The volume was calculated to be about 3.8 km3. He -filled valleys up to 200 m of sediment, covered a distance back up to 120 km and 20 km under water still flowed at the bottom of Puget Sound on.

At the Monts Dore in the French Auvergne put a lahar in the Tertiary more than 30 km back.

Galunggung Jökulhlaup Dirty thunderstorm 1980 eruption of Mount St. Helens Villarrica (volcano) Whangaehu River Washington (state) France Auvergne (region)

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