Debris avalanche
The term debris avalanche (ger .: debris avalanche ), even rubble or debris avalanche known, describes the geology a transport and deposition process that occurs when a part of a volcano collapses, almost all of the volcano or a volcanic island and slip.
It is the sudden and very rapid mass movement of a disjointed, unsorted large amount of rock and earth, which is mobilized and kept in motion by gravity. The collapse can be triggered by the rise of new magma , a volcanic explosion or an earthquake ; Ad hoc events such as instability due to erosion and overloading of the flanks due to continued eruptions are less common .
Although the term debris avalanche is kept neutral, it has so far been used almost exclusively in connection with mass movements that emanate from volcanoes. Few publications use the clearer term volcanic debris avalanche . Non-volcanic "debris avalanches" are usually as landslides , while the term landslide often for Muren disposals is used.
Volcanic debris avalanches almost always move at high speeds (often several hundred km / h) and over long distances (up to several tens of kilometers). They can therefore be very destructive. Debris avalanches triggered underwater or sliding into the sea can trigger devastating tsunamis or other mass transport phenomena. They are not uncommon disasters; It is estimated that 75% of all Andean volcanoes with altitudes above 2500 m have already collapsed the volcanic structure behind them.
definition
Volcanic debris avalanches are triggered by a partial or total collapse of a volcanic building. As with lahars , their formation is not directly linked to a volcanic eruption, although volcanic eruptions can also be a trigger. Basically, they are landslides that occur on a volcano (see Lahar and Debris Flow ). Rockslides are incoherent and unsorted masses of rock and earth that are primarily mobilized or kept in motion by gravity. The movement takes place as flowing, whereby the avalanche can be dry or wet. The components can be greatly reduced in size during movement; however, increasingly larger debris is typically retained , which can have a diameter of more than 100 m. Subaeric debris avalanches, i.e. debris released on land, can also contain considerable amounts of organic material, especially if the moving terrain was forested. The flow behavior of a debris avalanche can change on its way, or a debris avalanche can also trigger other mass transports.
Compared to non-volcanic landslides, volcanic debris avalanches have some special features. Volcanoes, especially stratovolcanoes , are composed of deposits of soft, fragmented, e.g. Sometimes non-lithified (solidified) pyroclastic sediments are built up, which alternate with massive lava flows or pyroclastic flow deposits. The loose sediments, but also the porous volcanic rocks, can contain considerable amounts of water, usually significantly more than sedimentary rocks or metamorphic rocks . Often these rocks or loose material have also been chemically and mineralogically altered by hot and acidic groundwater and / or rising aggressive gases. The hardness and internal strength of the rocks can be reduced. However, clay minerals , for example, can also be produced, which have a high level of lubricity. Furthermore, the geometry and the internal structure of a volcano already favor gravitational transport processes. A volcano often overlooks the surrounding clear the layers fall from the site of the eruption off up to 30 ° and more. Stratovolcanoes in particular have a typical alternation of lava flows and pyroclastic sediments, whereby the pyroclastic sediments can serve as sliding horizons. Stratovolcanoes in particular with large volumes of loose sediments are also particularly prone to erosion and thus tend to be unstable. This also applies to volcanic islands, the coastline of which is incessantly eroded by the surf , so that the emergence of mass movements is favored.
The term originally only describes the transport and deposition process (see avalanche ), but not the deposition. Some authors therefore consistently refer to the deposits as debris avalanche deposits ( debris avalanche deposits ). In spite of this, the term has meanwhile also become established in many publications for the deposits resulting from this process.
Volcanic debris avalanches were only recently recognized as a separate volcanic transport and deposition phenomenon. Although the deposits were known much earlier, they were misinterpreted as deposits of lahars, ultra-powerful volcanic eruptions or pyroclastic density flows, lava flows or even moraines . Above all, the similarities in the texture and the internal structure of the deposits as well as the morphology of the deposit surface led to these misinterpretations. The discovery of submarine debris avalanches at the foot of volcanic islands and their catastrophic consequences for the closer and wider surroundings of the islands has enormously increased interest in this phenomenon, even in the non-specialist media.
Emergence
Volcanic debris avalanches can be triggered directly by the flanks of a volcano becoming unstable or by earthquakes. A debris avalanche can also be triggered indirectly by an eruption (earthquake and change in slope inclination due to rising magma) or directly (by a phreatic , phreatomagmatic or magmatic explosion). In the latter case, however, they predominantly contain no juvenile material, i.e. H. Pyroclasts from the triggering eruption, but only volcanic "old rock" from earlier eruptions.
According to the causes of the discharge of rubble dunes, three types are distinguished:
- Besymjanny type (igneous explosions precede the avalanche),
- Bandai type (phreatic explosions precede the avalanche),
- Ounce type (the finish is caused by an earthquake).
In some cases, the departure of a debris avalanche can trigger the departure of another avalanche.
Debris avalanches usually or initially occur under water-undersaturated conditions (in contrast to water-saturated lahar ). The water in the pore spaces therefore does not initially play a major role in transport. Overall, however, the transport and sedimentation body of a debris avalanche behaves similarly to a viscous mass or pulp. By absorbing water and clay, but also by breaking up the components of the matrix and mobilizing pore water (possibly also by melting the ice contained in it), a debris avalanche can turn into a lahar during transport. Submarine, a debris avalanche can also change into a debris flow through the absorption of sediment.
The amphitheater, a semicircular demolition structure, is typical of the delivery area or the starting area of a volcanic debris avalanche. The depth, width and height of an amphitheater are variable, depending on the volume of the debris avalanche. It can even include the (earlier) summit or the central vent. In extreme cases, almost an entire volcanic building or island can collapse, as happened around 1888 with Knight's Island northeast of Papua New Guinea.
Deposits from a volcanic debris avalanche
The debris avalanche deposits are usually much longer than they are wide. In the vertical, the relatively narrow sliding area with few deposits and the actual deposit area can be subdivided. The actual run-out area is typically lobate or lobe-like. In contrast to the deposits of a debris flow , the deposits of a debris avalanche are much thicker, but the flow width is usually smaller.
The deposition of a debris avalanche is characterized by two facies, the block facies and the matrix facies. The blocks consist of broken and deformed rock blocks from the delivery volcano. The size varies from several hundred meters in diameter (or even over a kilometer) to less than a meter in diameter. Internal fractions, which are called jigsaw fractions (for example, 'puzzle fractions'), are very characteristic . The “matrix” consists of a mixture of small rock components from different parts of the delivery volcano. Occasionally, fragments of paleo soils and plants are included. In this context, matrix simply means the slightly finer sediment between the larger blocks. There is no exact definition of the grain size in this case.
Characteristic geomorphological structures in the avalanche deposit area are small hillocks ( hummocks ) on the surface of the deposit. The shape of the hillocks is variable, and sometimes a parallel arrangement of the hillocks has been observed. However, there is no general trend in the arrangement of the hillocks. The deposit itself is often flanked by longitudinal deposits in the form of hills ( levees ) along the edges of the debris avalanche. They are mainly found in the middle section (in the longitudinal direction) and can protrude up to 40 m above the surface of the deposit. In the marginal (distal) part of the debris avalanche instead, a royal cliff edge (Engl. Marginally cliff ) may be obtained. The forehead of the debris avalanche is also often formed as a cliff (eng. Distal cliff ). In the case of a non-volcanic debris avalanche, the cliff at the forehead was approx. 20 m high.
Transport distances
The transport distance of a debris avalanche depends on the height of the collapse above the deposit. The largest previously observed transport distance of a subaeric volcanic debris avalanche was at least 45 km, submarine up to 130 km are proven. Most avalanches of debris, however, come to rest well before that. A ratio can be calculated from the maximum collapse height and the maximum transport distance. It is generally between 0.2 and 0.06 and is on average lower than for non-volcanic debris avalanches, i.e. H. At the same initial altitude, volcanic debris avalanches continue. This is explained by the fact that volcanic rock can be changed by hydrothermal water.
Classification in the area of mass transport
As already mentioned in the definition, the equivalent in the non-volcanic area is the landslide. In engl. In the linguistic area, the term debris avalanche is occasionally used in the non-volcanic area. However, terms such as rock avalanche , land slide i. w. S. used more often. Rarely found actually clearer concept of volcanic debris avalanche (Engl. Volcanic debris avalanche ). Rockslide and (volcanic) debris avalanche differ not only in terms of their components, but also often in terms of the distance between them and the height of the demolition. Both are in principle "dry", at least water-undersaturated, i. H. the water does not play a major role in the transport. However, through further absorption of mud and water, the avalanche of debris can become a lahar . The equivalent to this in the non-volcanic area is a debris flow.
Another transition area exists between debris avalanches and pyroclastic density flows . This occurs when avalanches of debris are triggered by magmatic eruptions, for example when a lava dome is demolished. It can be very hot just a little below the surface. Since a lava dome is very often directly related to an eruption, i.e. basically represents juvenile material, such an eruption can also be located at the (super) dense end of a pyroclastic density flow. An essential difference, however, is mostly to be seen in the fact that the material of a pyroclastic density flow is very strongly fragmented by the explosive eruption and is very hot in relation to the avalanche of debris. The term “glowing avalanche”, which was coined for the dense part of a pyroclastic flow, alludes on the one hand to the transport process and on the other hand to the temperature.
If volcanoes in high geographical latitudes or very high volcanoes are affected by a debris avalanche, a third transition area can exist, to the ice fall . If the summit areas are icy, avalanches of debris can contain significant amounts of ice, which initially behaves like solid rock. However, as the ice melts during transport, the avalanche of debris can quickly turn into a lahar.
Avalanches of debris as a trigger for other types of mass transport
Avalanches of debris that slide from volcanic islands into the sea can trigger other mass transports submarine. Presumably the Canary Debris Flow, the deposits of which lie northwest of the Canary Islands in the deep sea, was triggered by a debris avalanche.
Avalanches of debris from Etna , which slid into the Ionian Sea , not only triggered devastating tsunamis in the Mediterranean, but also presumably so-called mega turbidites , which were detected at the bottom of the Ionian Sea.
Disasters triggered by volcanic debris avalanches
Volcanic debris avalanches that slide into the sea can trigger huge tsunamis, which, depending on the volume of the avalanche, can be up to 100 m high. The collapse of a flank of the Lānaʻi volcano (Hawaii Islands) about 105,000 years ago caused a series of tsunamis that reached altitudes on the island that were 375 m above sea level.
After a long period of rest, the Unzen volcano erupted on February 29, 1792 . The volcano initially produced a smaller lava flow. On April 9, 1792, a smaller part of a 4,000 year old lava dome broke off 4 km from the eruption site. On May 21, 1792, an earthquake triggered an even larger avalanche of debris. The volume was about 0.5 km³. The avalanche flowed into the neighboring Ariake Bay, triggering a large tsunami there. Over 15,000 people died as a result of this avalanche of debris, around 11,000 of them from the tsunami.
Historical and prehistoric debris avalanches
On December 30, 2002, the western flank Sciara del Fuoco of Stromboli broke off and a small debris avalanche (<0.01 km 3 ) slid into the sea and triggered a small, several meter high tsunami, which fortunately did not cause any material damage or loss of Resulted in human life.
On May 18, 1980, the northern part of Mount St. Helens collapsed, sending a volcanic debris avalanche into the valley of the upper tributary of the Toutle River. The avalanche ran about 28 km and had a volume of about 2.5 km 3 . The speed of the avalanche, calculated from a series of photos, was around 50 to 70 m per second (around 180 to 250 km / h). The event triggered a lateral explosion. This was followed by a Plinian eruption that triggered a pyroclastic current .
300,000 to 360,000 years ago, part of Mount Shasta collapsed at an altitude of around 3500 m and triggered the largest known Quaternary debris avalanche to date . It moved a distance of at least 45 km.
Even larger avalanches of debris are known from volcanic islands (for example La Palma , Tenerife , El Hierro , Hawaii and many others). The deposits are now at the foot of these islands in the deep sea. Around the Canary Islands, Masson et al. (2001) the deposits of 14 debris avalanches, most of which occurred in the last million years. The largest debris avalanches in the Canary Islands had volumes of up to 500 km 3 . When these debris avalanches left, huge tsunamis must have been triggered, which could have reached and flooded the east coast of North America.
Individual evidence
- ↑ a b for example McGuire, 1996
- ↑ see Silver et al. 2005
- ↑ a b c Masson et al. 2002: p. 1
- ^ Pareschi, Boschi and Favalli, 2006
- ↑ Chiocci and de Alteriis, 2006, p.
- ↑ according to other sources 24 km / h ( [1] )
- ↑ according to other sources 2.7 km 3 ( [2] )
- ↑ according to other sources 110 to 240 km / h ( [3] )
literature
- Tadahide Ui, Shinji Takarada and Mitsuhiro Yoshimoto: Debris Avalanches. In: Haraldur Sigurdsson (Ed.): Encyclopedia of Volcanoes. Pp. 617-626, Academic Press, San Diego et al., 2000, ISBN 0-12-643140-X
- Hans Füchtbauer: Transport processes and sediment structures. In: Hans Füchtbauer (Ed.): Sediment Petrology, Part 2: Sediments and sedimentary rocks. 4th ed., Pp. 779-863, E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart 1988 ISBN 3-510-65138-3 .
- DG Masson, AB Watts, MJR Gee, R. Urgeles, NC Mitchell, TP Le Bas, and M. Canal: Slope failures on the flanks of the western Canary Islands. Earth Science Reviews, 57: 1-35, Amsterdam 2002 ISSN 0012-8252
- Hans-Ulrich Schmincke: Volcanism. 324 pp., Springer Verlag 2004, ISBN 3540436502
- Francesco Latino Chiocci and Giovanni de Alteriis: The Ischia debris avalanche: first clear submarine evidence in the Mediterranean of a volcanic island prehistorical collapse. Terra Nova, 18: 202-209, Oxford 2006 ISSN 0954-4879 ( doi : 10.1111 / j.1365-3121.2006.00680.x )
- William J. McGuire: Volcano instability: a review of contemporary themes. In: William J. McGuire, AP Jones & J. Neuberg (Eds.): Volcano Instability on the Earth and Other Planets. Geological Society Special Publication, 110: 1-23, London 1996.
- Eli Silver, S. Day, S. Ward, G. Hoffmann, P. Llanes, A. Lyons, N. Driscoll, R. Perembo, S. John, S. Saunders, F. Taranu, L. Anton, I. Abiari , B. Applegate, J. Engels, J. Smith and J. Tagliodes: Island Arc Debris Avalanches and Tsunami Generation. Eos, Transactions, American Geophysical Union, 86 (47): 485-487, Washington 2005 ISSN 0096-3941 ( online at academia.edu )
- MT Pareschi, E. Boschi and M. Favalli: Lost tsunami. Geophysical Research Letters, 33, L22608, doi : 10.1029 / 2006GL027790 Abstract .
Web links
- Debris Avalanche (PDF; 2.66 MB)
- Debris Avalanches and Volcanic Landslides