Vulkan
Ein Vulkan ist eine geologische Struktur, an der Magma (geschmolzenes Gestein) nahe an die Oberfläche eines Planeten (etwa der Erde) kommt. Der Name leitet sich von der italienischen Insel Vulcano ab. Die Römer hielten diese Insel für die Schmiede von Vulcanus, dem römischen Gott des Feuers.
Wenn das Magma zur Oberfläche durchbricht, spricht man von einem Vulkanausbruch. Dabei fließt das Magma manchmal selbst als Lava, oder es schmilzt umgebendes Gestein, das dann als Lava fließt. Weiterhin können Asche, Obsidian, Dampf und Gase abgegeben werden (je nach Zusammensetzung des Magmas und der umgebenden Gesteine). Vulkanausbrüche sind oft von Erdbeben begleitet.
Die bekanntesten aktiven Vulkane in Europa sind: der Ätna auf Sizilien, der Vesuv nahe Neapel und der Stromboli zwischen Sizilien und Neapel. Einige andere bekannte Vulkane sind der Hekla, der Krakatoa, Mt. Fuji, Mauna Loa, Mauna Kea sowie der Mt. Saint Helens. Auf der Venus und dem Mars gibt es ebenfalls große, jedoch bereits erloschene Vulkane. Aktive Vulkane sind außer auf der Erde bislang nur noch auf dem Jupitermond Io bekannt.
Weltweit gibt es etwa 1500 Vulkane, die als aktiv betrachtet werden. Vulkane werden in zwei Typen unterteilt:
- Rote Vulkane: Sie enthalten Magma, die wenig Siliziumdioxid und Gas enthält und bilden meistens rotglühende Lavaströme.
- Graue Vulkane: Ihre Magma ist reich an Siliziumdioxid und Gas. Sie stoßen große Mengen an (grauer) Asche und Staub aus. Nach einem Ausbruch erstarrt das Magma im Vulkanschlot und bildet einen Pfropf, der den Schlot verstopft. Beim nächsten Ausbruch baut sich daher oft ein hoher Druck auf, der den Vulkanberg an seiner schwächsten Stelle sprengt und sich in einer gewaltigen Explosion entlädt. Der Ausbruch des Mount St. Helens 1980 ist dafür ein gutes Beispiel.
Literatur und externe Links
- Cliff Ollier, Volcanoes (1988 Basil Blackwell, Oxford, UK).
Noch zu übersetzen und einzuordnen:
One way of classifying volcanoes is by the type of material erupted, which also affects the shape of the volcano:
- The Lava Shield: Hawaii and Iceland contain examples of volcanoes which extrude huge quantities of lava which gradually builds up to form a tall wide mountain. Their lava is very fluid and solidifies as basalt. The largest lava shield on Earth, Mauna Loa, is 30,000 feet high (it sits on the sea floor); and 75 miles in diameter. Olympus Mons is a huge shield volcano on Mars, and the tallest mountain in the solar system.
- Smaller versions of the lava shield include the Lava Dome, Cone, and Mound.
- If the magma contains a lot (>65%) of silica the lava is called acidic and is very viscous (not very fluid) and is pushed up in a blob which will then solidify, Lassen Peak in California is an example. This type of volcano has a tendency to explode because it easily gets plugged up. Mt. Pelée on the island of Martinique for example.
- If, on the other hand the magma contains relatively small (<52%) amounts of silica, the lava is called basic, and it will be very fluid, capable of flowing like water for long distances. A good example of this is the Great Þjórsárhraun lava flow which was produced by an eruptive fissure almost in the geographical center of Iceland roughly 8.000 years ago, and it flowed all the way down to the sea, a distance of 130 kilometers, and covered an area of 800 sq.kms.
- Cinder Cones result from eruptions that throw out pieces of rock that build up around the vent. These can be relatively short lived and produce a hill perhaps 1000 feet high.
- Strato-Volcanoes such as Mt. Fuji in Japan, Vesuvius in Italy, Mount Erebus in Antarctica, and Mount Rainier in the northwestern United States are tall conical mountains composed of both lava and rocks.
Volcanoes are usually situated either at the boundaries between tectonic plates or over hot spots. Volcanoes may be either dormant (having no activity) or active (near constant expulsion and occasional eruptions), and change state unpredictably.
Volcanoes on land often take the form of flat cones, as the expulsions build up over the years. Under water, volcanoes often form rather steep pillars and in due time break the ocean surface in new islands.
Science has not yet been able to predict with absolute certainty when a volcanic eruption will take place, but significant progress in judging when one is probable has been made in recent time. Volcanic activity is often accompanied by earthquakes, hot springs, fumaroles, solfatare and geysers. Low-magnitude earthquakes often precede eruptions.
Surprisingly, there is no consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of activity. Given the long lifespan of such volcanoes, they are very active. By our lifespans, however, they are not. Complicating the definition are volcanoes that become restless but do not actually erupt. Are these volcanoes active?
Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic time. It is important to note that the span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States, it reaches back less than 300 years, and in Hawaii, little more than 200 years.
Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again.
Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. For example, since calderas have lifespans sometimes measured in millions of years, a caldera that hasn't produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct. Yellowstone caldera in Yellowstone National Park is at least 2 million years old and hasn't erupted for 70,000 years, yet scientists do not consider Yellowstone as extinct. In fact, because the caldera has frequent earthquakes, a very active geothermal system, and rapid rates of ground uplift, many scientists consider it to be a very active volcano.