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دوشنبه ششم شهریور 1391 :: نویسنده : پانته آ

What is a volcano?

Volcanism, or volcanic activity, is the venting of liquid magma at the surface of the earth. Volcanoes are hills or mountains that form around the vent and consist of cooled magma, rock fragments, and dust from the eruptions. Volcanoes differ widely in shape, structure, and size. Pieces of rock that are blown out of a volcano are called pyroclasts or pyroclastic debris. Pyroclastic flows are dense, cloudlike mixtures of hot gas and pyroclastic debris that flow down a volcano's sides like an avalanche. These flows can be especially

deadly for example, 30,000 people were killed by a pyroclastic flow on the Caribbean island of Martinique in 1902.

Craters and calderas

The crater is the circular depression at the top of the volcano . A caldera is a larger depression at least 1 kilometer in diameter that forms at the top of the volcano when the summit is destroyed during an eruption or when the crater floor collapses into the magma chamber below.

Types of volcanoes

There are three kinds of volcanoes: composite, shield, and cinder cone.

a) Composite volcanoes (stratovolcanoes) have been the sources of some of the more famous and destructive eruptions, such as those of Mount St. Helens, Vesuvius, and Damavand. Built up over millions of years, they consist of alternating layers of lava and pyroclastic debris that can approach slopes as steep as 45 degrees . They are characterized by long periods of inactivity that can last for up to hundreds of thousands of years. How violent an eruption is depends on the temperature of the lava and the amounts of silica and dissolved gas in the lava.

b) Shield volcanoes are broad, cone-shaped hills or mountains made from cooled lava flows. The sides are very gently dipping and rarely exceed 10 degrees from the horizontal because the lavas have a low viscosity and spread quickly after eruption. (Viscosity is defined as resistance to flow; a lava with high viscosity flows slowly)

c) A cinder cone (pyroclastic cone) is composed of pyroclastic material (not lavas) ejected from a vent and commonly has slopes of about 30 degrees.

Volcanic domes

If a magma is thick and viscous and does not easily flow, it may form a volcanic dome. Volcanic domes are steep sided or rounded and form near the volcanic vent.

Lava floods

Nonvolcanic lavas called lava floods or plateau basalts are often associated with deep cracks in the continental crust. The lithification of ejected rock fragments and other pyroclastic material creates a variety of fragmental textures. Dust and ash are the finest-grained particles, followed by cinders (pea sized), lapilli

(walnut sized), and bombs or blocks, which can be up to a meter across or larger. A tuff is composed of fine-grained pyroclastic material. One distinctive extrusive rock texture occurs in flood basalts and submarine lava flows. Flood basalts cool and contract to form vertical, parallel, generally six-sided columns called columnar structures or columnar jointing . As a submarine lava flow cools, blobs of lava may break through the exterior and harden immediately in the cold water, forming small rounded shapes called pillow structures. These are especially useful to the geologist for determining that the rock was formed on the ocean floor and for indicating the base of the flow.

Volcanic necks and pipes

A volcanic neck is the rock that formed in the vent of a volcano at the end of its eruptive life. It remains "standing" after the flanks of the volcano have eroded away. In some locations, cylindrical dikes called pipes extend from the asthenosphere to the Earths surface. They are conduits that once carried magma on its way to erupt from a volcano, but they are now filled with solidified magma. An unusual type of pipe contains a rock called kimberlite, which is the only known source of diamonds.





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جمعه ششم مرداد 1396 09:15 قبل از ظهر
I really like it when people come together and share thoughts.

Great blog, continue the good work!
شنبه بیست و هفتم خرداد 1396 12:29 قبل از ظهر
Your style is unique compared to other folks I have read stuff from.
I appreciate you for posting when you have the opportunity, Guess I will just book mark this blog.
یکشنبه سی و یکم اردیبهشت 1396 08:37 بعد از ظهر
It's hard to find knowledgeable people on this topic, but
you seem like you know what you're talking about! Thanks
 
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