Скачать или смотреть Volcanoes: Composition, Structure and Stages

A volcano is a vent, or opening, in the Earths's crust from which hot rock ejects. It is fueled by magma (molten rock found deep in the Earth) that flows upwards in fissures in the Earth’s crust. Most volcanoes are the size of mountains.
The substances which eject through opening are gases, ash, lava, and rock material. The most common gases volcanic gases are water vapour, carbon dioxide, sulfur dioxide, and hydrogen sulfide. Small quantities of other volatile elements and compounds also are present, such as hydrogen, helium, nitrogen, hydrogen chloride, hydrogen fluoride, and mercury.
More than 80 percent of the Earth's surface--above and below sea level--is of volcanic origin. Gaseous emissions from volcanic vents over hundreds of millions of years formed the Earth's earliest oceans and atmosphere, which supplied the ingredients vital to evolve and sustain life. Over geologic eons, countless volcanic eruptions have produced mountains, plateaus, and plains, which subsequently eroded and weathered into majestic landscapes and formed fertile soils.
Volcanoes are often described as active, dormant or extinct.
An active volcano is one that has erupted in some way relatively recently. Likely within the last few hundred or few thousand years.
A dormant volcano is one that hasn't erupted for some time, perhaps several hundred or thousand years, but is still is considered likely be erupting sometime in the future.
Extinct volcanoes are ones that are considered incapable of erupting.
0:00 Introduction
0:51 Definition
1:57 Composition
3:24 Volcanic Ash
3:52 Rock materials
4:17 Volcanic Earth
6:48 Structure
8:16 States
10:25 Global distribution of Active volcanoes
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