Introduction

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Introduction

A volcano is a location where magma, or hot melted rock from within a planet, reaches the surface. It may happen violently, in a massive supersonic explosion, or more quietly, as a sticky, slow lava flow.

Volcanoes have been a part of earth's history long before humans. Compare the history of human beings, a few million years in the making, to that of the Earth, over four billion years in the making. Volcanoes were important contributors to the early earth atmosphere by releasing gases such as nitrogen (N₂), carbon dioxide (CO₂), and ammonia (NH₄).

**Volumes of Some Well-Known Volcanic Eruptions**
	Eruption	Date	Volume in km³
eruptions observed by humans	Paricutin, Mexico	1943	1.3
	Mt. Vesuvius, Italy	79 A.D.	3
	Mount St. Helens, Washington	1980	4
	Krakatoa, Indonesia	1883	18
inferred by study of deposits	Long Valley, California	pre-historic	>450 & <700
inferred by study of deposits	Yellowstone, Wyoming	pre-historic	400

Note that volcanic eruptions that occurred before historic times were several orders of magnitude larger (more than 1000 km³ in erupted volume) than ones observed by humans.