The composition of the gases, rocks, and melted rocks known as lava that come out of a volcano depend on where the volcano is located. Earth’s top layer is broken into two kinds of plates, continental and oceanic. Along the west coast of the United States, for example, the oceanic Pacific plate meets the continental North American plate. Oceanic plates can sometimes become so heavy that they sink back into the earth’s mantle. Mantle rock heats up, melts, and comes back to Earth’s surface through volcanoes. When molten rock rises through an oceanic plate, like in the Aleutian Islands off Alaska, the lava and the volcanic gases escape easily. These volcanoes produce lots of lava flows, but typically no large explosions.
On the other hand, when rocks from sunken plates rise through a continental plate, like they do at Mount St. Helen’s, Washington, they melt through some of the continental rock. This mix of lava cannot flow easily and often clogs up a volcano with the gases accumulating and building up the pressure inside. Eventually, the pressure gets to be so much that the volcano breaks and explodes, often violently.
The temperature of volcanic magma is very high, typically around 2,000° F. At some areas of volcanic activity, such as Yellowstone National Park, geysers can produce hot steam from boiling water. Other types of volcanoes not erupting lava can produce a mix of steam and hot, toxic volcanic gases. Some volcanoes can appear dormant and harmless by not producing any lava or steam, but they can still be very dangerous by emitting carbon dioxide (CO2). Colorless and odorless, CO2 can flow down the flanks of a volcano and accumulate near the base, sometimes in large enough amounts to suffocate humans and other animals. The gas can also penetrate from below the soil and kill plants and trees.
-- Gabi Laske, research geophysicist, Institute of Geophysics and Planetary Physics