How many active volcanoes are there?

Currently, there are somewhere between 850 and 1,500 known active volcanoes on our planet; there are 63 active volcanoes in the United States, mostly in Alaska, Hawaii, and in the Pacific Northwest. At any given time, about 10 or 12 volcanoes are erupting planet wide.

What is a fumarole?

Volcanic gases escape from fumaroles, or vents, around volcanically active areas. They can occur along tiny cracks or long fissures in a volcano, in groups called clusters or fields, and on the surfaces of lava and pyroclastic flows. Fumaroles have been known to last for centuries. They can also disappear in a few weeks or months if their source cools quickly. For example, Yellowstone National Park and the Kilauea volcanoes have many fumaroles and associated deposits; some have been there for years, while others have just recently appeared.

What is tephra?

Tephra is the name given to all the material that erupts from a volcano, excluding lava. Tephra comes in all shapes and sizes, and is also referred to as pyroclastic material ("fire particles"). A pyroclast is material that is ejected during the explosive eruption of a volcano in the form of fragments; pyroclastic material that is hot enough to fuse together before it falls to the ground is called welded or volcanic tuff. Geologists classify tephra according to size. The following lists the most common types of tephra:

Ash—Ash is material smaller than approximately a tenth of an inch (2 millimeters) that is emitted from an erupting volcano; it can also contain lapilli (also called cinders or "little stones"), which is between 1 and 25 inches (2 and 64 centimeters). In a large eruption, ash can accumulate to a great thickness and spread out for thousands of miles (usually in the direction of the prevailing winds).

Block—Blocks are solid rock emitted from an erupting volcano. They can be anywhere from the size of a baseball to the size of a boulder as large as a house.

Bombs—Bombs are volcanic rocks that are still molten inside; they are shaped by their passage through the air. (They form the brilliant arcs seen in time-lapse photography of volcanic eruptions.) Typically ranging from baseball to basketball size, they can be as large as a house. Bombs (and blocks) can be ejected from a volcano with initial velocities greater than 1,000 miles (1,609 kilometers) per hour, and can travel more than 3 miles (5 kilometers), with some exploding and gushing molten rock when they eventually strike the ground. There are also certain types of bombs, including spindle bombs (very fluid magma chunks that are streamlined as they fly through the air) and bread crust, which is formed from viscous magma, creating rounded blobs that often have fractured surfaces.

What is a phreatic eruption?

Phreatic eruptions are steam-driven explosions that occur when groundwater or surface water is heated by magma, lava, hot rocks, or new volcanic deposits. The intense heat of such material can cause water to boil and turn to steam, generating an explosion of steam, water, ash, blocks, and bombs.

How much volcanic activity occurs underwater?

There is a huge amount of volcanic activity taking place underwater—we just can't see it. Some geologists estimate that approximately 80 percent of all Earth's volcanic activity occurs on the ocean floor.

What are black smokers?

Black smokers are actually deep-ocean hydrothermal (hot water) vents, named after the dark, soot-like material ejected from "chimney" formations on the ocean floor. The material is actually superheated water (around 662°F [350°C]) with very high concentrations of dissolved minerals—mostly sulfur-bearing minerals or sulfides from lava on a mid-ocean ridge volcano. As the hot water meets the cold ocean waters, the minerals precipitate out, settling out around the surrounding rock. Over time, the hollowed-out chimneys grow taller as more minerals precipitate out.

Black smokers tend to occur in volcanic vent fields that are typically tens of yards across, with fields ranging from pool-table size (43 square feet [4 square meters]) to tennis court size (8,288 square feet [770 square meters]). For example, vent fields are found on the Juan de Fuca Ridge, a mid-ocean ridge in the Pacific Ocean. Many vents have been discovered since the first site was found in 1977 near the Galapagos Islands (in the small research submersible Alvin), and there are probably many more. But scientists have only explored a small portion of the Earth's mid-ocean ridges.

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