At least 1,500 active volcanoes dot the globe, and some of them have been continuously erupting for years. They form when molten rock called magma up and breaks through a weak area of Earth’s surface. The chambers that contain magma can quietly lurk underground for hundreds of years, and then erupt with surprising fury. Here are some amazing facts about volcanoes.

1.    The volcanic rock pumice is the only rock that can float in water. It is usually grey and full of bubbly holes, which form when hot gases jet furiously out of the rock as it cools.

2.    The most formidable volcanoes are called supervolcanoes. A supervolcanic eruption can rain hellfire across thousands of miles and cause worldwide climatic changes, such as a drop in global temperature due to the release of tons of ash particles into the atmosphere. These monsters rear there ugly heads only ever few hundred thousand years, however. One of the biggest is in Yellowstone National Park, and scientists say it may be due for another eruprtion.

3.      The largest volcanic eruption ever observed was of Mount Tambora , on the island of Sumbawa, in Indonesia. Its eruption in 1815 killed about 100,000 people. Indonesia is thought to have the largest number of historically active volcanoes – a total of 76, according to the U.S. Geological Survey.

4.     Most volcanoes occur near the edges of tectonic plates, the massive rock slabs that make up Earth's surface. But some volcanoes, such as the Yellowstone supervolcano, lie over other "hot spots" where magma wells up from deep within the Earth.

5.     Known as the land of fire and ice, Iceland perches atop the volcanoes of the Atlantic’s mid-oceanic ridge. The latest eruption of the Eyjafjallajokull volcano (in April 2010) pales in comparison to the violent 1783's eruption of Mount Skaptar, which devastated the island's farming and fishing reserves and caused a famine that killed a fifth of the country's people.

6.     The 1991 eruption of  Mount Pinatubo in the Philippines was even worse. This volcano spewed 22 million tons of sulfur dioxide, which encircled the entire planet and brought the global temperature down at least 0.5 degrees C, according to the USGS.

7.    Volcanoes can grow. As lava and ash accumulate, they add layers and height to the volcanic surface.  This is one common way mountains are built.

8.    Volcanoes can become extinct. If scientists do not expect a volcano to ever erupt ever again, they consider it to be extinct. Volcanoes that are not currently active but may erupt again are labeled dormant.

9.    The sheer force of some volcanic eruptions can cause its magma chambers to collapse, forming a giant, bowl-shaped crater called a caldera.

10.    The biggest volcano on Earth is Hawaii’s Mauna Loa. One of the five volcanoes in Hawaii, it towers 13,000 feet above sea level. The Hawaiian Islands were actually created by a hot spot.

11.    Volcanoes make sunsets more colorful. When Alaska's Kasatochi volcano erupted in 2008, people all over the world saw unusually beautiful  orange and coral hues in sunsets. This visual phenomenon is the result of fine ash particles in the atmosphere scattering the sun's rays.