Scientists classify volcanoes using one of three descriptors: active, dormant or extinct. An active volcano is one that has erupted within the last 10,000 years. Volcanoes are considered active if they are currently erupting or showing signs of eruption. A dormant volcano has erupted in history but hasn't done so in the past 10,000 years. But it does have the potential to erupt again. An extinct volcano is one that professionals don't believe will erupt again.
When magma reaches the Earth's surface, it is then known as lava. But lava isn't the only substance coming from the opening -- volcanic ash and rock come up with the liquid, as well as various volcanic gases. Small holes, or fumaroles, in the inside of the volcano emit these gases, which include harmless water and carbon dioxide. But volcanoes do spew out several gases that are poison and cause pollution. These include sulfur dioxide, hydrogen chloride and hydrogen fluoride.
Volcanoes are not just dangerous for the lava, which can reach 1,000 degrees Celsius, but for the threat of natural disasters and health issues after an eruption. For instance, a volcanic eruption can trigger tsunamis, flash floods, earthquakes, rock falls and mudflows. Eruptions can also cause lava flows, which harm all in the flow's path, hot ash flows, falling ash and avalanches. This volcanic ash, which is made from pulverized rock, is acidic and gritty. As such, it can cause damage to the lungs of babies, those with respiratory issues and older people.
Volcanoes are measured using a Volcanic Explosivity Index, or VEI. The VEI measures the amounts of substances released from the volcano as a result of an eruption and uses a scale of 1 to 8. Supervolcanoes, the most intense volcanoes, are rated as 8. These volcanoes can cause worldwide climatic changes, such as a decrease in global temperature, and rain down their spewed substances for thousands of miles. Fortunately, supervolcanoes erupt only once every few hundred thousand years.