Volcanoes

A volcano is an opening in Earth's crust where magma, gas, and ash escape to the surface. Think of shaking a soda and popping the cap. That fizz rushing out is a lot like an eruption, just way hotter.

And eastern Idaho is volcano country. You live between Craters of the Moon and Yellowstone. That's not a coincidence.

Most volcanoes form in three places:

• Subduction zones: a sinking ocean plate melts and feeds volcanoes. Most of the volcanoes around the Pacific Ocean form this way, a ring called the Ring of Fire. About 75% of Earth's active volcanoes are on it.
• Divergent boundaries: plates pull apart and magma rises, like at mid-ocean ridges and in Iceland.
• Hotspots: a plume of hot mantle rises under the middle of a plate. Hawaii and Yellowstone are hotspot volcanoes.

• Magma chamber: the underground pool of magma.
• Vent: the opening where magma escapes.
• Conduit (pipe): the passage from the chamber to the vent.
• Crater: the bowl-shaped pit at the top.
• Caldera: a huge crater that forms when a volcano collapses into its emptied magma chamber. Crater Lake in Oregon is one. Yellowstone is a giant one, about 30 by 45 miles.

It comes down to two things: viscosity (how thick and sticky the magma is) and gas.

• Low-silica (mafic) magma is runny, like syrup. Gas escapes easily, so eruptions are calm, with flowing lava. This is called an effusive eruption. Hawaii's volcanoes work this way.
• High-silica (felsic) magma is thick, like peanut butter. Gas gets trapped, pressure builds, and then BOOM, an explosive eruption.

• Shield volcanoes: wide, gently sloping, built by layer after layer of runny lava. Mauna Loa in Hawaii is the largest volcano on Earth. Many low shield volcanoes dot the Snake River Plain.
• Cinder cones: small, steep, cone-shaped hills made of chunks of lava blasted into the air. They usually erupt just once. Craters of the Moon has lots of them.
• Composite volcanoes (stratovolcanoes): tall, steep, and built from alternating layers of lava and ash. They're the most dangerous. Mount St. Helens, Mount Rainier, and Mount Fuji are examples.

• Lava: Pahoehoe (say "pah-HOY-hoy") is smooth and ropey. A'a (say "AH-ah") is jagged and sharp. You'll see both at Craters of the Moon. Here's a tip: a'a is what you'd say if you walked on it barefoot.
• Tephra: all the bits blasted into the air, from fine ash to golf-ball-sized lapilli to huge volcanic bombs.
• Gases: mostly water vapor, plus carbon dioxide and sulfur dioxide.

• Pyroclastic flows: a racing avalanche of hot gas, ash, and rock that can go over 100 mph and reach 700 °C. The deadliest hazard. In AD 79, one buried the Roman city of Pompeii.
• Lahars: volcanic mudflows of ash mixed with water or melted snow. They flow like wet concrete down river valleys.
• Ash fall: collapses roofs, ruins crops, damages lungs, and wrecks jet engines.
• Lava flows: usually slow enough to walk away from, but they destroy anything in their path.
• Volcanic gases: large eruptions can even cool the global climate by blocking sunlight.

• Mount St. Helens, 1980: the blast flattened about 230 square miles of forest and killed 57 people. Ash fell on parts of northern Idaho.
• Craters of the Moon: its last eruptions were only about 2,000 years ago. Geologists say it could erupt again in the future.
• Yellowstone supervolcano: its last huge eruption was about 631,000 years ago. Its hot springs and geysers are powered by the magma below. Scientists watch it constantly, and a giant eruption is very unlikely in our lifetimes.

Volcanoes usually give warnings before erupting. Scientists watch for small earthquakes, the ground bulging upward (measured by GPS and satellites), and changes in gas coming out. The USGS Yellowstone Volcano Observatory monitors Yellowstone 24/7.

Volcanoes aren't all bad: volcanic soil is some of the most fertile on Earth, and volcanic heat provides geothermal energy.