Maars

A maar is a volcanic crater that forms when magma contacts ground water to produce a steam explosion.

What is a Maar? A maar is a shallow volcanic crater with steep sides that is surrounded by tephra deposits. The tephra deposits are thickest near the crater and decrease with distance from the crater. A maar is formed by one or more underground explosions that occur when hot magma comes into contact with shallow ground water to produce a violent steam explosion. These explosions crush the overlying rocks and launch them into the air along with steam, water, ash and magmatic material. The materials usually travel straight up into the air and fall back to Earth to form the tephra deposits that surround the crater. If the tephra lithifies, it will become an igneous rock known as tuff. If tephra surrounding a maar lithifies, it will become a rock known as “tuff.” Tuff is composed of rock fragments and large pieces of tephra in a matrix of volcanic ash. Image by Roll-Stone of Wikimedia. The crater floor of a maar is usually below the original ground surface. After the eruption, an inflow of ground water often turns the crater into a shallow lake. Most maars are a few hundred to a thousand meters in diameter and less than one hundred meters in depth. The largest maars, located on the Seward Peninsula of Alaska, are up to 8000 meters across and up to 300 meters in depth. See Google map at right. How Common are Maars? Maars are more numerous than most people realize. After cinder cones, maars are the second most common volcanic landform. [2] If you search the Smithsonian Institution’s Global Volcanism Program database, you will be able to find hundreds of maars. [3] Maars are underrepresented as volcanic landscape features because they are small in size and lack rocky vertical development that would make them resistant to weathering and erosion. Because they are relatively small, shallow depressions, they can be easily filled with sediment and not recognized as volcanic features.

The Largest Known Maar

The largest known maar on Earth is Devil Mountain Maar Lake, located on the northern part of the Seward Peninsula of Alaska. It was produced by a hydromagmatic eruption that occurred about 17,500 years ago. The blast spread tephra over an area of about 2,500 square kilometers. The tephra is several tens of meters thick near the maar and decreases with distance away from the maar. [4] You can explore five of the world’s largest maars in the Google satellite image in the right column of this page.