Volcanoes produce a wide variety of natural hazards that can kill people and destroy property. This simplified sketch shows a volcano typical of those found in the Western United States and Alaska, but many of these hazards also pose risks at other volcanoes, such as those in Hawaii. Some hazards, such as lahars and landslides, can occur even when a volcano is not erupting.
A sphincter is a ring of muscle which holds any kind of biological opening closed. Sphincters are an important part of almost any system in a body. They help to both regulate the exit, entrance and circulation of fluids, gasses, and solids.
Many sphincters are used every day in the normal course of digestion and vision. For example, the lower esophageal sphincter (aka: cardiac sphincter), which resides at the top of the stomach, keeps stomach acids and other stomach contents from pushing up and into the esophagus.