What is a volcano? In general terms, describe some related features.

Volcanoes are not just spectacular natural shows of fire and ash; they also create distinct geological structures. At the heart of a volcanic system is magma, which is molten rock beneath the Earth's crust. Once magma breaches the surface during an eruption, it's known as lava. It's the lava that builds up over time, layer by layer, to form the mountainous shape we associate with volcanoes.

Lava cools down to form various types of hardened rock, which can be studied to learn more about the volcano's history. Additionally, explosive eruptions can spew out volcanic ash, composed of fine particles of rock and volcanic glass. If an eruption is particularly violent, it can leave behind a caldera, a large depression formed when a volcano's summit collapses or is blown apart.

Other volcanic features may include hot springs and geysers, fueled by the heat of the Earth's interior. Structures like volcanic domes and spatter cones are also common, representing the various ways in which volcanoes build terrain. These features are more than just beautiful or dangerous; they're vital clues to understanding our planet's geological activity.

Volcanoes come in an array of shapes and sizes, which have been categorized into several types based on their characteristics and eruption styles.

One common type is the stratovolcano (or composite volcano), known for its conical shape and explosive eruptions. These are often found at tectonic plate boundaries, such as Mount Fuji in Japan or Mount St. Helens in the USA.

Shield Volcanoes

Another type is the shield volcano, which is characterized by its broad, gently sloping sides formed by the flow of low-viscosity lava. An example is Mauna Loa in Hawaii.

Cinder Cones

Cinder cone volcanoes are the simplest type of volcano. Built from ejected lava fragments that solidify in the air, they have steep sides and usually host a single eruption.

Finally, there are lava domes, which are formed from the slow eruption of viscous lava. They typically have steep sides due to the slow-moving lava that doesn't travel far from the vent, such as the one formed by Mount St. Helens after its famous 1980 eruption.

The term 'volcanic activity' encompasses more than just the dramatic eruptions often featured in media. Volcanic activity refers to the various processes associated with the movement of magma and gases from beneath the Earth's surface to its exterior.

The most visible sign of volcanic activity is an eruption, which can range from gentle lava flows to catastrophic explosions that can have global consequences. But volcanic activity also includes less dramatic, but still critical events like fumarolic activity (the release of gases) and low-level persistent release of lava in some environments.

The period between eruptions—when a volcano is considered inactive or dormant—is a deceptive time as it can be a crucial phase for building up the pressure that leads to future eruptions. Monitoring these quiet periods is essential for volcanic hazard assessment and mitigation.

Understanding the various signs of upcoming volcanic activity, such as changes in seismic activity, ground deformation, gas emission rates, and local hydrology, is key for predicting eruptions and protecting communities living in the vicinity of volcanoes.