7.4: Volcanic Processes and Landforms

Features of Volcanoes

Volcanoes serve as a window into the Earth’s dynamic interior, born from molten rock that rises from the depths to demonstrate the planet's constantly recycling nature. While these displays of fiery fountains and massive ash clouds are awe-inspiring, they pose a significant threat to those living in volcanically active regions.

A volcano is a geological structure featuring a vent or a group of vents through which magma, gases, and ash from deep within the earth are released. While often associated with a conical shape formed by accumulated debris, volcanoes can also appear as vast, flat plateaus created by massive flood basalt flows from surface fissures. These extensive lava flows often cover wide areas and are later shaped by the erosive force of streams. Unlike smaller parasitic cones that branch off near the surface, a true volcano is defined by a primary feeder conduit that draws magma from great depths, typically exceeding 30 kilometers.

The internal plumbing of a volcano begins with a magma chamber, a large underground reservoir of molten rock. Because this magma is less dense than the solid rock around it, it rises toward the surface, often driven by intense pressure. It travels through a central vent, which acts as the primary pipe for the ascending material. At the summit, a bowl-shaped depression called a crater serves as the main exit point where lava, ash, and volcanic gases are expelled into the environment.

Figure \(\PageIndex{1}\): Aerial view of Mount St. Helens. (Public Domain; Courtesy USGS).

Lateral vents are found on the sides of some volcanoes where lava is extruded. An extremely large crater is some times found at the summit of a volcano. These massive craters called calderas are created when pressure builds inside the volcano until the top is literally blown off, magma drains back into the central vent and the top of the cone collapses in. Crater lake formed in the caldera of Mount Mazama.

Outpourings of magma can occur as a fluid-like lava or as fiery clouds of ash and rock fragments. The ability of lava to flow depends on its viscosity or "stickiness". The viscosity of magma depends on temperature, silica content, and incorporated gases. Basaltic magma has approximately 50% SiO_², the smallest amount of incorporated gas and readily flows across the surface. Andesitic magma contains 60% SiO_² and has a substantial content of incorporated gas, giving it a moderate viscosity. Rhyolytic magma is 70% SiO_² and contains the largest amount of gas. Rhyolytic magma produces the highest viscosity lava and is responsible for violent eruptions.

Figure \(\PageIndex{2}\): Hydrocholoric acid steam rises from the ocean as a Pu`u `O`o lava flow reaches sea water. (CC BY-SA 4.0 ;  Michael Ritter via The Physical Environment).

Volcanic eruptions, particularly those of an explosive nature, release various fragments of rock and lava known as tephra. Among the most dangerous phenomena are pyroclastic flows, which consist of high-speed gases and rock fragments. These ground-hugging flows can reach temperatures of 500 degrees Celsius and travel down volcanic slopes at speeds of 100 kilometers per hour, effectively carbonizing everything they encounter. In addition to solid debris, eruptions discharge several types of gases into the atmosphere. While steam is the most prevalent, volcanoes also release carbon dioxide, sulfur dioxide, and hydrogen chloride. The environmental impact of these gases is significant; for instance, when sulfur dioxide reacts with atmospheric moisture, it creates acid rain. This natural acid deposition has historically caused extensive damage to vegetation, such as the areas surrounding the summit of Mauna Loa. Furthermore, when lava enters the ocean, the extreme heat causes a chemical reaction with saltwater, generating a hazardous steam cloud containing hydrochloric acid.

According to the U.S. Geological Survey, the classification of a volcano is determined by the depth of its plumbing system rather than its outward shape. A true volcano is fed by a primary conduit that extends deep into the earth, often more than 30 kilometers, which distinguishes it from smaller parasitic vents that branch off at shallower depths. This deep-rooted structure allows a volcano to remain active and continue growing through numerous eruptive cycles, even as individual vents on its surface may cease to function.