7.4.2: Types of Volcanoes and Landscapes

Types of Volcanoes

Volcanoes are categorized according to the specific nature of their eruptions, which generally fall into two groups: effusive or explosive. Effusive eruptions are characterized by the steady outpouring of highly fluid lava that flows easily across the landscape. These are often referred to as quiet eruptions because the magma is thin enough to allow volcanic gases to escape without building up significant pressure.

In contrast, explosive eruptions are violent events caused by more viscous magma that can become trapped within the volcanic vent. This blockage causes gases to reach extreme pressure levels until they are suddenly released, blasting lava, ash, and various rock fragments into the air. These distinct eruptive styles are responsible for shaping different varieties of volcanic cones. According to the U.S. Geological Survey, the behavior of an eruption is largely determined by the composition of the magma, specifically its silica content and gas concentration, which dictates how the volcano will ultimately develop its physical form.

Effusive Eruptions

Effusive eruptions are characterized by the production of extensive lava flows consisting of fluid, low-viscosity molten rock. Because the magma involved in these events contains relatively low levels of silica, it moves easily through the volcanic vent and spreads broadly across the terrain. As this lava travels over the landscape, it typically develops into one of two distinct physical forms. One variety, known by the Hawaiian name pahoehoe, exhibits a smooth, glistening, and ropy texture as it advances and hardens. The second variety, called AA, is more viscous and pasty, resulting in a jagged, rough and fragmental surface. In an AA flow, these sharp fragments are transported along the top of the moving molten core and eventually pile up in heaps at the very front of the flow.

Figure \(\PageIndex{1}\): View NNW of Mauna Loa, a shield volcano. (CC BY-SA 4.0; Courtesy Michael Ritter via The Physical Environment).

Shield volcanoes are formed by effusive eruptions involving highly fluid basaltic lava. Because this lava flows easily and covers great distances before it solidifies, it creates broad landforms with gentle slopes rather than steep peaks. The resulting shape is wide and slightly contoured, resembling a medieval warrior's shield laid flat on the ground. The volcanoes comprising the Hawaiian Islands serve as the primary example of this type of volcanic structure (USGS, 1997).

While a significant amount of lava is released from the central summit caldera, these volcanoes also experience flank eruptions. In these instances, molten rock is discharged from lateral vents and fractures along the sides of the mountain, further extending its width. As the exterior of a moving flow cools and hardens, it can form a protective crust that insulates the liquid material inside, creating a lava tube. These tubes act as underground pipes that allow the lava to remain hot and travel even further from the source. In some cases, lava may also gather in depressions to form temporary ponds or persistent lava lakes (USGS, 1997).

Figure \(\PageIndex{2}\): Shield volcano similar to those found in the Hawaiian Islands. (CC BY-SA 4.0; Courtesy Michael Ritter via The Physical Environment).

Figure \(\PageIndex{2}\): is a cross-sectional view of a shield volcano, showing its internal structure and the pathways by which magma reaches the surface.

At the base of the volcano is a large underground reservoir labeled magma chamber, where molten rock accumulates beneath the Earth's surface.

A vertical conduit rises from the magma chamber to the summit of the volcano. This conduit is labeled central vent and serves as the primary pathway by which magma reaches the surface.

At the top of the volcano, surrounding the central vent, is a depression labeled summit caldera. This feature forms when volcanic activity causes the summit area to collapse or subside.

On the side of the volcano, a secondary opening labeled flank eruption is connected to the magma chamber by a branching magma conduit. This illustrates that magma does not always erupt from the summit; it can also emerge from fractures on the volcano’s flanks.

A broad sheet of molten rock spreading across the surface is labeled lava flow. The lava originates from the flank eruption and moves downslope away from the vent, adding new layers to the volcano.

The volcano has a wide, gently sloping profile rather than a steep cone shape, which is characteristic of shield volcanoes formed by repeated eruptions of fluid basaltic lava.

The diagram illustrates the major components of a shield volcano and how magma moves from an underground magma chamber to the surface through central and flank vents. It shows how repeated lava flows build the volcano over time and explains why shield volcanoes develop broad, gently sloping sides. This type of volcanic structure is typical of hot-spot volcanoes such as those found in Hawaii.

Explosive Eruptions

Volcanoes characterized by explosive activity represent a distinct category of volcanic landforms. These intense eruptions typically occur when the underlying magma is highly viscous and contains significant amounts of trapped gas under extreme pressure. The thickness or resistance to flow in the lava is determined primarily by its silica levels, as magma with higher silica content is much stickier than low-silica varieties.

This type of volcanic activity is frequently observed throughout the Ring of Fire, which is a major volcanic chain stretching from the Pacific Northwest of the United States through the Aleutian Islands and toward Japan. In these systems, the thick magma often becomes obstructed as it moves upward through the central vent. This blockage causes internal gases to reach critical pressure levels, eventually resulting in a massive and forceful release of material (USGS, 1997).

Figure \(\PageIndex{3}\): Mt. St. Helens, a composite volcano. (CC BY-SA 4.0; Courtesy Michael Ritter via The Physical Environment).

Explosive volcanoes are capable of discharging immense quantities of ash and various pyroclastic materials. These eruptions are frequently joined by massive releases of toxic gases and tiny particulates known as nuees ardentes, which translates to glowing clouds or avalanches. These dense clouds can surge down the sides of a volcano at speeds of up to 60 miles per hour. Because of their extreme velocity and heat, it is nearly impossible to escape them once they begin their descent.

Figure \(\PageIndex{4}\): A nearly perfect cinder cone in the Cinder Cones and Lava Beds Natural Landmark Area in the north-central portion of the Mojave National Preserve. (Public Domain; Courtesy USGS).

Explosive eruptions are typically associated with two specific types of volcanic structures: cinder cones and composite volcanoes. Cinder cones are primarily formed from accumulated layers of pyroclastic debris, which consists of rock fragments that were originally trapped within the central vent before being ejected. A well known example of this volcanic form is Mount Paricutin, which famously began erupting in the middle of a cornfield in Mexico in 1943.

Figure \(\PageIndex{5}\): Cinder Cone; Example: Mt. Paricutin. (CC BY-SA 4.0; Courtesy Michael Ritter via The Physical Environment).

Figure \(\PageIndex{5}\) is a cross-sectional view of a cinder cone volcano, showing its internal structure and the materials that build the cone. At the summit is a bowl-shaped depression labeled crater. This opening marks the site where volcanic material erupts from the volcano. Running vertically through the center of the cone is a conduit labeled central vent filled with rock fragments. This vent serves as the main pathway through which magma and volcanic gases travel from below the surface to the crater.

The sides of the volcano are composed of layered deposits labeled pyroclastic layers. These layers consist of volcanic fragments such as ash, cinders, and volcanic bombs that were ejected during eruptions and accumulated around the vent. The repeated deposition of these materials creates the steep, cone-shaped profile characteristic of cinder cones. The cone is relatively small and steep-sided compared with a shield volcano. The pyroclastic material is shown sloping outward from the central vent in all directions.

The diagram illustrates how a cinder cone volcano forms through the accumulation of pyroclastic material around a central vent. During explosive eruptions, fragments of magma are thrown into the air and fall back to the ground around the volcano, gradually building a steep-sided cone with a summit crater. Cinder cones are among the most common volcanic landforms and are typically composed of loose volcanic fragments rather than extensive lava flows.

Figure \(\PageIndex{6}\): Composite or Stratovolcano; Example: Mt. Rainier & Mt. St. Helens. (CC BY-SA 4.0; Courtesy Michael Ritter via The Physical Environment).

 

Figure \(\PageIndex{6}\) is a cross-sectional view of a composite volcano (stratovolcano), showing its internal structure and how repeated eruptions build the volcanic cone.

At the summit is a bowl-shaped depression labeled crater, which surrounds the main opening where volcanic material erupts.

A vertical conduit labeled central vent extends from deep beneath the volcano to the crater. This vent serves as the primary pathway through which magma, gases, and volcanic materials rise to the surface.

The interior of the volcano is composed of alternating layers of pyroclastic material and lava. These layers are shown as stacked bands that slope away from the central vent. Pyroclastic layers consist of volcanic ash, cinders, and other fragmented material produced during explosive eruptions, while lava layers form when molten rock flows from the volcano and solidifies. The repeated accumulation of these alternating deposits creates the steep, cone-shaped profile characteristic of composite volcanoes.

On the side of the volcano, a narrow intrusion labeled radiating dike extends outward from the central vent through the volcanic cone. Dikes form when magma moves into fractures within the volcano and solidifies, creating sheet-like bodies of igneous rock.

The volcano has steep slopes compared with a shield volcano, reflecting the more viscous magma and the combination of explosive and effusive eruptions that build the cone.

The diagram illustrates how a composite volcano is constructed through successive eruptions of lava and pyroclastic material. The central vent transports magma to the surface, while alternating layers of volcanic debris and lava flows gradually build a tall, steep-sided volcanic cone. Composite volcanoes are commonly associated with subduction zones and are known for potentially explosive eruptions due to their silica-rich, viscous magma.

Composite volcanoes, also known as stratovolcanoes (Figure \(\PageIndex{6}\)), are characterized by their highly explosive eruptions. These structures develop through a series of alternating eruptive phases that discharge either pyroclastic debris or thick lava flows. This process results in the distinct internal layering of volcanic material that defines their form.

Stratovolcanoes are some of the most prominent and tallest peaks on the planet, with famous examples including Mount Fuji, Mount Saint Helens, and Mount Kilimanjaro. Geologically, these volcanoes are frequently situated near convergent plate boundaries, specifically in regions where one tectonic plate is subducting beneath another.

Volcanic Landscapes

Geological processes involving volcanism produce a distinct array of landforms. When a volcano becomes dormant, the magma remaining within its central vent eventually cools and solidifies. Over thousands of years, the surrounding cone may be stripped away by erosion, revealing the hardened interior rock known as a volcanic neck. Furthermore, magma can move into nearly vertical fractures that radiate away from the main vent. Once the overlying surface is eroded, these features appear as linear, fin-like ridges called dikes. A prominent example of this geological structure is Shiprock in New Mexico, where a massive volcanic neck is surrounded by visible radiating dikes.

Historically, vast regions of the Earth have been submerged under immense floods of basaltic lava. One notable instance is the Columbia River basalts, where approximately 170,000 cubic kilometers of molten rock inundated parts of southeastern Washington, eastern Oregon, and southern Idaho. This lava, reaching temperatures of 1100 degrees Celsius, moved across the landscape at an average speed of 5 kilometers per hour. By the time the eruptions ended, more than 500,000 square kilometers of the Earth's surface were covered. As rivers and streams later eroded this massive accumulation of rock, it formed a high-elevation lava plateau known today as the Columbia Plateau.

Figure \(\PageIndex{7}\): Shiprock, New Mexico in background with dikes in left foreground. (CC BY-SA 4.0; Courtesy Michael Ritter via The Physical Environment).