Distribution of Volcanic Activity

There are between 1400 and 1500 active volcanoes on our planet right now, but they are not evenly distributed throughout Earth’s surface. Some volcanoes cluster in specific regions. One infamous area that loosely spans the border between continents and the Pacific Ocean is known as the Ring of Fire. This region spans about 40,000 kilometers and it has significantly more volcanic and seismic activity compared with other places on the planet.

Why are so many volcanoes bunched together? Most volcanoes are formed by tectonic processes such as subduction at convergent boundaries or rifting at divergent boundaries. Therefore, the distribution of volcanoes naturally coincides with plate boundaries. This connection between volcanism and plate boundaries is called interplate volcanism. There are some volcanoes, however, that occur very far away from plate boundaries. They are typically the result of hot spot volcanism, which can occur on both continental and oceanic lithosphere.

Below we will explore some of the broad geologic environments that result in volcanism and the types of volcanoes we might expect from each.

Mid-Ocean Ridges

New lithosphere is continuously being formed at divergent boundaries. As two tectonic plates spread apart, the magma from the asthenosphere will rise to the surface and form brand new oceanic crust made of basalt and gabbro. The best example of this process at work can be found at the Mid-Atlantic Ridge, which is Earth’s longest mountain range. There are two sets of divergent boundaries at the Mid-Atlantic Ridge: the North American-Eurasian plate boundary and South American-African plate boundary. Such a long spreading center along the ocean floor is responsible for the majority of Earth’s volcanism, but because it is deep underwater, it poses little to no risk to society.

Continental Rifts

The lithosphere does not only spread apart along mid-ocean ridges! Although the continental lithosphere can be up to 100 km thick, the movement of tectonic plates can pull apart the crust. As a result, this crust will thin over time, and the lower layers of the crust and the mantle will rise. These rocks undergo decompression melting, and basaltic magma will erupt from surface volcanoes. These regions are called continental rift valleys. Most continental rift valleys form at divergent boundaries where ocean basins have not yet opened. These places are sites of continental break-up, and we can see this process occurring today at the East African Rift.

Convergent Boundaries

Convergent boundaries that form between two plates of oceanic lithosphere or at a boundary between oceanic and continental lithosphere will result in the subduction of the denser oceanic lithosphere toward the mantle. Within the subduction zone, magma will form in the overlying plate due to flux melting.

The magma that forms at the subduction zone is initially mafic; however, if it rises toward a plate composed mostly of continental crust, it will become intermediate or even felsic in composition. Stratovolcanoes typically form at convergent boundaries. The Pacific Ring of Fire is dominated by subduction zones with these explosive volcanoes.

A description of the Pacific Ring of Fire along western North America is below:

• Subduction at the Middle American Trench creates volcanoes in Central America.
• The San Andreas Fault is a transform boundary.
• Subduction of the Juan de Fuca plate beneath the North American plate creates the Cascade volcanoes like Mount St. Helens, Mount Rainer, Mount Hood, and more.
• Subduction of the Pacific plate beneath the North American plate in the north creates the long chain of the Aleutian Islands volcanoes near Alaska.

Hot Spots

Hot spots are areas of partial melting deep within the mantle that rise and erupt at the surface. They are not associated with a plate boundary, and as such, the volcanic activity that results from hot spots is called intraplate volcanism (“intra” meaning within a plate). Hot spots may form brand new islands if they erupt new material over oceanic lithosphere. These islands are usually just “the tip of the iceberg” as they are often very large shield volcanoes that have been formed by low-viscosity, mafic lava. Additionally, these islands form in chains because the lithosphere will move over millions of years due to plate motion, but the hot spot will stay in place. The most famous example of hot spot volcanism may be the Hawaiian island chain, in which the hot spot remains active over Mauna Loa and Kilauea (above).

Some hot spots form beneath continental lithosphere. Although the partially-melted mantle composition is ultramafic at depth, as it rises to the surface and mixes with the materials in within the continental crust, it becomes mafic, then intermediate. In some areas, it might even become felsic. This mixing of molten materials is a recipe for potential disaster! As the magma becomes more silica-rich, it also becomes more viscous with volatile gases, which make for a very explosive eruption. In the United States, there is one active continental hot spot volcano we are currently monitoring: the Yellowstone Caldera (above), which is likely to have a super-volcanic eruption in the next 100,000 years.

Backyard Geology: Sunset Crater National Monument Park

The Sunset Crater National Monument Park is a site of fairly recent (geologically-speaking!) volcanic activity. The park encompasses two cinder cone volcanoes; the Sunset Crater Volcano and Lenox Crater Volcano which have produced basaltic lavas and scoria from their previous eruptions. The last eruption from Sunset Crater was about 950 years ago, and it was said to result in lava fountains that were nearly 3 times higher than the Statue of Liberty. Even though Sunset Crater erupted in the past 1000 years, it is classified as extinct and it is not expected to erupt again as the hot spot has since moved eastward. We call it the “Sunset Crater” because mafic igneous rocks such as scoria have a lot of iron in them that easily rusts or oxidizes under Earth’s atmosphere from their original blackish-gray color to bright reds and purples, and pinks [10].