Distribution of Volcanic Activity

There are between 1400 and 1500 active volcanoes on our planet right now, and they are unevenly distributed throughout Earth’s surface. Some volcanoes cluster in specific regions. The Ring of Fire is an infamous area loosely spanning the border between continents and the Pacific Ocean (Fig. 4.5.3). This region spans about 40,000 kilometers and it has significantly more volcanic and seismic activity compared to other places on the planet.

Why are so many volcanoes bunched together? Tectonic processes form most volcanoes 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-Oceanic 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 Mid-Atlantic Ridge exemplifies this process, and it is Earth’s longest mountain range! There are two sets of divergent boundaries at the Mid-Atlantic Ridge: the North American-Eurasian plate boundary to the north and the South American-African plate boundary to the south. 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 spreads in mid-ocean ridges and in the continents! Although the continental lithosphere can be up to 100 km thick, divergent tectonic forces can break it apart. As a result, the continental crust will thin over time, and the lower layers of the crust and the mantle will rise because they are buoyant. These uprising rocks in the lower lithosphere and mantle undergo decompression melting (section 4.1) and basaltic magma will erupt from newly created volcanoes. The spread-apart regions on continents 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

Recall from Ch. 2 that in convergent boundaries between two oceanic plates or between an oceanic and continental plates, the denser oceanic lithosphere sinks toward the mantle. Within the subduction zone, magma will form due to flux melting (Section 4.1).

The magma generated at the subduction zone is derived from the mantle and thus shares the same chemical composition; it is mafic. Mafic magmas typically have higher temperatures than felsic magmas, they are hotter. If this mafic magma rises toward a continental plate, which is felsic in composition, it will melt parts of it. This is because the continental crust melts at lower temperatures than the oceanic crust, so the mafic magma can melt the felsic crust and mix with it. The new melt will have a different composition, it will become intermediate between mafic and felsic, or even felsic as more continental crust gets mixed and melted into it. The result is a very viscous magma that will produce stratovolcanoes. Stratovolcanoes typically form at convergent boundaries. The Pacific Ring of Fire, a continuous subduction zone, receives its name due to the abundance of these explosive volcanoes.

A description of the Pacific Ring of Fire along western North America is below (Fig. 4.5.3):

• 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 Rainier, 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

A volcanic “hotspot” is an area in the mantle from which heat rises as a thermal plume from deep in the Earth. High heat and lower pressure at the base of the lithosphere (tectonic plate) facilitates melting of the rock. This melt, called magma, rises through cracks and erupts to form volcanoes. As the tectonic plate moves over the stationary hot spot, the volcanoes are rafted away and new ones form in their place. This results in chains of volcanoes, such as the Hawaiian Islands (Video 4.5.1, from IRIS and Fig. 4.5.4). These islands are usually just “the tip of the iceberg” as they are often very large shield volcanoes that lie under the water (Fig. 4.5.4). Shield volcanoes form by low-viscosity, mafic magma.

Video 4.5.1 Hotspot track animation. Observe the movement of the oceanic plate while the thermal plume remains in place.

Some hot spots form beneath the 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 volcanic activity (geologically-speaking!). 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, we classify it as extinct and we do not expect it to erupt again as the plate 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].