Chapter 5, Problem 1GST

To determine

The volcanoes on the basis of plate boundaries (convergent/divergent/intraplate) it is associated with.

1. a) Crater Lake
2. b) Hawaii’s Kilauea
3. c) Mount St. Helens
4. d) East African Rift
5. e) Yellowstone
6. f) Mount Pelée
7. g) Deccan Traps
8. h) Fujiyuma

Answer to Problem 1GST

1. a) Crater Lake – convergent plate boundary
2. b) Hawaii’s Kilauea – intraplate volcanism
3. c) Mount St. Helens – convergent plate boundary
4. d) East African Rift – divergent plate boundary
5. e) Yellowstone – intraplate volcanism
6. f) Mount Pelée – convergent plate boundary
7. g) Deccan Traps – intraplate volcanism
8. h) Fujiyuma – convergent plate boundary

Explanation of Solution

Volcanism occurs at three different plate boundaries, namely convergent, divergent, and intraplate. Most of the volcanoes occur along the former two settings (which are formed by plate tectonic movements), while the latter occur from a deep source within the mantle called intraplate volcanism.

Convergent plate boundary (destructive boundaries) – It is formed when two crustal plates move towards each other and collide. It has three different settings, when an ocean-continent subduction takes place; the denser oceanic plate slides beneath the continental plates to form subduction zones.

Examples: Andes and Cascade Mountains ranges.

When a continent-continent collision takes place, the subducting plate edges are compressed, folded, and pushed upwards forming orogenic belts or mountain ranges. Examples: Himalayas and Alps.

When ocean-ocean subduction takes place, the denser and older plates slide beneath the less-dense, new plates; this results in the formation of ocean trenches, earthquakes, and island arcs. Examples: Aleutian Islands, and the Japanese island arc.

Divergent plate boundary (Constructive plate boundaries) – They are formed when two lithospheres move away from each other. In case two oceanic lithospheres are drifting apart, a new ocean basin is formed by the seafloor spreading.

Example: Atlantic Ridge and East Pacific Rise

In case of a continent-continent rifting, a new ocean basin forms as the continent splits, spreads, the central rift collapses, and the ocean fills the basin. Example: East African Rift.

Intraplate volcanism (Hot-spot volcanism) - An intraplate volcanism occurs from enormous outpourings of fluid, basaltic lavas from the source deep within the Earth's core–mantle boundary, called mantle plumes; it remains in a fixed location relative to the moving tectonic plates. The surface manifestation of mantle plumes is termed as hot spot.

When an oceanic crust moves over a fixed mantle plume, it forms a chain of volcanic structure known as Hawaiian Islands.

If a large mantle plume ascends beneath a continental crust, it may result in vast outpourings of fluid basalts and form basalt plateaus such as Deccan plateau and Columbia plateau.

1. a) Crater Lake – Crater lake, Oregon is formed when violent eruptions collapse the summit of a big composite volcano accompanying a disastrous ejection of silica-rich pyroclastic fragments which partially empties the magma chamber. The crater is then filled with rain and groundwater.
2. b) Hawaii’s Kilauea – It is a shield volcano; the largest of the Hawaiian Islands formed from the mantle plume. Hence, it is a product of intraplate volcanism.
3. c) Mount St. Helens – It is an active stratovolcano in Washington. This was created when the Juan de Fuca plate subducted beneath the North American plate, producing the line of volcanoes along the Cascade Mountain Range.
4. d) East African Rift – It is an active continental rift zone in East Africa. It is a product of continent-continent rifting.
5. e) Yellowstone – It is a volcanic caldera and super volcano in Yellowstone National Park in the Western United States. The source is Yellowstone hotspot.
6. f) Mount Pelée – It is an active stratovolcano on the Caribbean island of Martinique. It is formed by the result of a subduction zone.
7. g) Deccan Traps – The Deccan Traps are a large igneous province located on the Deccan Plateau of west-central India and are one of the largest volcanic features on Earth. They consist of multiple layers of solidified flood basalt.
8. h) Fujiyuma – Mt Fuji is an active stratovolcano located on Honshu Island, Japan. The major source of it is said to be subduction of the Pacific Place beneath the Philippine Plate.