What is the tectonic setting of Mt St Helens?
Mt St Helens is a major stratovolcano in the Cascades Range, all of which have formed as a result of the ongoing subduction of the Juan de Fuca tectonic plate beneath the western coast of North America.
Shield volcanoes are usually found at constructive or tensional boundaries. They are formed by eruptions of thin, runny lava. Eruptions tend to be frequent but relatively gentle.
- The Himalayan mountain range and Tibetan plateau have formed as a result of the collision between the Indian Plate and Eurasian Plate which began 50 million years ago and continues today. 225 million years ago (Ma) India was a large island situated off the Australian coast and separated from Asia by the Tethys Ocean.
Tectonic Setting Typical ype(s) of volcanoes Divergent plate boundary mid-ocean ridge; fissures and vents; shield volcanoes Intracontinental exension varied: fissures and vents, flood eruptions, cinder cones, stratovolcanoes, caldera complexes Convergent plate boundary stratovolcanoes Hot spots
- There are three main places where volcanoes originate: Hot spots, Divergent plate boundaries (such as rifts and mid-ocean ridges), and. Convergent plate boundaries (subduction zones)
Stratovolcanoes are common at subduction zones, forming chains and clusters along plate tectonic boundaries where oceanic crust is drawn under continental crust (continental arc volcanism, e.g. Cascade Range, central Andes, Campania) or another oceanic plate (island arc volcanism, e.g. Japan, Philippines, Aleutian
- Volcanoes are associated with three types of tectonic structures: convergent plate boundaries, divergent plate boundaries and hot spots. California has all three. The SAF is a transform plate boundary (strike slip fault) and so is not accompanied by volcanic activity.
- Plates move away from one another at divergent boundaries. This happens at mid-ocean ridges. Plates move towards one another at convergent boundaries; one plate is forced below another in a process called subduction. Earthquakes and composite volcanoes are common at this type of boundary.
- At diverging plate boundaries, earthquakes occur as the plates pull away from each other. Volcanoes also form as magma rises upward from the underlying mantle along the gap between the two plates. We almost never see these volcanoes, because most of them are located on the sea floor.
Shield volcanoes are formed by lava flows of low viscosity - lava that flows easily. Consequently, a volcanic mountain having a broad profile is built up over time by flow after flow of relatively fluid basaltic lava issuing from vents or fissures on the surface of the volcano.
- Shield volcanoes are usually found at constructive or tensional boundaries. They are formed by eruptions of thin, runny lava. Eruptions tend to be frequent but relatively gentle.
- Five volcanoes make up the island of Hawai`i: Kohala, Mauna Kea, Hualalai, Mauna Loa, and Kilauea. Volcanoes that will never erupt again are considered extinct. Dormant volcanoes have not erupted in historic time (the last 200 years in Hawai`i) but probably will erupt again.
- Unlike the composite volcanoes which are tall and thin, shield volcanoes are tall and broad, with flat, rounded shapes. The Hawaiian volcanoes exemplify the common type of shield volcano. They are built by countless outpourings of lava that advance great distances from a central summit vent or group of vents.
Updated: 3rd October 2019