Volcanoes and Plate Tectonics

Volcanism is mostly focused at plate margins Pacific Ring of Fire This map shows the margins of the Pacific tectonic plate and surrounding region. The red dots show the location of active volcanism. Notice how the majority of the volcanism is focused in lines along the plate boundaries? For this region is the area known as the “Pacific Ring of Fire”.

Divergent Convergent Transform Remember that there are three types of plate boundaries

Spreading ridges – As plates move apart new material is erupted to fill the gap Divergent Boundaries

Island Arc Many volcanoes occur near boundaries where two oceanic plates collide. Through subduction, the older, denser plate sinks beneath a deep-ocean trench into the mantle. Eventually, the magma breaks through the ocean floor, creating volcanoes. These volcanoes create a string of islands called an ISLAND ARC.

Island arc of Aleutian trench

Firstly, what are hotspot volcanoes and how do they form? They are hot mantle plumes breaching the surface in the middle of a tectonic plate. What are Hotspot Volcanoes? A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time. The source of this volcanism is a mantle plume of hot mantle material rising up from near the core-mantle boundary through the crust to the surface (see left diagram). A mantle plume may rise at any location in the mantle, and this is why hotspot volcanoes are independent from tectonic plate boundaries. The Hawaiian island chain are an example of hotspot volcanoes (see right photograph).

The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. The volcanoes get younger from one end to the other.

Hotspot’s commonly form volcanic island chains (like the Hawaiian islands). These result from the slow movement of a tectonic plate over a FIXED hotspot. Persistent volcanic activity at a hotspot will create new islands as the plate moves the position of the “old” volcanic island from over the hotspot. Therefore at one end of the island chain you see the youngest, most active volcanic islands (directly over the hotspot) and along the island chain the extinct volcanoes become older and more eroded (see diagram). This way geologists can use hotspot volcano chains to track the movement of the tectonic plate through time.

Properties of Magma Element: a substance that cannot be broken down into other substances. Compound: a substance made of two or more elements that have been chemically combined. Each substance has a particular set of physical and chemical properties. These properties can be used to identify a substance or to predict how it will behave.

Physical Properties Any characteristic of a substance that can be observed or measured without changing the composition of the substance. It always has the same physical properties under particular conditions. EX: density, hardness, melting point, boiling point. Chemical Properties Any property that produces a change in the composition of matter. EX: ability to burn Reaction with other substances that cause one to change color, produce a gas or form a new, solid substance.

Viscosity Viscosity: a physical property of liquids. It is the liquid’s resistance to flow. The greater the viscosity of a liquid, the slower it flows: honey. The lower the viscosity of a liquid, the more easily it flows: water.

Viscosity of Magma The viscosity of magma depends upon its silica content and temperature. Silica is a compound and one of the most abundant materials in the Earth’s crust. More silica in the magma, the higher the viscosity of the magma.

Types of Lava Pahoehoe fast-moving hot lava, low viscosity, low silica Aa slow-moving cooler lava, high viscosity, high silica

Life Cycle of a Volcano Active, or live, volcano is one that is erupting or has shown signs that it may erupt in the near future. Dormant, or sleeping, volcano is one that is not doing anything but could become active again. Extinct, or dead, volcano is unliely to erupt again.

Three types of volcanoes

Lava Plateau/Calderas A lava plateau is made up of many layers of thin, runny lava that erupt from long cracks in the ground. Crater Lake, above, fills an almost circular caldera. A caldera forms when a volcano’s magma chamber empties and the roof of the chamber collapses.