Plate Boundaries and Volcanic Eruptions

This page focuses on the types of plate boundaries and volcanic eruptions, which are crucial concepts in understanding what happens at a destructive plate boundary and other tectonic processes.

Types of Plate Boundaries

1. Destructive/Convergent Boundaries:
   • Oceanic/Oceanic: The older, more dense plate subducts below the other, which is then heated and melts as it is forced deeper into the chamber. This process forms oceanic trenches.
   • Continental/Continental: Causes a powerful collision, as both plates are equally dense, preventing subduction. This process forms mountain ranges, such as the Himalayas.

Example: The Mariana Trench, located in the western Pacific Ocean, is an example of a destructive plate boundary where oceanic-oceanic subduction occurs.

2. Constructive Boundaries:
   • Continental/Continental: Plates move away from each other. The overlying plate is stretched thin until it breaks, causing earthquakes. This process forms rift valleys, and streams/rivers flow into the ridge to form long lakes.

Highlight: Understanding constructive boundaries is essential for grasping what happens at a constructive plate boundary in A-level Geography.

3. Conservative Boundaries:
   • Mainly found on the ocean floor.
   • Plates slide past each other, either at different speeds or in different directions.
   • No volcanic activity occurs, but may cause shallow focus earthquakes.

Example: The San Andreas Fault in California is a well-known conservative plate boundary example.

Types of Volcanic Eruptions

1. Hawaiian Type: Lava slowly and easily escapes the vent. It is very runny and basaltic, resulting in gently sloped volcanoes.

2. Strombolian: Gas explosions occur more frequently. Small but very frequent eruptions.

3. Vulcanian: More violent but less frequent than Strombolian. Plugs of cooled lava may be ejected in blasts.

4. Vesuvian: Extremely strong explosions, often occurring after a volcano has been lying dormant for a while. Gas and ash clouds fall over vast areas.

5. Krakatoan: Exceptionally violent eruptions.

6. Plinian: Usually the most violent, with massive amounts of lava, gas, and pyroclastic flow. Part of the volcano may be removed during the eruption.

Vocabulary: Plinian eruption - An extremely violent volcanic eruption characterized by a tall, umbrella-shaped cloud of ash and gas that can reach the stratosphere.

Understanding these types of eruptions is crucial for students studying volcanic hazards in A-level Geography and learning about the three types of volcanic eruptions.

Types of Volcanoes

This page provides an overview of various volcano types and their characteristics, which is essential for understanding tectonic hazards in A-level geography.

Shield Volcanoes are formed from basaltic rock and have gently sloping cones due to less viscous lava. They are typically found in hot spots and above double oceanic plate boundaries, with gentle and predictable eruptions.

Vocabulary: Basaltic rock - A type of igneous rock formed from cooled lava, typically dark in color and low in silica content.

Cinder Cone Volcanoes are the most common type, characterized by symmetrical cones that may occur as single or secondary cones. They are formed by airborne fragments of lava ejected from the vent, building up layers.

Calderas are formed when gases build up under a blocked vent, resulting in internal eruptions that create huge craters at the summit. They are found on destructive plate boundaries and cause very explosive and unpredictable eruptions.

Example: The Yellowstone Caldera in Wyoming, USA, is one of the largest known calderas in the world.

Acid/Dome Cone Volcanoes have steep sides and form from very viscous lava. They are often found on continental crusts and cause explosive and unpredictable eruptions. The rock type associated with these volcanoes is Rhyolitic.

Composite Volcanoes are the most common on land and are found on destructive plate boundaries. They cause explosive and unpredictable eruptions.

Highlight: Understanding the different types of volcanoes is crucial for revising A-level geography and comprehending the theory of plate tectonics.

The page also mentions volcanic hotspots, which are caused by a concentration of radioactive elements inside the mantle. A plume of magma rises from these hotspots to eat into the plate above, creating an active volcano where lava breaks through.

Volcanic hazards are also discussed, including:

1. Ash Clouds: Small pieces of pulverized rock and gas shot into the atmosphere.
2. Volcanic Gases: Sulphur dioxide, water vapor, and carbon dioxide emitted from volcanoes.
3. Lahars: Volcanic mudflows that usually run down valleys on volcanoes.
4. Pyroclastic Flows: Fast-moving currents of super-heated gas and ash, reaching temperatures of 1000°C and traveling at speeds up to 450 mph.
5. Volcanic Bombs: Thick viscous lava fragments ejected from the mouth of a volcano.

Definition: Pyroclastic flow - A dense, fast-moving flow of solidified lava pieces, volcanic ash, and hot gases that occurs during certain volcanic eruptions.