1A2 Geography27th January 2015

Objectives:Students will be introduced to landforms associated with volcanic activity: Mid-ocean ridges, volcanic islands, and volcanic mountains.

Homework:

1. In your copies/hardbacks, draw a volcano, labelling the magma chamber, vent, crater, and layers of ash & lava.

2. Write a short description for each of the key words to be used during today’s class (see hand-out).

3. Memorise the names of the major tectonic plates in Fig. 4.2 on page 66 of your textbook.

Volcanic Activity

• During our last class, we learned that the Earth’s outer layer, the crust, is not one whole piece. It is divided into sections called plates.

• Due to convection currents in the magma, these plates do not remain stationary – they move.

Volcanic Activity

• As a result of this movement:• Plates can collide with one another

(destructive plate boundaries).• Plates can separate (constructive plate

boundaries).• This can result in volcanic activity.

Pacific Ring of Fire

• Much of this activity takes place on the edge of the Pacific Plate in an area known as the Pacific Ring of Fire.

• Volcanic activity can result in the creation of certain landforms.

Volcanic Landforms

• For the remainder of today’s class, we will be focusing on the creation of three landforms:

1. Mid-ocean ridges.2. Volcanic islands.3. Volcanic mountains.

1. Mid-Ocean Ridges

• Magma can reach the Earth’s surface when it rises through gaps between plates.

• There, it cools and hardens.

• This process continues over time. The magma builds up and forms new land in a ridge-like shape.

1. Mid-Ocean Ridges

• Under the Atlantic Ocean, the American Plates are pulling away from the Eurasian and African Plates.

• This movement is resulting in the creation of fissures (gaps), that allow magma to reach the surface, where it has cooled and built up to form an underwater chain of mountains – the Mid-Atlantic Ridge.

Volcanic Islands

• Over time, the mid-ocean ridges can increase in size as magma continues to flow from the fissures.

• As a result, these mountains can appear above the water’s surface forming volcanic islands, e.g. Surtsey.

Volcanic Mountains

• Sometimes, cracks will form in the Earth’s crust.

• These will be filled by magma rising from the mantle.

• As this magma rises, it melts the solid rock around it, creating a huge reservoir, called a magma chamber, in the crust.

Volcanic Mountains

• The build-up of pressure in the chamber can force magma to the surface through a narrow tube, called a vent, in the Earth’s crust.

• This causes eruptions – often explosive.

Volcanic Mountains

• Lava (magma that has reached the surface), ash and gas are thrown into the air.

• This lava and ash settles around the vent and gradually builds up to form a cone shaped volcanic mountain.

• Further eruptions add layers to the mountain.• It is eventually left with a hole at the top,

called a crater, through which the lava flows.