Volcanoes (Notes)

Volcanoes forming at subduction zones of convergent boundaries

-When an oceanic and another plate collide, the heavier oceanic plate subducts into the mantle. -Fluids and gases are released from the crust as it sinks down. The fluids then increase the temperatures which melt the sinking crust.
-The silica and magma formed rises and burns a path through the overlying crust, An explosive eruption occurs when magma reaches the earth's surface.
-A chain of volcanic islands (island arc) forms at the subduction zone of 2 oceanic plates (Japanese Island Arc).
-A chain of volcanic mountains (volcanic arc) forms at the subduction zone of oceanic and continental plates (Andes volcanic arc).

Volcanoes form at constructive plate boundaries

-When 2 oceanic plates separate, magma rises through the fissure formed on the earth's surface, which when reaching water firms and hardens.
-Overtime a mid-ocean ridge of volcanic mountains are built up (Mid-Atlantic ridge, Mount Kilimanjaro).

Volcanoes form at hotspots

-A volcanic island chain is formed at a hotspot, which is located at a fixed point in the mantle where the earth's plates move over it.
-When a weak area of a crust moves over it, plumes of magma rise from the mantle to the surface and a volcano is formed.
-The volcano will go extinct as the plate keeps on moving away from the hotspot.
-The process is repeated with each volcanic mountains getting older as it moves away from the hotspot.
-Example: Hawaiian island chain

Fissure eruptions

-Basic lava flows from a linear crack in the earth's crust. Fissures are up to 50cm in width having been widened by the force of the magma.
-The lava slowly spreads over a large area (>50km). Repeated flows will result in a lava plateau.

Shield volcano

A wide, gently sloping mountain made of layers of lava and formed by quiet eruptions.

Stratovolcano

The most common volcano. Known for its layers of lava flows and cinder. Built up by violent eruptions that leave the tephra followed by calm eruptions that leave lava flows. They build tall and steep volcanoes.

Caldera

A large crater caused by the violent explosion of a volcano that collapses into a depression.

Batholith

A mass of rock formed when a large body of magma cools inside the crust.

Dyke

Magma vertically intruded, cooled and solidified within the earth's crust.

Laccolith

A massive igneous body intruded between preexisting strata.

Sill

A slab of volcanic rock formed when magma squeezes between layers of rock.

Lopolith

Magma pushed its way into the bedding plane of a sedimentary rock forcing the overlying strata downwards, forming a small inverted dome-shaped structure.

Ways of predicting eruptions

-Historic patterns

-Cone temperature

-Gas measurements

-Changes in ground level

-Changes to groundwater

-Seismic activity

Negatives of volcanoes

-Lost lives

(20,000 died in Pompeii after Mount Vesuvius eruption)

-Weather patterns

-Man-made and natural landscapes destroyed.

(Mount St Helens eruption reduced its summit by 481m and triggered a landslide destroying 600km2 of trees)

-Clouds of ash and poisonous gases can be ejected from the volcanoes.

Positives of volcanic activity

-New land

(Iceland)

-Building materials

(Granite and Basalt)

-Fertile soils

(Volcanic soil in Brazil and Italy used for fruits and vegetables)

-Mineral deposits

(Gold rush in USA)

-Jobs

(Mount Vesuvius and Hawaiian islands being tourist destinations)