VULCANICITY

•          Vulcanicity

•          Process whereby magma flows into the earth’s crust or onto the surface

•          When magma reaches the surface, it is called lava

•          The type of landform that arises depends on:

•                          1.            Location where the magma cools or solidifies

•                          2.            Nature of the magma (viscosity, temperature)

•                          3.            Means by which the magma
                reaches the surface

Location where the magma cools

•          When magma enters lines of weakness such as faults in the earth’s crust, it may cool underground to form intrusive landforms

•          When the magma (lava) cools and solidifies on the surface, it forms extrusive landforms

Viscosity of the magma

•          Viscosity refers to the magma’s resistance to flow

•          It depends on the silica content and the temperature of the magma

•          Basic magma has low silica content, higher temperatures and lower viscosity (i.e. more fluid)

                è When the magma reaches the surface, it moves         quickly and covers large areas before solidifying

•          Acid magma has high silica content, lower temperatures and higher viscosity (i.e. less fluid)

                è The lava moves slowly and solidifies quickly

Means by which the magma reaches the surface

•          Acid magma is associated with explosive eruptions

•          As the magma rises from deep within the earth, the dissolved gases in the magma expand due to the lower pressure

•          Because of its viscosity, acid magma does not allow these gases to escape

•          The gases build up and
eventually lead to
explosive eruptions

•          Such eruptions generate
a lot of ash and cinders

Types of volcanoes

•          The shape and size of the volcano formed depend on the nature of the lava and the way the lava erupted

•          Types of volcanoes:

                 – Acid lava cone

                 – Basic lava cone

                 – Ash and cinder cone

                 – Composite volcano

Acid lava cone

•          Volcano formed from acid lava

•          Acid lava is viscous and solidifies quickly

•          The lava is unable to move far away from the crater before it solidifies

•          The volcano formed has steep sides and a narrow base

•          A volcanic plug may be formed when viscous magma solidifies within the pipe of the volcano and is exposed after years of denudation

Basic lava cone

•          Also called a shield volcano

•          Basic lava is fluid and has high temperatures of between 1,100°C and 1,200°C

•          The lava spreads far from the crater before solidifying

•          The resultant volcano has a broad base and gentle slopes and is often
of low height

Case study: Hawaiian Islands

•          The Hawaiian Islands in the Pacific Ocean are the tops of massive undersea shield volcanoes

•          They were formed far away from the plate boundary as the Pacific Plate has been moving slowly over a hot spot

•          Hot spots are fixed locations deep within the mantle from which columns of magma rise to the surface

•          The magma rises to the surface by melting through the crust

•          As the plate moves, the volcano over the hot spot is slowly carried away and eventually becomes extinct while a new one is formed in its place

•          The hot spot is believed to be currently under the island of Hawaii, also known as the Big Island

•          The entire Big Island is a shield volcano with three active peaks – Mauna Kea, Mauna Loa and Kilauea

Ash and cinder cone

•          Acid lava tends to trap large amounts of gases, resulting in violent eruptions

•          Volcanic rock fragments, called pyroclasts, are ejected with great force

•          Ash refers to pyroclasts smaller than 4mm while the larger ones are called cinders

•          Larger pyroclasts fall nearer the summit and form steep slopes

•          Finer pyroclasts get blown
farther and form gentle slopes

Composite volcano

•          Made up of alternate layers of viscous lava, and ash and cinders

•          The gases trapped in the acid magma eventually lead to a violent eruption

•          This ejects ash and cinders which settle around the crater

•          The eruption clears the vent, allowing lava to flow out freely

•          The lava cools and solidifies over the ash and cinders  deposited earlier

•          Repeated eruptions of ash and cinders followed by lava flows build the composite volcano

•          The violent eruptions may cause cracks along the sides of the volcano

•          Lava may escape through these cracks, resulting in parasitic cones

•          Mount Fuji in Japan and Mount Mayon in the Philippines are composite volcanoes

Distribution of vulcanicity

•          Most volcanoes are located near convergent plate boundaries and coincide with earthquake zones

•          There are also volcanoes in the Caribbean. Below are two shuttle radar maps of Grenada and St. Lucia showing the locations of volcanoes on the islands.

•           

Case study: Montserrat eruptions

•          Montserrat is one of the islands of the Lesser Antilles

•          It is the result of the North American Plate subducting beneath the Caribbean Plate

•          The Soufrière Hills volcano erupted violently in 1995, emitting ash and steam

•          The eruption started a
series of larger eruptions
that lasted over two years

•          19 people were killed

•          The capital town of
Plymouth was destroyed