14.2 Types of volcanic eruption Flashcards
What is viscosity?
An indication of how well a substance flows
What causes volcanic eruptions to produce diverse landforms?
- Where the eruption occurs
- The type of lava
- Variety of materials produced eg ash, gases
- How the eruption takes place
What are the two types of volcanic eruption?
Effusive and explosive
Effusive eruptions: where do they occur?
Divergent plate boundaries
Effusive eruptions: what type of lava?
Basalt
Effusive eruptions: characteristics of the lava?
Basic (low % silica), low viscosity, higher temperature at eruption
Effusive eruptions: style of eruption?
Gas bubbles expand freely, limited explosive force
Effusive eruptions: materials erupted?
Gas, lava flows
Effusive eruptions: frequency?
Tend to be more frequent and can continue for many months
Effusive eruptions: shape of volcano?
Gently sloping sides, shield volcanoes
Explosive eruptions: where do they occur?
Convergent plate boundaries
Explosive eruptions: type of lava?
Rhyolite (more acid), andesite (less acid)
Explosive eruptions: characteristics of the lava?
Acid (high % silica), high viscosity, lower temperature at eruption
Explosive eruptions: style of eruption?
Violent bursting of gas bubbles when magma reaches the surface, highly explosive, vent and top of cone often shattered
Explosive eruptions: materials erupted?
Gas, dust, ash, lava bombs, tephra
Explosive eruptions: frequency of eruption?
Long periods with no activity
Explosive eruptions: shape of volcano?
Steep-sided stratovolcanoes, caldera
What type of eruptions does Iceland owe itself to?
Effusive eruptions
Two products of explosive eruptions?
Calderas and stratovolcanoes
What are stratovolcanoes?
Also known as composite cone volcanoes – made up of layers of ash and acid lava with concave symmetrical profiles – associated with explosive eruptions
What are the complex internal networks of lava called inside a stratovolcano?
Sills and dykes
What is the difference between a sill and a dyke?
Sill = horizontal protrusion of magma into surrounding older rocks Dyke = vertical protrusion of magma into surrounding older rocks
What are the vents like in stratovolcanoes + why + what can this lead to?
The vents are often filled with a mass of solidified magma because acid magma does not flow easily = build-up of pressure causing explosive eruption
What is a caldera?
A large-scale volcanic crater formed because of an explosive eruption which emptied the magma chamber causing the volcano sides to subside
Example of a caldera:
1883 eruption of Krakatoa in Indonesia left a 7km wide caldera
What do convergent plate boundaries and explosive eruptions often give rise to?
Chains of stratovolcanoes eg Indonesia has 130 active stratovolcanoes along its archipelago
Two products of effusive eruptions:
Lava plateaux and shield volcanoes
How do lava plateaux form?
When basic magma erupts from multiple fissures, vast areas can be covered by free-flowing lava – flood basalts
Example of a flood basalt?
Deccan Plateau in central India covers more than 500,000 km2
What is a shield volcano?
A volcano with a broad base and gently sloping sides
How did the Hawaiian chain of islands form?
Due to the formation of a hotspot, thousands of kilometres from the nearest plate boundary
What is a hotspot?
A fixed area of intense volcanic activity where magma from a rising mantle plume reaches the Earth’s surface
Two examples of places with a hotspot:
Hawaii and East African Rift Valley
Most common type of volcano formed at a hotspot:
Shield volcano
Why do volcanoes of Hawaii become extinct?
Over millions of years the pacific plate moves northwest away from the hotspot, so the volcanoes lose their source of magma eg Kauai
How quickly does the Pacific plate move over the Hawaiian hotspot?
Moves northwest at a rate of 10cm/year
Two examples of active volcanic peaks in Hawaii:
Mauna Loa and Mauna Kea = both reach over 4000m above sea level
What Hawaiian volcano is currently rising?
Loihi is directly over the hotspot and is currently 970km below sea level
How long is the rift valley running through East Africa?
4000 km long
How have active volcanoes been created in East Africa?
As a result of rifting – magma has forced its way to the surface
Example of a volcano formed through rifting:
Mount Kilimanjaro
What is a super-volcano + what do they exist as?
A volcano that erupts more than 1000km3 of material in a single eruption
They exist as giant calderas
Example of a super volcano:
The Yellowstone super volcano in Wyoming has a caldera measure 75km in diameter
When was the most recent super-volcano eruption?
27,000 years ago – Taupo, North Island, New Zealand
When did volcanic activity start in Yellowstone?
2 million years ago – when the giant caldera formed
When did Yellowstone last erupt?
70,000 years ago
What two key factors need to be considered when measuring and assessing a volcano?
- Magnitude = amount of material erupted
- Intensity = speed at which the material is erupted
What scale are volcanoes measured on?
Volcanic Explosivity Index
Scale is 0-8
Combines magnitude and intensity
What factors are considered when measuring explosivity?
Volume of erupted material, heigh of ejected material, duration
What VEI was Eyjafjallajökull in 2010?
VEI of 4
