Vulcanicity. Objectives to identify the basic structure of volcanoes and understand how they form.

Transcription

1 Vulcanicity Objectives to identify the basic structure of volcanoes and understand how they form.

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3 Some key terms to start.. Viscosity how well a substance (lava) flows. Acid lavas have a high viscosity, are sticky and do not flow far. Basic lavas are runnier. Stratovolcano diagram Pyroclastics the ash, gas, lava and rock blasted from inside the volcano. These are intrusions of lava into gaps in the rock layers. They can solidify in time.

4 Types of eruption Explosive eruptions Effusive eruptions Location Convergent boundaries Divergent boundaries Type of lava Lava characteristics Style of eruption Rhyolite (more acid) Andesite (less acid) Acid (high % silica), high viscosity, lower temperature at eruption Violent and explosive, vent and cone often blown apart Basalt Basic (low % silica), low viscosity, higher temperature at eruption Gas bubbles expand freely, limited explosive force Materials erupted Gas, dust, ash, lava, bombs, tephra Gas and lava flows Frequency of eruption Shape of volcano Tend to be long periods of inactivity Steep sided, strato volcanoes, calderas Frequent and can last for long periods (up to many months) Gently sloping sides, shield volcanoes, lava plateaus.

5 Explosive volcanoes..

6 Strato volcanoes / composite volcanoes Layers of ash, pyroclastics and lava that builds up. Often steep sided. Inside the vent the highly viscous sticky lava builds up forming a plug. As gas and material builds up behind it pressure builds. This is released in huge explosions blowing it apart. A bit like a champagne cork!!!!! Often found in chains e.g Andes mountains or archipelago islands (Japan) on convergent boundaries.

7 Calderas These are craters normally over 2km wide. They form during explosive eruptions. The interior collapses in as the magma is blasted out leaving a vast crater. Mount Mazama's summit was destroyed by a volcanic eruption that occurred around 5677 BC. The eruption reduced Mazama's approximate 3,700 m height by about 1 mile 1,600 m.

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10 Match the description to the diagram.. 1. Icelandic persistent eruptions along a fissure, lava with low viscosity, can build up vast horizontal plains. 2. Plinian huge explosions, highly viscous lava plugs the vents, vast amounts of gas blasted up and out the sides of the volcano. Volcano side may be destroyed. Vast columns of ash and gas may blanket a huge area, with large quantities of pyroclastics. 3. Strombolian explosive but primarily lava and gas. Gas columns form, some pyroclastic explosions. 4. Vulcanian violent gas explosions clearing blocked vents of lava. Tall columns of ash and gas formed with large amounts of lava bombs and fragments of rocks thrown out at the start of the eruption. 5. Hawaiian eruptions large amounts of lava produced, frequent eruptions, occasional pyroclastic and gas eruptions. 6. Pelean / Vesuvian eruption violent and powerful eruptions with lava flows covering the local area. Large ash falls and pyroclastics in the local area.

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12 Effusive volcanoes

13 Lava plateau When basic (runny) magma erupts from multiple fissures. Basalt Can cover vast areas Examples - Deccan plateau in India ( km²) and Columbia plateau in NW USA ( km²) They are flat uniform slope of 1, but denudation has changed this. None in past 50 million years.

14 Shield volcanoes These are found at divergent boundaries. Many are unseen on the ocean floor. Basic runny lava spreads over a wide area creating flat but wide volcanoes. Iceland was created by this process.

15 Hot spots A hot spot is a FIXED area of high volcanic activity. Here magma rises through the mantle as a mantle plume. As the plate moves the hot spot stays still. So chains of islands form. The best example is the Hawaiian island. Normally the volcanoes are shield volcanoes. An exception is Mt Teide in the Canary islands, a stratovolcano.

16 Super volcanoes A volcano that erupts upwards of 1000 km³. These exist as giant calderas and produce vast ash deposits. Debate surrounds the potential impacts.

17 Tasks 1. How does the viscosity of lava affect volcanic structure and eruption type? (8) 2. Describe the formation of a caldera (6) 3. Examine the potential for a super volcano event in one named place (12)

18 Explosive eruptions Effusive eruptions Location Convergent boundaries Divergent boundaries Type of lava Lava characteristics Rhyolite (more acid) Andesite (less acid) Acid (high % silica), high viscosity, lower temperature at eruption Basalt Basic (low % silica), low viscosity, higher temperature at eruption Style of eruption Materials erupted Frequency of eruption Shape of volcano Violent and explosive, vent and cone often blown apart Gas, dust, ash, lava, bombs, tephra Tend to be long periods of inactivity Steep sided, strato volcanoes, calderas Gas bubbles expand freely, limited explosive force Gas and lava flows Frequent and can last for long periods (up to many months) Gently sloping sides, shield volcanoes, lava plateaus.

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