Lecture 5 - Volcanic Hazards

Question 1

Describe the Features of a Fissure Volcano.

Answer 1

-Long, linear vent
-Mostly effusive eruptions producing basaltic lava
-Fed by dykes connecting magma chamber to the surface
-Found in rift zones and spreading centres, e.g. Iceland and East African Rift Valley

Question 2

Describe the Features of a Shield Volcano.

Answer 2

-Very shallow slopes
-Made entirely of lava flows (effusive)
-Lava is very hot (not very viscous)
-Typically basaltic lava with low gas content
-Found at spreading centres and intraplate hotspots, e.g. Mauna Kea, Hawaii

Question 3

Describe the Features of a Dome Volcano.

Answer 3

-Form by small bulbous masses of lava
-Lava is cooler and therefore more viscous (basaltic, rhyolitic)
-Grows by slow expansion from within; lava pours out the top and flows down the sides
-Common on convergent boundaries
-They comprise 6% of the volcanoes on Earth
-Mostly effusive but can be explosive

Question 4

Describe the Features of a Cinder Cone Volcano.

Answer 4

-Most common type of volcano
-Small, often found near larger volcanoes
-Made of unconsolidated ash, pyroclastic material and ejected rock fragments
-Steep-sided (30-40 degrees)
-Cone-shaped, circular base and crater on top with single vent
-Can be isolated or form on the sides of other volcanoes (parasitic)
-Gas-rich magma
-Grow very quickly

Question 5

Describe the Features of a Stratovolcano.

Answer 5

-Strato (from strata) as they consist of layers of lava and pyroclastic material
-Felsic-intermediate magma composition
-Often explosive
-Steep-sided
-Cone-shaped with a crater at the top; lava flows from this crater or from fissures on the sides, solidifying to form dykes which add strength to the volcano
-Conduit system brings magma from the chamber to the surface

Question 6

Describe the Features of a Caldera Volcano.

Answer 6

-Large eruptions which empty the magma chamber, causing the ground above to collapse into the space left thus forming the caldera
-Steep-walled, basin-shaped depression, often so large that you wouldn’t realise you’re in a volcano

Question 7

What is the meaning of an Effusive Eruption?

Answer 7

-Lava flows that ooze out of the vent, magma is low viscosity typically Mg/Fe rich, low water/gas content and hotter

Question 8

What is the meaning of an Explosive Eruption?

Answer 8

-Produces tephra, highly viscous magma (cooler) which is typically Si-rich and rich in water and gas
-Gas expands rapidly which generates an eruption with magma arranged into pyroclastic fragments

Question 9

What are the 3 types of Eruptive Material?

Answer 9

-Hydrothermal; just hot water, no magma, explosive with pulverised rock producing ash and are very short-lived
-Phreatic; Heat from old magma interacts with water, explosive and produces ash
-Phreatomagmatic; New magma and water which is very explosive

Question 10

Describe a Fissure Eruption.

Answer 10

-Effusive
-Flows out of parallel fissures
-Mostly basaltic lava
-Builds large plateaus of lava

Question 11

Describe a Hawaiian Eruption.

Answer 11

-Effusive
-Lava flows out of central vent or radial fissures
-Mostly basaltic lava
-Builds shield Volcanoes

Question 12

Describe a Strombolian Eruption.

Answer 12

-Mildly explosive, small sized eruptions
-More viscous, basaltic/andesitic magma
-Explosions caused by bursting gas bubbles (slugs) in volcanic conduit
-Ejection of cinders and lava bombs which build cinder cones

Question 13

Describe a Vulcanian Eruption.

Answer 13

-Explosive
-Short, violent small explosions of viscous magma
-Andesite or rhyolite
-Caused by fragmentation of plug in volcanic conduit
-Produces ash and pyroclastic density currents

Question 14

Describe a Pelean Eruption.

Answer 14

-Viscous magma, produces lava domes that collapse
-Andesitic/ rhyolitic composition
-Produces many pyroclastic density currents

Question 15

Describe a Plinian Eruption.

Answer 15

-Largest/most explosive eruption
-Fragmentation of glassy, viscous magma
-Rhyolitic magma
-Lots of ash, pyroclastic density currents
-Ash column can rise 50km
-Usually completely empties the magma chamber

Question 16

What is Flood Basalt?

Answer 16

-Large-scale effusive eruption
-Short duration, high lava volume (>1 million km cubed)
-Produces large lava flows (>100,000km squared)
-Very hot, so magma is very runny producing a ‘flood’
Mostly basaltic, hence the name
-Creates lava plateaus and mountain ranges
-Only 11 have occurred in the last 250 million years, but 5 have been connected to mass extinctions

Question 17

How do Flood Basalts Form?

Answer 17

-Large mantle plumes cause hot material from the core to rise
-Plume hits the base of the crust which raises the temperature
-This produces huge amounts of melt which may tear continents apart
-They are associated with supercontinent cyclicity

Question 18

What was the Central Atlantic Magmatic Province?

Answer 18

-Huge flood basalt which formed 201 million years ago
-Covered 10 million km squared (2% of the Earth)
-Split Pangea

Question 19

What are the Deccan Traps?

Answer 19

-Thick layer of basaltic lava flows which formed 66 million years ago covering 1.5m km squared in under 30,000 years
-Produced a rift valley which split the Seychelles from India
-It released huge volumes of toxic gases, cooling the world by 2 degrees C nearly wiping out the dinosaurs

Question 20

What are the Siberian Traps?

Answer 20

-Largest volcanic event in the last 500 million years (occurred 250 mya)
-Eruptions lasted 2 million years covering 7 million km squared and erupted 4 million km cubed of lava
-This was likely caused by a mantle plume

Question 21

Describe the Extinct Volcano Olympus Mons.

Answer 21

-No known active volcanism on Mars
-Mars is half the size of Earth and has already lost a lot of its heat
-Largest mountain in the Solar System

Question 22

Describe the Moon Io orbiting Jupiter.

Answer 22

-Most volcanically active object in the Solar System
-Very few impact craters which suggests it has a young surface
-Closest moon to Jupiter so heat source comes from tidal forces

Question 23

Describe Saturn’s moon Enceladus.

Answer 23

-Enceladus is characterised by cryovolcanoes which are common in the outer solar system
-Lava here is made of water, liquid nitrogen and CO2, and contains organic molecules like amino acids
-Continuous geyser volcanism from here formed and maintains Saturn’s E ring
-The structure consists of an ice crust, liquid water mantle and a core of solid rock