Extrusive processes Flashcards
What is the volcanic spectrum?
Effusive to explosive
What is the eruptive process controlled by?
Amount of dissolved gasses in magma
Viscosity
What are some examples of dissolved gasses which can be trapped in magma?
Water
CO2
SO2
What is viscosity?
fluids resistance to flow & ability to degas
What is viscosity of lava related to?
Silica content
Lava temperature
What does a higher silica content mean?
higher viscosity
What will the viscosity of a cooler lava be?
higher
How do volcanic systems form at convergent boundaries?
Partial melting oceanic crust
Melt rises and collect in magma chamber (pressure builds)
Erupts on surface forming volcanic cone
Post eruption- magma solidify and plug vent
Magma continually accumulates and cools
What will eruptions at convergent boundaries/ subduction zones be like?
Most violent eruptions
What will magma be like at convergent boundaries?
High viscosity of intermediate to felsic magma
high volatile content (water dehydration melting)
What type of colour rock will be produced from felsic magma?
lighter coloured rock (rhyolite)
What will felsic magma be enriched by?
lighter elements (silicon, oxygen, aluminium, sodium, potassium)
What is the viscosity of felsic magma like?
high viscosity- sticky doesn’t flow easy
What does having high viscosity magma mean for the explosivity at convergent coundaries?
pressure can build making eruption more explosive
What type of volcanoes is most associated with convergent boundaries?
stratovolcanoes
What is an example of a stratovolcanoes?
Mt St Helens
What are stratovolcanoes also known as?
composite volcanoes
Why are stratovolcanoes known as composite volcanoes?
comprised of interbedded (andesitic) lava flows and pyroclastic debris
What do the layers on composite volcanoes reflect?
gradual calm and then more violent phases of volcanic development
What leads to a well formed cone shape for composite volcanoes?
Moderate to high viscosity
High angle of repose for pyroclastic debris
What happens to the neck and vent of a volcano post eruption?
plugged with fragments of pyroclastic debris and lava
What will begin to form over time below the plugged section of the neck and vent?
pressure increases and build a lava dome
What happens when magma and expanding gases being to move in the volcano when it is plugged?
plug can rupture leading to eruption
What happens to material around the vent when an eruption occurs?
pulverised
What is the charging element of pyroclastic debris?
gas
What is pyroclastic debris?
airborne volcanic fragments
What speed can pyroclastic debris reach?
200km/h
What allows pyroclastic debris to reach such high speeds?
minimal drag and frictional resistance
What can fast moving pyroclastic debris be called?
pyroclastic flow or glowing avalanches
What happens when water mixes with loose ash in the atmosphere?
lahars- destructive mudflows are created
How long might it take for fine ash to come down from the atmosphere?
depends on ejection height but can range from months to years
When did Mt St Helens erupt?
1980
What was different about the pyroclastic flow for the Mt St Helens eruption?
occurred on the north flank with a lateral eruption. Created largest mass movement as whole north face collapsed
How could geoscientists tell an eruption was imminent at Mt St Helens?
Shallow earthquakes sign of magma moving combined with bulging and deformation of volcano
What path will magma take when moving through a volcano?
the weakest point in the structure
How much of the surrounding area was devastated by the 1980 Mt St Helens eruption?
150 miles*2
How long was Mt St Helens dormant before the 1980 eruption?
120 years
What are cinder cone volcanoes like?
small volcanic cones generally developed on flank of large volcano (found in clusters)
What are cinder cones mainly composed of?
pyroclastic cinders (pea sized blobs of lava) cooled mid air
What will there rarely be evidence of with with cinder cones?
lava flows
What are the typical dimensions of cinder cones?
rarely over 250m in height or 500m in diameter
What happens when a caldera erupts due to be so explosive?
the top of he volcano collapses in
What are huge amounts of pyroclastic debris and gases released from in calderas?
ring fractures
What is a caldera?
the crater left after eruption when the volcano collapses in because of the mass ejection of magma
What is an example of a past caldera?
Crater lake (oregon) last Mt Mazama eruption 7000yrs ago
What was the scale of crater lake caldera?
Relatively small with 10km diameter
What is the last eruption of Mt Mazama associated with?
ash deposition across western US and Canada
How many super volcanoes are there?
20
When was the last super volcano eruption?
27,000 years ago Taupo New Zealand
What are tuffs description?
fragments 2mm in diameter
look sedimentary but lithified volcanic ash
Welded
What does welded mean with volcanic deposits?
material was still hot enough that they partially recrystallised when emplaced and consolidated
What are Breccias?
rock formed from angular gravel and boulder-sized clasts cemented together in a matrix
What is the structure of breccias like?
rock formed from angular gravel and boulder-sized clasts cemented together in a matrix
How does pumice form?
when lavas have a lot of water and other volatiles
What is pumice like?
Glassy texture
Light (mostly air many void spaces)
Frothy appearance
What are cinders? (volcanic deposits)
pebble sized particles
What are volcanic bombs?
baseball to car sized chunks (due to cooling in air will have head and tail shape)
What are mantle plumes?
stationary plumes of heat in the mantle that dome the lithosphere and produce magma by decompression
What does the magma plume process lead to on land and in oceans?
Rift formation within continental plates
Volcanic island chain on oceanic plates
What will usually happen first from a mantle plume before hotspot volcanoes?
Flood eruptions
What happens with flood eruptions?
very fluid basaltic lava creates a extensive lava plateau
What are some examples of flood eruptions?
Deccan traps- India in cretaceous
Siberian traps- Permian, Triassic boundary
What are the general characteristics of mafic magma?
Usually darker rock produced
Low silica Fe and Mg rich
Less viscous - less sticky
What type of eruption will you get with mafic lava?
effusive as lava flows
What is created with hotspot activity under continental crust?
rift systems
What is the characteristic geometry of rift systems?
“triple junction”
What is triple junction geometry?
where three lithospheric plates meet
How does continental crust cause assimilation of magma to share similar characteristics with subduction volcanoes?
Under continent more rock to get through for magma to get to surface so more time for assimilation of material
Where is the greatest volume of volcanic rock produced?
along oceanic ridges at divergent boundaries
What type of basalt is extruded underwater at divergent plate boundaries?
pillow basalt
How does pillow basalt shape form?
rapid exterior quenching of lava bodies
What are eruptions like at mid ocean ridges?
gentle due to low viscosity and low volatile content
What type of magma will form shield volcanoes?
mantle derived mafic lava
hot
non-viscous (runny)
dry (limited volatiles)
How do shield volcanoes form?
Basaltic lava builds up in flat layers creating a large volcano with gentle slopes
What is an example of a shield volcano?
Typical of Hawaii (Mauna Kea)
What type of lava is mafic lava?
Block lava
What are the characteristics of block lava?
less mobile more viscous
Rough irregular surface
What happens when block lava cools?
goes to angular chunky texture causes lava to thicken and break apart as flow continues downslope
What are vesicles in block lava?
few and irregular in shape
What is the texture of blocky lava referred to as?
a-a (sound you make when walking on a flow)
What is another name for ropey lava? (mafic lava)
pahoehoe
What is ropey lava like?
more mobile
less viscous
wrinkled but smooth and shiny
What are vesicles like in ropey lava?
more abundant
small spherical shape
