Topic 6 Flashcards
TOPIC QUESTION What are some volcanic hazards?
- Intrusive igneous activity
Granitic and andesitic melts are higher in silica and therefore more viscous than basaltic magmas - Gas content
Magmas contain dissolved gases (violations - water vapour, CO2, SO2, H2S) as magma rises pressures decreases - gases come out of solution to form bubbles
What are three types of magma?
- Basaltic or mafic
- lowest silica content 50%
- 1000 - 1200 C temperature - Andesitic or intermediate
- 60% silica concentration
- 800 - 1000 C temperature - Rhyloite, felsic or granite
- 70% silica concentration
- 650-800 C temperature
______________ magma is finegrained while _____________ magma is coarse grained
Extrusive, intrusive
TOPIC QUESTION What are the three types of caldera formed by volcanic eruptions?
- Crater lake type - collapse of the summit of a large stratovolcano
- Hawaiian type - caused by the collapse of the top of a shield volcano by subterranean damage
- Yellowstone type - caused by collapse of a large area by discharge or pyroclastic material
What are the characteristics of explosive or violent eruptions?
Magma is torn apart as it rises and reaches the surface in pieces known as pyroclasts
Describe the effusive eruption style of a volcano
Magma rises through the surface and flows out of the volcano as a viscous liquid or lava
TOPIC QUESTION Hot lava that is low in viscosity will result in what type of eruption style?
Effusive
How does a magma’s composition affect a volcano’s eruption?
- lava with a higher viscosity and silica content tend to have explosive eruptions
- low viscosity lava tend to have effusively be eruptions
Multiple eruptions with andesite magma usually firm what types of volcanos?
Composite cone or stratovolcanos
Eruptions that are high in gas and steam gas into the atmosphere firm what type of volcano?
Cinder Cone (scoria cone)
TOPIC QUESTION A low viscosity basaltic lava flow would form what type of volcano?
Shield volcano
TOPIC QUESTION What are the characteristics of a stratovolcano or composite cone?
- are made of alternating layers of lava and ash (other volcanos just made of lava)
- eruptions from these volcanos may be pyroclastic flow rather than lava
- flow can roll down sides very fast with temperatures over 400C
What are the different types of Hawaiian lava flows?
- A’A’ flows
- Pahoehoe flows
- Lava tubes
What are the characteristics of pahoehoe flows?
- low viscosity, basaltic flow
- cools on contact with atmosphere causing thin plastic like surface skin to form
- sometimes inflated with molten lava to give braided rope appearance
- due to low viscosity the flows are thin and travel long distances from the vent
What are the characteristics of lava tubes?
- once lava makes contact with surface skin becomes solid
- lava continues to flow beneath the surface in channels (called lava tubes) which carry lava from the vent to the flow edge
- surface skin insulated lava keeping it hot while moving through the tubes
- usually basaltic magma
- can travel great distances
“Fire fountains”
What are the characteristics of A’A’ flows?
- made from higher viscosity basaltic and andesitic lava
- initially makes smooth skin on surface but quickly broken up by molten lava and gases to form rough jagged surface
What are the characteristics of silica rich volcanos?
- pyroclastic materials
- Eruption cloud is an explosion be vertical cloud of hot gases and ash
- Gravity driven deposits
- Pyroclastic flow
- Fast moving >100 km/h of superheated gas and rock reaching 1000C in temperature
- High density flow
- Pyroclastic surge is a low density flow with higher gas to rock ratio
- Resulting rocks = tuffs and volcanic breccias
What are the characteristics of andesitic magma?
- 60% silica content
- 800 - 1050C temperature
- viscosity is intermediate
- gas content intermediate (3-4%)
Solidified volcanic rock is andesite
Solidified plutonic rick is diorite
What are the characteristics of basaltic magma?
- 50% silica
- 1000-1200C temperature
- low viscosity
- low gas content (1-2%)
- tend to form shield volcanos, basaltic plateaus and composite cones
- solidified volcanic rock is basalt
- solidified plutonic rock is gabbro
What are the characteristics of felsic (granitic) magma?
- 70% silica
- 650-800C temperature
- high viscosity
- high gas content
- tend to form lava domes
- solidified volcanic rock is rhyolite
- solidified plutonic rock is granite
TOPIC QUESTION How do gases effect eruption style?
- gases create explosions if they cannot easily escape from viscous magma
- they can be released without explosion from less viscous magma in an effusive style
TOPIC QUESTION What is the effect of the temperature of the lava on the eruption style?
- the higher the lava temperature the lower the viscosity of the lava, high temperature lava will be less viscous and have an effusive eruption style
- low temperature lava is less viscous and more likely to have an explosive eruption style as they do not flow easily
What factors impact magma viscosity?
- The amount of silica in the magma higher silica = higher viscosity
- Temperature of eruption
Higher temperature = lower viscosity - Silica tends to polymerize in magma which increase the viscosity so more silica more polymerization and higher viscosity
Describe the anatomy of a volcano?
Volcanic activity begins with a fissure which widens into a circular conduit which becomes an opening at the surface or a vent
Describe the composition of a volcanic cone?
Cone shaped structure made of lava and pyroclastic material
TOPIC QUESTION What are the characteristics of a cinder cone volcanos?
- Made of unconsolidated material (rejected pyroclastic material)
- have a large crater
- Steep sided 30-40 degree angle < 300m high
- Single, short lived eruptive event (days or years)
- Fields (Arizona) or parasitic cones (on the flanks of a larger volcano)
TOPIC QUESTION What are the characteristics of a stratovolcano (composite cone)?
- Dangerous/ violent
- Periodic eruptions
- conical shape/steep summit
- eruption of gas rich and esitic lava or rhylotic pyroclastic - lava is viscous in nature
- Some fluid basaltic video
What is a lahar?
A potentially deadly mud flow made volcanic debris and water (rain or melted glacial ice)
May occur with or without eruption
I.e. Mount St. Helens done today
What triggers Hawaiian type eruptions?
- New magma enters the chamber
- This is detected by inflation of volcano
- Causes eruption along new fractures
- A precursor for many eruptions is a sharp increase in seismic unrest (earthquakes tremors) followed by a quiet period
Describe the Hawaiian type caldera?
- steep walled with a flat bottom
- formed by gradual subsidence as magma drained laterally
- Mount Loa and Kilauea both have large summit calderas
Describe a Yellowstone type caldera?
- caldera is 70km in diameter
- resurgence following collapse
- volcano capable of an eruption with an eject volume greater than 1000km3
- can result in volcanic winter
- created 630,000 years ago
- eruption of 1000km2 of pyroclastic material
- found in long valley Yellowstone
TOPIC QUESTION How is a caldera formed?
- By collapse of the summit of a large stratovolcano (crater lake type caldera)
- By collapse of the top of a shield volcano by subterranean drainage of magma (Hawaiian type caldera)
- By collapse of a larger area caused by huge discharge of pyroclastic material (Yellowstone type caldera)
What is a caldera?
A large circular depression >1 km in diameter
TOPIC QUESTION Describe the composition of a shield volcano
Shield volcanos are broad dome shaped structures with gently sloping sides
They are composed of accumulated basaltic lava flows that is low viscosity and runny
They have large circular depressions (caldera) at the summit
They have frequent but gentle eruptions
i.e. Olympus Mons on Mars
What are pyroclastic materials?
When volcanos erupt explosively they eject pulverized rock, lava and glass fragments from the vent
The particles produced are called pyroclastic materials or tephra
Can form into pyroclastic rocks (tuffs or volcanic breccia)
