4. Seamounts & Islands Flashcards
How many seamounts are there globally?
Wessel et al. (2010) using satwllitw altimetry data estimated that 100,000 seamounts >1km tall remain to be charted
Seamounts <1km difficult to discriminate from abyssal hills
How are Mid-Ocean Ridge seamounts formed and why do they have flat tops?
Formed from local excess melting within the upwelling mantle
Seamount trail trajectories are fixed in the mantle
simple truncated cone shapes, commonly with calderas or collapsed pits
sides are usually gullied, near the 30 degree angle of repose of talus
Flat tops
- 1988 an effect of limited pressure (grow to limit which magma can erupt) (incorrect)
- 2001 theory of presure in ,agma chamber causign uplift and injection of dykes (incorrect)
- 2000: **used a higher resolution/frequency multibeam sonar **central eruption and ponding of lava within older calderas
Why does a high frequency mean a higher resolution?
The width of the beams produced by these sonars is inversely proportional to the length of the transducer arrays and proportional to the wavelength. A high frequency allows a narrower beam for a given transducer array size
Describe Island Arc Seamounts
Typically point cones formed from gaseous central eruptions
sides of the cones are slopes of volcanic talus formed as the centrally erupted material fragments
How do seamounts grow into islands?
During growth towards sea level, we expect increasingly explosive eruptions from expanding water in the lava and lava-water interactions
Once above sea level, lavas flowing from land will tend to fragment
Both these processes will leave a layer pf fragmented lava above the seamount series
The material erupted in shallow water is highly friable and erodes rapidly
Violent lava-water eruptions occur, generating fragmented material until the cone of fragments isolates the vent from the sea
Lava then extrudes, covering and amouring the cone
What are volcanic rifts?
zones of dyke intrusion and surface rifting.
Rift eruptions of Hawaii often coincide with caldera florr deflation, hence lava draining central magma chamber into dykes.
Underwater they continue and build ridges
Landslides
sonar data -> massive debris aprons around volcanic island,chutes
Hawaii - abundant landslides of two types: debris avalanches (fast-moving) and slumps (slow-moving)
Conditions at time of failure poorly known (pore water pressure, earquakes?)
Tsunami Hazard
Major landslides in Canaries every ~125-170ky, perhaps every 10ky globally
small events are less damaging but more frequent, perhaps also a hazard
Origins of volcanic rift zones
- The stresses inside a ridge are likely to be extensional along its centre because of gravitational loading (sagging under its weight)
- This may produce some of the extensional stress regime within the ridge to encourage dykes to intrude along them
- However, this would require the ridges to have quite extreme elastic properties and intrusion of dykes would most likely offset those extensional stresses rapidly, so another mechanism is needed
- Suggested that movement along the decollement could offset the effect of dyke intrusions, allowing further dyke intrusions into the centre
Submarine Lavas
- Degassing of magma lowers its sulfur content
- This can occur by venting in lava lakes, the magma then draining back into dykes and erupting elsewher
- Some submarine emplaced lavas have <0.0% S, suggesting they had degassed
- Some lavas emplaced from a subaerial source have >0.08% S suggesting no degassing
Dendritic Flow Lobe emplacement
suggests the a’a flow became repeatedly pressurized and produced new breakouts, leading to a sequence of flow lobe emplacement somewhat like a pahoehoe flow on land
Guyots
flat-topped seamounts of several kms in height
formed at sea level
at low lattitudes: drowned atolls - thought to occur because of an environmental stress (coral dying)
at high lattitudes: no coral so though to be formed by coastal erosion and subsidence
