Schol ZEALANDIA Flashcards
the two plates of Zealandia
Indo-Australian and Pacific plates
more buoyant continental, denser oceanic
types of boundaries in Zealandia
convergent (subduction)
transform (lateral movement)
describe the differences between troughs and trenches
both occur at SUBDUCTION zones…
troughs = oceanic and continental
big difference in density, hence SHALLOW angle (drags overriding plate slightly).
shallow depression.
trenches = oceanic and oceanic
similar density, hence STEEPER angle of subduction, deforms the overriding plate more.
deeper depression
describe the features of the Kermadec subduction zone, and the hazards
upper North Island
fastest converging, most seismically active plate boundary on earth (58mm/year)
has the highest density of submarine volcanoes
Islands (Raoul, Macauley) are peaks of volcanoes
tsunami hazards due to earthquake activity.
submarine cone collapse –> pyroclastic flows impacting water
oceanic/oceanic –> Kermadec Trench (8km deep)
older pacific plate subjects under younger Aus Plate
describe the features of the Hikurangi subduction zone and the hazards associated with it
on the East of the NI
denser oceanic PP subducts continental AP
potentially the largest earthquake / tsunami hazard for NZ (as there would only be a 3 minute tsunami warning)
World’s shallowest ‘slow slip’ events occur off the coast of Gisborne
The Taupo volcano zone on the AP is parallel to the HSZ
describe the features and hazards of the Puysegur subduction zone
below the SI
oceanic/oceanic, AP subducts PP, trench 6km deep
Alpine fault starts at the Northern end of the trench.
Macquarie Ridge lies to the east. (different system)
highly seismically active, with magnitude 7.2 and 7.8 earthquakes occurring in the last 20 years.
describe the processes and features of the Southern Alps
continental AP (moving NE) and PP (moving SW), transform
PP overrides, uplifting land to form the Alps
forms transform fault (Alpine)
metamorphic rocks of deep origin are uplifted.
two types of volcanism in Zealandia and where they occur
subduction zone (Kermadec, Hikurangi) hotspot (Auckland field)
describe the general process of a volcanic eruption for a subduction zone
PP subducts AP
at a certain depth, rock melts
magma rises through cracks/vents, erupts.
Describe the Taupo Volcanic zone system
formed along the Hikurangi
leading edge of the subduction zone bends down, stretches the overriding plate, which thins.
large magma chamber closer to the surface
hence geothermal activity near Taupo.
material brought to the surface is from both PP and AP, transfer of continental crust from PP to AP.
andesite (Ruapehu, Ngaruhoe) and rhyolite (Rotorua, Taupo) volcanoes.
Describe the Kermadec Volcanoes
mostly submarine basalt and andesite cones (forming the kermadec Ridge)
increasing number of rhyolitic caldera volcanoes discovered
describe the process of a hotspot volcano
caused by a plume of basalt magma below crust, in an area typically far from a plate boundary.
low viscosity ‘lava fountain’ eruption
forms chains of volcanoes - hotspot remains in the same place but the plate moves, linear pattern.
shows late movement direction (so a chain of volcanoes suggests tectonic activity)
describe the Auckland Volcanic Field
- -> how volcanoes are created
- -> plate tectonics interaction
- -> types of volcanoes
A plume about 100km from the surface.
each volcano is a different path the magma took to the surface - all from the same chamber
subduction of the east stretches overriding AP, causing cracks under Auckland, allowing magma to reach surface.
mostly scoria cones (basalt + water)
what are slow slip events?
the release of pressure (tectonic stress) at a SUBDUCTION BOUNDARY over a long time
(causes movement associated w/ a mag 6+ eq over months)
usually cannot be detected on seismographs, identity land movement with GPS
the pros and cons of slow slip events
can both relieve and transfer stress.
relieve: reduces risk / delays a major earthquake
potential use - predict eq
can load tress onto another part of the boundary, causing fault ruptures –> large eq
eg. Kapiti slow slip led to 2014 Eketahuna eq by loading stress onto nearby fault, causing it to break.
describe the Hikurangi system in terms of the slow-slip plate boundary
between AP and subducting PP, there is the steady creeping zone (normal mvmt), the slow slip zone and the stuck plate interference.
this is lower in the SI.
stuck = adhesion between rocks
huge tectonic stresses built up over time may be released in a decoupling event, producing a mega thrust EQ (mag 8+)
slow slip events further north (Gisborne/Hawkes Bay) may increase pressure on ‘stuck’ section.
what is a turbidity current?
a rapid, downhill flow of denser sediment-laden water (underwater landslide)
sediment travels downslope (towards the abyssal plane), picking up more sediment as it goes, so it deposits a large amount.
what causes a turbidity current?
sediment builds up on continental shelf (the shallow area off the coast of a landmass), loosened by a geological event (earthquake)
what information do turbidites reveal?
Each turbidite layer is indicative of a geological event.
easy to identify, as larger particles settle first (density flow), forming classic turbidite layers.
hence distinct boundary between each turbidite unit.
contain BOTH SHALLOW AND DEEP WATER ORGANISMS.
may be a layer of pelagic (normal Si/Ca type sediments) between turbidites, but it takes a long time to settle, typically a thin part of the core sample.
At what location are turbidity currents common?
Kaikoura Canyon
describe the features of the Kaikoura Canyon
60km long, 2km deep submarine canyon, 500m off the coast of Kaikoura.
very shallow continental shelf –> deep abyssal plane.
joins the Hikurangi Channel, which meanders on the Hikurangi plateau
Why are turbidity currents common in the Kaikoura canyon?
the canyon is the sink for the coastal transport system
carries eroded material deposited by rivers draining from the Southern Alps.
how are turbidity currents created in the kaikoura canyon?
every few hundred years, EQ loosens the materials built up at the head of the canyon.
flows down the Hikurangi channel at 30km/h, depositing layer in the Hik Trough
what do sediment cores from the Hikurangi trough show?
regular deposition events.
eg. 2016 kaikoura EQ, 10cm of new, loose deposit.
