Extreme Earth Events (EEE) summary notes
Introduction to plate tectonics:
1. Earth has 3 distinct layers
a. Crust – thin, solid, outer layer
b. Mantle – part solid, part semi-liquid
c. Core – inner core solid, outer liquid
2. Tectonic plates consist of crust and upper (solid) mantle
3. NZ straddles Pacific plate (PP) and Australian plate (AP).
4. Two types of crust:
a. Oceanic, thinner, denser and younger
b. Continental, thicker, buoyant, older
5. NI on AP and continental crust, SI also continental, on AP and PP
6. When one plate meets another:
a. Move apart or diverge, but not around NZ
b. Move sideways past each other e.g. along Alpine Fault
c. Move together or collide – in 3 ways
i. Oceanic crust subducts under oceanic crust at Kermadec
trench and Puyseger trench
ii. Oceanic subducts under continental under east coast of NI
and Fiordland
iii. Continental and continental collide forming Southern Alps
7. Overview of NZ
a. Kermadec trench, oceanic crust of PP subducting under oceanic
crust of AP, forming very deep trench, ridge and volcanoes
b. Middle NI, oceanic crust of PP subducting under continental crust of
AP, forming Hikurangi trough, and Taupo volcanoes
c. Fiordland - oceanic crust of AP subducting under continental crust
of PP, forming steep sided fiords
d. Puysuger Trench - oceanic crust of AP subducting under oceanic
crust of PP, forming deep trench.
e. Auckland – a bubble of dry mantle magma forms a hot spot
creating a volcanic field
f. Southern Alps – continental crust colliding with continental crust
forming young sharp pointed mountains. These are also uplifted by
vertical movement of Alpine Fault. (AF also has considerable
horizontal movement).
Volcanoes:
1. Made by 2 processes:
a. Subduction – oceanic crust diving under either oceanic crust
(Kermadecs) or under continental crust (Taupo Volcanic zone).
Volcanoes are parallel to tectonic plate boundary. Water from
subducted sediment superheats and lowers mp of surrounding rock
forming magma. Magma rises through weakness in crust to form
volcanoes, mainly andesitic and rhyolitic. Gases released as
magma rises, may make eruptions violent.
b. Hot spots – a bubble of “dry” magma comes from deep (100 km)
underground, forming Auckland volcanic field. Very little gas,
basalt lava.
2. Volcanic eruptions are influenced by:
a. Stickiness of magma – which relates to the amount of silica in the
magma
b. Amount of dissolved gases in magma – often relates to the amount
of wet sediment subducted
3. Types of magma:
a. Basaltic, 45-55% silica, high T, runny, shield volcanoes
b. Andesitic, 55-65% silica, lower T, sticky or viscous, cone volcanoes
c. Rhyolitic, 65-75% silica, lowest T, very sticky/viscous, caldera or
dome volcanoes
4. Shape of volcano related to magma type and explosiveness
a. Shield – basaltic magma, low silica, lava runs a long way before
cooling because lava runny and very hot, so shield shape formed.
Found in hot spots, e.g. Rangitoto
b. Cone or stratovolcanoes – andesitic lava, medium silica, sticky lava,
not so hot as basalt so doesn’t run as far forming a cone shape,
also ash erupted so cone formed by layers of ash and lava, e.g.
Ruapehu
c. Caldera and dome volcanoes – rhyolitic magma which is very
viscous/sticky so it traps gas.
i. Calderas – violent eruptions because viscous magma traps
gases which expand putting pressure on ground above which
cracks. Pressure is released violently via weakest point, huge
ash clouds and pyroclastic flows, and ground collapses into
now empty magma chamber.
ii. Dome – degassed rhyolitic magma, gentle eruptions, magma
oozes out
5. Geothermal systems need heat source e.g. magma, cold water to
percolate down e.g. rain, and hot water to rise forming geysers,
fumeroles, mud pools, boiling springs.
Earthquakes:
1. Volcanic ones caused by rising magma
2. Tectonic ones – tectonic plates very slowly move together, distorting the
land. To relieve the distortion large cracks called faults form.
3. Earthquakes occur along faults or tectonic plate boundaries when blocks
of rock either side move abruptly. Seismic waves are released, shaking
the ground.
4. Focus where earthquake starts underground. Epicentre where waves first
reach surface.
5. 3 types of faults, strike slip (sideways movement), normal (land slumps
down), reverse (one side rides up over other side).
6. Seismic waves:
a. P waves, fastest, longitudinal, can pass through solid and liquid
b. S waves, slower, transverse, can only pass through solid
c. Seismographs record seismic waves forming a seismogram.
d. Epicentre can be determined by finding out time lag between P and
S at 3 seisomographs.
e. Richter scale measure energy released, good for small to moderate
earthquakes
Modified Mercalli measures shaking – depends on eye witness
accounts and doesn’t work where there are no buildings or under
the sea.
7. Where a plate is starting to subduct very shallow earthquakes occur
because crust is brittle and breaks easily. As plate gets deeper
earthquakes get deeper.
8. Distribution of deep earthquakes shows how steeply plate is
dipping/subducting. In NI PP dipping towards north-west, in bottom SI
AP dipping towards east.
9. Where 2 continental crusts are colliding as along Alpine Fault, so deep
earthquakes as no plates are subducting.
10.
Shallow earthquakes also found along tectonic boundary, axial fault
and volcanic areas.
11.
The Alpine fault is the boundary between PP and AP continental
crust. Movement in earthquakes is severe, with strong lateral plus
vertical movement. The vertical movement helps raise the Southern Alps.
12.
Liquefaction happens in saturated, loose sandy soils which when
shaken settle so water is squeezed out and strength in soil is lost.
Tsunamis:
A tsunami is a destructive ocean wave that occurs due to any geological event
happening under water such as:
 A strong off shore earthquake
 Volcanic eruptions
 Submarine avalanches or landslides
Stages of a tsunami:

Water is displaced

Waves form that travel very fast and have very long wavelengths.

As they get close to shore first wave slows down and rest of water
bunches up behind.

Amplitude increases and wavelength decreases.

This results in either giant breaking waves or very strong surges of water

They are so destructive because the whole water column is moving, not
just the surface of an ocean as in a storm. E.g. if the sea is 1000m deep,
the wave is 1000m deep.
Earthquake generated tsunamis:
1. Fault line movement, must involve either sudden dropping or rising of
seafloor.
2. Tectonic thrust, the subducting plate suddenly thrusts under so overriding plate rebounds.
Volcano generated tsunamis:
1. Volcanic earthquakes
2. Undersea eruptions
3. Pyroclastic flows, landslides or lahars happening underwater or flowing
into water
4. Caldera collapse
Landslides or submarine avalanches:
New Zealand has steep continental slopes and deep cut canyons offshore that
accumulate sediment. An underwater or land earthquake or just accumulation
of sediment can make the accumulated sediment collapse causing tsunamis.