Y12 Geology Flashcards
Earth structure
Inner core - solid (Fe, Ni)
Outer core - liquid molten metal
Mantle - mainly silica and minerals
Crust - solid top layer
Lithosphere
Rigid solid surface layer, made up of the crust and the top layer of the mantle.
Broken into tectonic plates with
Asthenosphere
Viscous plastic-like layer of semi molten rock
Lithosphere floats on the asthenosphere
Convection current process
Outer core heats mantle
Less dense magma rises
Reaches asthenosphere, pushed sideways by more rising magma
Pulls tectonic plates with it
Magma cools, denser, sinks —> convection currents
NZ tectonics
NZ sits on the boundaries of the Indo-Australian and the Pacific plates
Indo-Aus pushed NNE
Pacific pushed WNW
Diverging plate process
Plates spread apart due to convection currents
Magma reaches surface, cools
New ocean crust formed - basaltic, forms a mid Ocean ridge
NZ plate tectonics
Converging by about 4cm per year
Builds up stress within the lithosphere
Describe continental crust
Less dense (granite)
Thicker - 40km
Mainly above sea level
Describe oceanic crust
Denser - basalt (high in Fe)
Thinner - 5-10km
Mainly below sea level
Detailed tectonics of NZ
North of NI: Kermadec trench
Denser oceanic pacific subducts under continental Aus plate
Hikurangi fault to the east of NI, joins Kermadec trench
Converging boundary causes the softer AP to fold, creating mountain ranges in Eastern NI
Alpine fault across SI, transform movement between continental crusts, creates southern Alps
Puyseger trench: south of SI
Oceanic AP subducts continental PP
Softer AP folds, creating underwater mountain range
Volcano structure
Includes what is inside the cone and outside
Magma chamber,
Main vent up to a crater,
May be a second vent and secondary cone
Volcanic flow,
Ash cloud,
Volcanic bombs,
Pyroclastic flow
Volcano builds up layers of lava and ash over successive eruptions
Volcanic eruption process
Magma is buoyant, rises through the crust to erupt at the surface.
Large amount of gas + high viscosity magma = explosive eruption
Small amounts of gas a/o low viscosity magma = effusive eruption
Eruption column
Material being erupted upwards
Tephra and tephra plume
Erupted material of different sizes
Tephra plume = billowing clouds
Volcanic bombs
Large fragments
6cm +
Lahar
Mixture of erupted material, mud and water
Pyroclastic flow
columns of tephra that fall and spread at high speeds close to the ground, with devastating destruction
Ignimbrite is a pumice-dominated pyroclastic flow ejected from an explosive eruption
Can weld into rock
Can be found hundreds of kilometres from site of eruption
Where are volcanoes commonly located?
95% are found near converging plate boundaries (subduction zones)
5% are hotspots
80% are found along the subduction zone of the Pacific Ring of Fire
Phreatic eruption
Rising magma containing gas comes into contact with groundwater
Created superheated compressed steam, erupts explosively
Types of deposits from volcanoes
Air fall:
Material blankets the surface like snow
Pyroclastic flows:
Tend to follow low-lying areas of topography, filling valleys rather than a uniform blanket
Surges:
Tend to hug the ground, producing thicker deposits in valleys and thinner deposits over ridges (non uniform)
Types of magma
Basaltic:
Mantle magma
Low silica, low viscosity, low gas
Above 1200C to be molten
No crystallisation
Hotter hence runnier
Forms flatter volcanoes
Andesitic:
Mainly mantle magma with some partial melting or mixing
Intermediate silica, viscosity and gas
880-1000 degrees C
Flowing and moderately explosive
Rhyolitic:
Partial melting, more crustal material present, or fractional crystallisation removing Fe
High silica, viscosity and gas
750-850 C
Cooler hence more sticky, traps gas
Very explosive as heated trapped gas expands rapidly when it escapes the pressure of the earth
Eruption products of different types of magma
4 main types of rocks formed, classified by silica content
Basalt: thin flows hence forms shields, scoria cones and craters
Heavy, basic, mafic
Andesite: thick, rubbly lava flows.
Scoria/pumice and ash fall
(Dacite and) Rhyolite:
Pumice and ash fall
Ignimbrite (pyroclastic flow)
Light, acidic, Felsic
Basalt iron rich, less silica, hence dark coloured
Gradient to rhyolite, iron poor, more silica, light coloured
How is silica related to properties of lava and rocks
Higher silica, lower temp, more viscous, decreasing mobility of lava
Traps more gas, hence more explosive
High silica = acidic
Low silica = basic
Mafic and Felsic rocks
Mafic = MAgnesium and FErriC
Darker in colour
Basalt
Felsic: FELdspar and Silicates
Lighter elements (Al, K)
Lighter in colour
Granite and Rhyolite
Hot spot volcanos definition and process
An area of the mantle where a continuous plume of hot magma rises from the same spot being heated by the core.
Pressure builds up, crust cracks, magma forced to surface.
Plates move, chain of islands created
Auckland volcanic field is a hotspot
Factors that magma type depends on
Initial composition
(Mantle magma or subduction plate material)
Partial melting
Melting at lower temperatures due to water released from the subduction plate
Mixing
Magma melts crust, mixes
Fractional crystallisation
Magma pools in a chamber, minerals high in Fe crystallise and fall out
Reasons why volcanos are formed near subduction zones
Subjecting plate dives, assisted by GRAVITY
Stress on overriding plate - fault lines, areas of weakness
Convection currents cause spreading of the plate at the back arc zone
Melting and mixing of material causing magma to rise and putting pressure of crust
Shield volcanos
Relatively thin lava flows built up over central vent
Mostly balsatic magma
Hence relatively non-explosive eruptions
Roughly circular / oval
Composite volcanos / stratovolcanoes
Most common
Mainly andesitic magma, typically felsic hence high viscosity
Inter layering of lava flows and pyroclastic layers
Intermediate steepness (steep near the top and flatter at the base due to erosion)
Intermittent eruptions but more explosive than shield volcanoes
Scoria cones
Small, steep sided, with deep central craters
Gas-rich lava, ejected in fire fountains
Cools quickly forming brittle rough rock with gas holes
Scoria is high in iron, dark red colour due to oxidation
Caldera volcano
Formed after a massive explosive eruption that empties the magma chamber
Rhyolitic magma (high in gas)
Remaining volcano walls collapse, forming huge crater
Dome volcano
Formed from degassed rhyolitic magma
After violent eruptions, remaining chamber has no gas, magma no longer explosive
High silica rhyolite (high viscosity) so it cools very quickly without flowing far
Steep convex slope
Lava plateau
When lava erupts from long cracks or fissures, spreads out evenly
What is the Taupo volcanic zone and how was it formed?
TVZ is an extensional feature called a back arc basin, west of the subduction zone
extends from Mt Ruapehu to White Island
Formed due to the subduction of the PP
Has most of NZ’s active volcanoes
Back arc system
Secondary convection current is set up behind the subduction zone, resulting in a spreading zone
BOP area is pulled apart by the back arc - many areas below sea level
Back arc has created a 50km by 160km pool under the crust, reservoir for future eruptions
TVZ volcanoes
Stratovolcanoes: ruapehu, White Island and Ngaruhoe
Ruapehu:
Crater lake above main vent
Fills with melted snow
Has ruptured at times to send lahars (Tangiwai)
Caldera: lake Taupo
How are geothermal areas created
- WATER
- DEEP HEAT SOURCE (magma)
Water (rain and groundwater) in porous rock come into contact with magma
Hot water less dense
Rises to the surface
Hydrogen sulfide gives rotten egg smell
Define and explain geyser
Jets of hot water and steam
Magma chamber heat radiates upwards into surrounding rock
Water underground through fractures in rock
Water reaches hot rock, try to rise back to surface
Overlying cold water keeps pressure on
Causes superheating, until pressure pushes overlying water out of the hole and steam expands rapidly
During this process, hot water dissolved silica, carries it upward to line the fractures, creating a sealed system = plumbing system
Fumerole
Jets of steam
Three ways earthquakes are created
Volcanic activity - pressure of rising magma
Tectonic plate movement -
Plates stick together, stress builds, released as earthquakes
Landslide
Define:
Focus
Epicentre
Seismic waves
After shocks
Where the earthquake originated
Epicentre: point directly above focus
Seismic waves: vibration of energy passing through the ground
After shocks: further sudden movements in earth’s crust causing seismic waves
How does earthquake depth depend on location?
Shallow -
occur where PP is subducting
Middle of NI due to volcanic activity
Along alpine fault due to lateral mvt
Deep -
Further away from subduction zone on the overriding plate (NI)
Describe and explain the Benioff Zone
The planar zone of earthquakes produced by an interaction between subducting oceanic plate and continental
Upper part (close to subduction zone)
Rocks sliding past each other, shallow earthquakes
Lower part (under the overriding plate)
Oceanic plate semi-solid, blobs fall off, rocks shift - deep earthquakes
Types of seismic waves
Body waves and surface waves
Body waves: travel through earth
P waves - primary. Arrive first. Compressions (longitudinal waves)
S waves - secondary. Transverse.
Surface: move along surface, cause the most damage
Love wave - side to side
Rayleigh - circular motion
What are faults and why do they occur
Breaks/fractures in earth’s crust after movement
Occur as a result of the release of stress
Faults occur along fault lines as they are the weakest areas in the crust which stress can be released
Syncline and Anticline
Describes folds based on relative ages of rock layers.
Syncline: trough of a curve of a fold.
Youngest rock at the centre axis
Anticline: crest of a curve of a fold. Oldest rock at centre axis
Lateral fault
Also known as a ‘strike-slip fault’
Sideways movement at a plate boundary
AP moves NE relative
PP moves SW
Sinistral - LEFT side moves
Dextral - RIGHT side moves. Most in NZ
Normal fault
Crust material pulled apart
Land slumps down and away
Exposes a scarp face
Eg. TVZ
Reverse fault
Crust material pushes together
Land climbs up and over
Exposed surface = scarp
Eg. Marlborough fault, NI fault
Major fault zones in nz
NI: NI fault system
Runs from Wellington to BOP
Includes Taupo, Tarawera and Rotorua
Responsible for most of NZ’s volcanic activity
SI:
Marlborough fault system
Alpine fault
Macquarie fault - transition from lateral (strike slip) to subduction near the Puysegur Trench
Events causing a tsunami
Waves generated when a large body of water is displaced
Meteor impact
Landslides (submarine or into water)
Submarine volcano eruption
Underwater earthquake
Continental slope avalanche
Continental shelf avalanche
At the edge of a continental shelf there is a continental slope to the ocean floor.
Sediment deposited at top of slope, periodically plunges down, causing tsunami
What factors determine the speed and wavelength of a tsunami
Depth of ocean and size of earthquake
Deeper - when pulse reaches surface, wave front is so wide it is unnoticeable
Shallow - front of wave slows due to friction with ocean floor, shorter wavelength
Amplitude increases as water compressed, but wave slows (KE to GPE)
Greater force of earthquake —> greater amplitude of water displacement, hence faster wave
