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