Tectonic plates and volcanoes Flashcards
What are the effects of eruptions?
Lava flows Volcanic ash Pyroclastic flows Volcanic collapse Lahars Floods Volcanic gases
What are the four I dependant spheres that the world is divided into?
Atmosphere
Geosphere/lithosphere
Hydrosphere
Biosphere
Give a brief description of the atmosphere
Air that extends up from the Earths surface for about 100 km
Give a brief description of the hydrosphere
Contains all earths solid, liquid and gaseous water eg. As the ocean, rivers and glaciers
Give a brief description of the biosphere
All living organisms, wherever they live
What are the two types of crust?
Oceanic and continental
Where is oceanic crust found and characteristics
Under the ocean
Denser, thinner and younger
Where is continental crust found and characteristics
Is the land and includes the extended continental shelf around New Zealand
Buoyant, thicker, older
Oceanic crust characteristics
Crust depth: thin-5-12km
Age: young rocks, less than 200 million yrs old
Density: more dense
Rock type: mainly basalt
Continental crust characteristics
Crust depth: thick-40km average
Age: older rocks up to 4500 million yrs old
Density: less dense
Rock type: range of granites
Where do volcanic eruptions generally occur in nz?
In Auckland, taranaki and the taupo volcanic zone
Explain why gravity is so important on extreme earth events?
Gravity pulls everything towards the centre of the earth. Gravity causes material to fall, such as ash from a volcano or land after an earthquake shake. If land starts to slide after an earthquake, gravity makes it move even faster
What is magma?
Molten rock that rises from deep beneath the surface of the earth.
What are the types of magma?
Basaltic
Andesitic
Rhyolitic
Characteristics of basaltic magma
Amount of silica: 45-55% Viscosity: runny Temp: 1000-1200 •C Amount of gas: smaller Eruption style: effusive with runny lace eg. Auckland volcanic field
Characteristics of Andesitic magma
Amount of silica: 55-65% Viscosity: sticky Temp: 800-1000 •C Amount of gas: larger Eruption style: explosive eg. Cone volcanoes such as ngauruhoe, ruapehu
Characteristics of rhyolitic magma
Amount of silica: 65-75%
Viscosity: very sticky
Temp: 650-800 •C
Amount of gas: largest
Eruption style: extremely explosive eg. Taupo caldera
Dome- gentle and slow because gas has been released in previous eruptions leaving very little gas in the magma
What are the two types of explosions
Magmatic explosions
Phreatomagmatic eruptions
What are magmatic explosions
Occur when gas is dissolved into the magma. As the magma rises, it forms bubbles and when it reaches the surface the gas bubbles burst into the air. The more viscous the magma is the more gas it can trap and the more gas it can trap the more violent the explosion is.
What are phreatomagmatic eruptions
Occur when hot magma comes up beneath a water source (lake, sea, ground water). The heat of the magma makes the water flash into vapour which then has a volume 10,000 times that of the original liquid water
What are lava flows
Magma without much gas runs slowly covering anything but doesn’t cover large areas
What is volcanic ash
Fine particles carried by wind due to hot expanding then cooling and contracting gas. The ash is abrasive and corrosive
What are pyroclastic flows
Dense clouds of hot fragments and gas close to the ground, travel fast (50-150kmh-1) and hot (100-700•C)
What are volcanic collapses
Happen when rapidly forming volcano suddenly collapses due to instability caused by the pressure of more magma rising, shaking from an earthquake, or heavy rain saturating the volcano
What are lahars
Flows of boulders, ice, water and mud that occur during or after an eruption. Short lived but can be devastating
What are volcanic gases
Consist mainly of CO2, water vapour and sulfur dioxide with traces of other gases. These gases can be toxic or corrosive and dangerous to life
What is tephra
Fragments of material produced after a violent explosion
What are the 3 different classes of size tephra is separated into
Ash- particles smaller than 2mm in diameter
Volcanic cinders- between 2 and 64 mm in diameter
Volcanic bombs- larger than 64mm in diameter
Why can tephra be used for dating eruptions
Each eruption is different which results in the tephra having a unique chemical fingerprint, trace elements and characteristics for a particular eruption. Therefore analysis of the tephra can be used to identify different eruptions
How are subduction volcanoes formed?
Where the edge of the pacific plate subducts under the edge of the Australian plate assisted by gravity. When the pacific plate has dived deeply enough, rock melts, assisted by superheated water from subducted sediments, forming magma, which forces it’s way up through vents in the crust forming volcanoes
Two ways volcanoes are formed in nz?
Subduction eg. Volcanoes of taupo volcanic zone, mt taranaki, kermadec volcanoes
Plume of magma from the mantle in a hot spot eg. Volcanoes in auckland volcanic field
Name the three main types of volcano found in the taupo volcanic zone
Cones or stratovolcanoes eg. White Island, mt ruapehu, mt ngauruhoe
Caldera eg. Taupo, okataina
Domes eg. Parts of mt tarawera, mt maunganui
About cone volcanoes
Made up of Andesitic magma
Erupted lava is semi sticky and cools relatively quickly
Eruptions gentle just lava or explosive of pyroclastic flow of hot ash
Cone is formed when layers of ash and lava build up over time
About caldera volcanoes
Generally erupt rhyolite lava
Eruptions that form calderas create craters
Very violent and explosive eruptions because gas bubbles find it hard to escape sticky magma
The stage of caldera volcanoes
- Gases such as CO2 and water vapour from the melting of subducted limestone in the sediment form bubbles.
- Gas bubbles expand as they rise, filling the magma chamber with gaseous magma and putting pressure on the ground above forming cracks in the ground
- Magma moves up to surface through weakness in the crust and violently explodes due to high amount of gas emptying the chamber
- Weakens the ground causing roof of chamber to collapse forming a giant caldera which fills with water over time forming a lake. Pyroclastic flow sets as ignimbrite rock
About dome volcanoes
Made from rhyolitic magma from which gas has escaped because of previous violent eruption
Gentle eruptions
Forms a steep convex slope from thick, fast cooling lava
Mt ruapehu
Largest active volcano in NZ
A ring plain surrounds the mountain due to lahars, landslides and ash falls
3 summit craters have erupted in the last 10,000 years
The active vent is beneath crater lake which contains varying amounts of warm, acidic water fed by snow melt
Major eruptions have been 50 years apart: 1895, 1945, 1995
Minor eruptions have occurred 60 times since 1945
On 24 dec 1953 a train fell into a river killing 151 people and a bridge was swept away
How do geothermal areas form?
When rainwater or groundwater seeps down through cracks in the rock towards a heat source. The hot water rises as steam or both and emerges at the earths surface. Hot water reacts with the rock it’s comes in contact with and becomes enriched with dissolved minerals
What are the features of geothermal areas
Geysers Fumeroles Mud pools Boiling springs Sinter deposits Gases
What are geysers
Hot springs that eject jets of boiling water and steam into the air
Fumeroles
Steam and gas vents
Mud pools
Form where steam and gas rise to the surface under rainwater ponds
Boiling springs
Occur where geothermal fluids rise quickly to the surface
Sinter deposits
Are features of boiling springs, composed of almost pure silica but often containing traces of impurities or micro organisms which produce beautiful colours
Gases
Carbon dioxide and hydrogen sulfide are ground hugging gases that are heavier than air, and can accumulate in dangerous concentrations. The most common death traps are excavations in geothermal ground, or poorly ventilated bathing pools
Give a brief description of geosphere
The land which includes all minerals, rocks, sediments and soils
Where is the tectonic plate boundary in nz
NI is on the AP. The tectonic plate boundary cuts through SI, starting at kaikoura leaving Milford sound. The west coasts and nelson area on AP the rest of SI on PP. Also includes the kermadec trench to the north and pusegur trench to the south
Pacific plate and crust type
PP made up of OC to north of nz and east coast of NI. At kaikoura PP enters SI changes to CC
Australian plate and crust type
AP made up of CC in NIand most of SI. At Milford sound AP leaves SI becomes OC
What shape are volcanoes with basalt magma
Shield volcanoes, cinder cones, tuff rings
What shape are volcanoes with andesite magma
Cone volcanoes
What shape are volcanoes with rhyolitic magma
Caldera and dome volcanoes
What magma is erupted from subduction volcanoes
Andesitic or rhyolitic
What shape are subduction volcanoes
Cones, calderas and dome
Examples of cone volcanoes
Mt ruapehu, White Island, ngauruhoe, tongariro, taranaki
Examples of dome volcanoes
Mt tawarewa and mt maunganui
How are hot spot volcanoes formed
From a plume of molten magma that starts 100 km under Auckland. Basalt magma rises up through the crust at about 5km per hour forming a new volcanic in the auckland volcanic field
How geosphere causes volcanic eruptions
Oceanic crust of pacific plate subducts under the continental crust of the Australian plate
How does the hydrosphere cause volcanic eruptions
Wet sediments from the bottom of the ocean are subducted and the water from these becomes superheated. This lowers the melting point of the rock around it which forms magma
How does the biosphere cause volcanic eruptions
Carbonate rock which had been partially made from the compressed remains of plankton is melted underground releasing CO2
How does the atmosphere cause volcanic eruptions
Gases are released from magma as it rises through the crust. Ash is also erupted and both can rise tens of kms into the air
