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Why is material in the mantle semi-solid instead of completely fluid?
Because of the great pressure exerted upon it
What is the difference between extrusive and intrusive volcanic features?
- Extrusive - when lava and other materials reach the surface
- Intrusive - materials injected into the crust, may later be exposed at the surface by erosion of the overlying rock
What is magma?
Mixture of molten rock and gases, which may be viscous or basaltic
What does viscous (lava) mean?
Sticky, less runny
If something has a low viscosity then it is more runny
What 3 factors affect viscosity?
- 1. Temperature (the higher the temp, the less viscous it becomes)
- 2. Dissolved Gasses (more gas = less viscous)
- 3. Silica levels (low silica = less viscous)
What is the chemical composition of magma that has a high silica content?
What is the chemical composition of magma that has a low silica content?
Do basaltic/basic lavas produce explosive or nonexplosive eruptions?
Do andesitic or acid lavas produce explosive or nonexplosive eruptions?
Where is basaltic lava usually found?
- Constructive plate boundaries - originates from upward movement of mantle material
- Also found in hotspots and more developed rift systems
Where is andesitic lava usually found?
Destructive plate boundaries where the crust is being destroyed
Where is rhyolitic lava usually found?
- Collision plate boundary
- Destructive plate boundary
What are the characteristics of basaltic lava?
- Low viscosity (flows easily)
- Gas can easily escape
- Non-violent eruptions
- Frequent eruptions
- Long eruption
- 1200 degrees celsius
- Flows long distances
What are the characteristics of andesitic lava?
- High viscosity
- Often form blockages in vents
- Gas cannot escape as easily and pressure builds in volcano
- Violent eruptions
- Infrequent eruptions
- Short eruptions
- 800 degrees celsius
- Flows short distance, solidifies quickly
What are the characteristics of an acid/lava dome volcano?
- Dome shape
- Steep convex sides
- Narrow base, high cone
- May have secondary cone
What are the characteristics of a shield volcano?
- Gentle sloping cone
- Wide base, not high
- Made up of layers of basaltic lava
What are the characteristics of a cinder cone volcano?
- Slightly concave sides
- made of successive layers of ash and cinders
What are the characteristics of a composite cone/ stratovolcano?
- steep and symmetrical sides
- Made of alternating layers of acidic lava and ash
- Often has parasitic/secondary cone
What are the characteristics of a caldera?
- Steep walled
- Basin shaped depression
- Lake inside crater
- may have new volcanic cones growing inside
How are acid/lava dome volcanoes formed?
- Acidic and andesitic lava cools and solidifies near source which leads to dome appearance. Vent plugging, as lava is very viscous, may lead to formation of secondary vent on side of volcano which is a release of built up gasses that can be explosive.
- e.g. Puy region in France
How are shield volcanoes formed?
- Basaltic, low-viscosity lava flows and cools slowly as sheets of volcanic rock which build up the sides of volcano, over successive small and mild eruptions.
- e.g. Mauna Loa, Hawaii
How are cinder cone volcanoes formed?
- Steep sides where largest material falls and finer material is carried away to form gently sloping base.
- e.g. Paricutin, Mexico
How are composite cone/stratovolcanoes formed?
Alternating layers of asha and andesitic lava build up steep sides when the passage of magma becomes blocked, pressure builds and magma may flow from fissures on the volcanic flank, forming secondary cones. Produces both lahars and a nuée ardente
What is a lahar?
- Volcanic mudflows carrying material.
- Ash and debris mixes with water and moves rapidly over long distances - usually along river valleys
What is a nuée ardente?
A gas and ash cloud, which can move up to 200km/h
How are calderas formed?
Due to highly explosive volcanic activity. Magma enters chamber and gradually empties during explosion until there is no magma left to support the roof of the chamber. The mountain collapses and creates a caldera - a bowl shaped depression which fills with water and forms a lake or new magma
How is a lava plateaux formed?
- Formed during fissure eruptions, where the basaltic lava (known as flood basalts) pour out of cracks in the surface rather than from a central vent. They are layered structures because lava accumulates over older lava flows forming a broad plateaux rather than a mountain.
- Rivers often cut across the plateaux, forming gorges.
Where are acid/lava dome volcanoes usually found?
Destructive plate margin
Where are shield volcanoes usually found?
Constructive margins and hotspots
Where are cinder cone volcanoes usually found?
Near shield volcanoes on constructive margins or hotspots
Where are composite volcanoes usually found?
Where are calderas usually found?
Where are lava plateaus usually found?
List the seven different volcano types in order from most basaltic lava to most rhyolitic lava
What is a fumarole/solfatara?
- Both steam and gas vents which are common on the flanks of active volcanoes but are also found in geothermal regions, where the temperatures are generally close to the boiling point of water.
- In some cases they are hidden in the ground and can be broken into.
- The gases are dangerous as they are a sign of active volcanism
How are fumaroles/solfataras formed?
Gas escaping through tiny cracks or long fissures, usually in clusters and on the surface of lava flows and thick deposits of pyroclastic flows. They may persist for decades and disappear within weeks if they occur atop a fresh volcanic deposit that quickly cools
What are geysers?
Intermittent turbulent discharge of superheated water ejected and accompanied by a vapour phase. Commonly occur in clusters called geyser fields
How are geysers formed?
- Porous rock allows for easy access to groundwater.
- Water is heated by rock that sits above magma.
- As water is heated by the rock, it rises upwards within fractures in the overlying rock.
- Water above rises to surface to form a hot spring but is cooler than the water within the chamber.
- Its weight above the chamber increases the pressure within the chamber,
- This pressure increases the boiling point and allows intense heat to develop. Eventually the water within the chamber rapidly turns to steam and erupts upwards through the hot spring
- on the surface a tall eruption of steam and water is observed
- A period of recharge is required where water returns back into the pool and also recharges the chamber
What are hot springs/boiling mud?
- Hot springs are similar to geysers, in that they require a heat source and a supply of groundwater.
- Boiling mud spots are iconic in New Zealand. They are large pools of bubbling mixed earth and water.
How are hot springs/boiling mud spots formed?
- Hot spring: groundwater percolates deep into the crust, making contact with rock. This heats the water, causing it to rise upwards within fractures in the rock until it forms a pool on the surface.
- Some hot springs are gaseous and release hydrogen sulphide
- Boiling mud: Steam and gas tise to the surface under rainwater ponds. Acidic gases attack surface rocks, forming clay. This clay mixes with water to form a muddy, steam heated mud pool.
- Rainfall affects the appearance of mud pools. In dry conditions the mud is thick, when it is raining the mud is more fluid
What are the seven features of intrusive volcanic activity?
- Volcanic plug
What is a batholith?
- Largest type of intrusion, can reach 100s km across
- Has thick and viscous lava, doesn't flow easily
- Typical rock type is granite, which contains a high proportion of silica. Granite cools slowly and forms large rock types
How is a batholith formed?
- A vast amount of magma cools over time in layers
- Heat given out by a cooling batholith cooks the country rocks in a band all around the intrusion (metamorphic aureole) which changes their composition and they become metamorphic rock due to the process of contact metamorphism. This can result in the rocks becoming harder and more brittle.
What is a pluton?
- Igneous body of usually less than 100km²
- Bigger than a sill or dyke
- Can be individual or joined together (several plutons form a batholith.
What is a Lopolith?
- Saucer shaped intrusions
- Usually concordant with the country rock
- Lenticular in shape, depressed central region
- Formed by magma with a low silica content
How is a lopolith formed?
Magma is forced between laters of sedimentary rock so it is parallel to these layers. Shape is caused by crystal sagging. Made of up several layers, formed by different phases of intrusion. Beneath a lopolith, there will be a vertical 'funnel' along which the magma travels
What is a laccolith?
- Usually concordant, dome upward intrusion.
- Land above also domes
- Formed close to the surface, similar to sills
- may force a feeder dyke beneath (a funnel for magma to travel through)
- Formed from high viscosity magma, which causes a buildup of pressure
How are laccoliths formed?
Magma being intruded is under very high pressure and forces country rock above to dome
What is a dyke?
A discordant intrusion into sedimentary rock, cutting through layers of sedimentary rock
How are dykes formed?
Magma is forced along cracks or weaknesses in rock, or when the force of the intruding magma opens up new cracks in the rock
What is a sill?
- Parallel or concordant to the layers in country rock
- Vary in thickness from 1cm to hundreds of metres
- Can cause contact metamorphism
- Thinner than batholiths, so less heat & less country rock affected
How is a sill formed?
Magma forced along between layers of sedimentary rock
Define: country rock
The original rock of an area, into which magma has intruded
Define: metamorphic rock
Rock which has had its composition changed
Define: contact metamorphism
Change which occurs due to contact of material/substances
What is a volcanic plug?
A closed vent due to magma cooling and solidifying or not flowing easily and so the vent becomes blocked.
What are the primary impacts of a volcanic event?
- Lava flows
- pyroclastic flows
- volcanic gases
What are the secondary impacts of a volcanic event?
- Climate change
- Volcanic landslide
What is the difference between a volcanic hazard and a volcanic effect?
Volcanic hazards have an impact upon people/the built environment whereas volcanic effects do not
What are lava flows?
- Fissure eruptions of basaltic magma
- Can reach speeds of 50km/h can spread tens of kilometres
- Rare events
- e.g. Kilauea in Hawaii -erupting since 1983
What are pyroclastic flows? (same as neuée ardents)
- Mixture of hot rock fragments, lava particles and ash buoyed up by hot gases
- Moves at up to 40km/h
- Material is denser than surrounding air, so moves close to ground
- e.g. Mount Pelée, 1902 - killed all but 2 of 29,000 inhabitants
What is ash/tephra fall?
- Volcanic ash = <2mm diameter
- Tephra = >2mm diameter
- Form when magma is fragmented or when solid rocks are broken up by water turning into steam
- Ash is carried high into the atmosphere and by prevailing winds.
- e.g. Mt St Helens, Washington state USA
What are the hazards of ash/tephra fall?
- Breathing difficulties
- Building collapse
- Agriculture reduced - animals may eat or die
What are volcanic gases?
- Active volcanoes produce large amounts of water vapour, carbon dioxide, sulphur dioxide, chlorine, hydrogen sulphide, hydrogen, helium, carbon monoxide and hydrogen chloride
- but rarely reach toxic levels
Why are volcanic gases dangerous?
They are heavier than air and collect in depressions where concentrations can become high enough to cause suffocation
What are volcanic landslides?
- Gravity driven slides of masses of rock and loose volcanic material which occurs as a result of heavy rainfall or earthquakes
- ground deformations of volcanic slopes by rising magma can also cause landslides
What are tsunamis?
- Giant waves
- secondary effect of volcanic eruption, especially caldera events
What are the four main responses to a volcanic hazard?
- Do nothing
- Move to a safer location
- Attempt to prevent the hazard
- Adapt lifestyle to hazard
What is the hazard management cycle?
What factors does the response to a volcanic hazard depend on?
- The nature of the hazard
- The frequency of hazard
- Magnitude of hazard
- Population pressure
- Level of economic development
What are the benefits of volcanicity? (4)
- Lava flows can create new land
- Hot rocks allow geothermal power (eg in Iceland)
- Igneous rocks contain valuable mineral deposits
- Volcanic sulphur is used in manufacturing processes including pharmaceutical
- Igneous rocks (eg granite) make good building materials
- Forms points of interest - tourism
What are the hazards of volcanicity? (4)
- Lava flows burn and bury crops
- Can cause tsunamis
- Ash fall may ruin crops or disrupt transport
- Violent eruptions lead to pyroclastic flows which destroy life
- Volcanic dust absorbs solar energy and so lowers atmospheric pressures
What is the Volcanic explosivity index (VEI)?
An open ended chart used to describe the scale of a volcanic eruption. Each value, from 0 to 8, represents an eruption ten times larger than the previous one (logarithmic)
what is a mantle plume?
- Long lived areas of high levels of heat flow within the mantle.
- Consists of an upwelling long thin conduit and bulbous head and produces huge volumes of magma
What is a conduit?
A channel for conveying water or other liquid
What does low viscosity mean?
Not very thick, semi fluid in consistency. Low magnitude of internal friction
What are lava lakes?
- Rare. Form when lava collects in craters and are a sign that volcanoes are active but not erupting. There is enough magma to create fill a crater floor but not enough to create a flow which extends from the crater.
- Large gas bubbles form in the lake
What is vog?
- Gas carried by the wind.
- Once it reacts with the air, it creates sulphur dioxide.
- May lead to respiratory problems and may damage crops and contaminate water
What is laze?
Formed when lava reaches the sea, as often happens in Hawaii, creating steam full of hydrochloric acid