Igneous Rocks and Processes Flashcards
Define Euhedral
Describes a complete crystal, has the perfect crystal shape
Define Subhedral
Describes a partially complete crystal
Define Anhedral
Describes a crystal with no crystal shape
Define Porphyritic
Crystals have different sizes
Define Equicrystalline
Crystals are the same size
Define Groundmass
Small crystals between phenocrysts in porphyritic rock
Define Phenocryst
The big crystal in a porphyritic rock
Define Vesicles
Trapped gases in the rock creates hole/cavities
Define Amygdales
Vesicles which have been filled in with crystals
Define Crystal Alignment
Crystals are aligned
Define Random Orientation
Crystals are not aligned
Define Flow Banded
Form lines
What is the equation to calculate the degree of magnification?
Size of image/Size of real object
What are two trends of oceanic crust?
Trend 1 - the crust gets older the further you move from the ridge
Trend 2 - oceanic lithosphere gets thicker the further you move from the ridge
How do pillow lavas form?
When lava is erupted into water it causes the lava to cool and crystallise very quickly. This causes it to form a pillow shape and the edges of the pillows will be glassy
What are six features of a pillow lava?
Rounded top, Pointed keel, Fine grained basalt, Vesicles, Glassy rind, 1m tall
How can pillow lavas be used as a way up indicator?
The pillow has a rounded top and pointed bottom. If the keel can be seen it is the wrong way round
What are the two ways of describing the tops of basalt lava flows (flow tops)?
A’a and Pahoehoe
Define A’a
Has a rubbly brittle top due to having high viscosity and cooling quickly
Define Pahoehoe
Smooth and ropy due to having a lower viscosity and cooling slower
What are three features of an A’a flow top?
Thick flow units (2-10m), High flow-front velocity, Forms large channels
What are three features of a Pahoehoe flow top?
Thin flow units (0.2-2m), Low flow-front velocity, Forms lava tubes
How do columnar joints form?
When an igneous body cools and crystallises the unit contracts inwards, creating cracks arranged in a polygonal shape and columns of rock
What two heat transfer processes are involved in the creation of columnar joints?
Conduction and Radiation
Define Intrusive
Inside the crust
Define Extrusive
Outside the crust
Define Concordant
Follows bedding planes
Define Discordant
Cuts across bedding planes
Define Chilled Margin
Inside the igneous body where the magma has cooled very quickly against the country rock
Define Baked Margin
Outside the igneous body where the magma has heated up the country rock, causing it to become harder and more crystalline
What are the four types of igneous body?
Lava flow, Sill, Dyke, Pluton
Define Lava Flow
An extrusive, concordant body with sheet like morphology, two chilled margins and one baked margin (can have a reddened top due to oxidation)
Define Sill
An intrusive, concordant body with sheet like morphology, two chilled margins and two baked margins
Define Transgressive Sill
Intruding along bedding planes and then cutting up into higher layers
Define Dyke
An intrusive, discordant body with sheet like morphology, two chilled margins and two baked margins
Define Pluton
An intrusive, discordant body with a circular morphology, one baked margin and one chilled margin
What is the difference in thickness between a sill and a lava flow?
Sills are usually thicker than lava flows
What is the difference in baked margins between a sill and a lava flow?
Sills have two baked margins whereas lava flows only have one
What is the difference in chilled margin between a sill and a lava flow?
Sills have two chilled margins whereas lava flows only have one (though the top may be chilled or made of scoria)
What is the difference in crystal size between a sill and a lava flow?
Sills are medium sized crystals (as they are hypabyssal and cool slowly) whereas lava flows are fine sized crystals (as they are volcanic and cool quickly)
What is the difference in arrangement of crystals between a sill and a lava flow?
Crystals are not arranged in a sill, whereas the movement of lava does cause them to be aligned
What is the difference in weathering of the upper surface between a sill and a lava flow?
No weathering of a sill whereas oxydation can chemically weather a lava flow, making it have a reddened top
What is the difference in included fragments between a sill and a lava flow?
Sills can have xenoliths as it rips up material above and below, whereas no xenoliths are found from above in a lava flow (though could come from below)
Define Absolute Dating
A method of giving a material/rock a numerical age
Define Isotope
An element with the same number of electrons and protons, but different number of neutrons
Define Half-Life
The time taken for the number of unstable parent nuclei to decay to stable daughter elements
Define Parent Isotopes
The radioactive/unstable isotope
Define Daughter Isotopes
The product of radioactive decay/stable isotope
What can a mass spectrometer measure?
The abundance of different isotopes within a whole rock or an individual mineral
What will scientist compare to determine the absolute age of a sample?
The abundance of parent isotopes compared to daughter isotopes
What is the half life for 40K-40Ar (Potassium-Argon)?
1,250 million years
What is the half life for 87Rb-87Sr (Rubidium-Strontium)?
48,800 million years
What is the half life for 147Sm-143Nd (Samarium-Neodymium)?
106,000 million years
What is the half life of 283Ub-206Pb (Uranium-Lead)?
4,470 million years
What are three problems associated with radiometric dating of igneous rocks?
The older the rock the harder it is to detect and count the amount of parent isotope, Small/shallow intrusions cool and crystallise quickly meaning the closure temperature is reached rapidly and the rocks becomes a closed system, Weathering of igneous rocks may cause the soluble element to be removed leading to false ages
What volcanic landforms are associated with basaltic lava?
Shield shape volcanoes - low angle (5-10 degrees) and up to 9km high
What volcanic landforms are associated with andesitic lava?
Stratocone shape - steep angle (30 degrees) but up to 3km high
What volcanic landforms are associated with rhyolitic lava?
Stratocone or caldera shaped - large crater
What are three factors controlling the type of volcanic activity?
The viscosity in terms of % silica content, The volatile content (carbon dioxide, sulphur dioxide, water), The quantity of mass (amount of rock ejected)
What three things is viscosity affected by?
Silica content, Temperature of the magma, Volatile content
What does the Volcanic Explosivity Index provide?
A relative measure of the explosiveness of volcanic eruptions
What is VEI based on?
Observation e.g. volume of products and eruption cloud height
What scale does VEI use?
Logarithmic - each point on the scale represents a ten-fold increase in the observed ejecta
What are six types of volcanic hazard?
Lava flow, Pyroclastic flow, Air-fall tephra, Volcanic gases, Lahars, Blast (lateral blast)
Describe Lava Flow
Can be any composition or speed, damages property more than life, usually can be outrun
Describe Pyroclastic Flow
Very dangerous due to unpredictability, usually restricted (but not always) to valleys, high mortality rate and extensive damage
Describe Air-Fall Tephra
Formed from rock fragments during an explosive eruption, fall from eruption cloud and can settle anywhere, causes buildings to collapse, contaminates water, creates fertile land
Describe Volcanic Gases
From any volcano, from the crater or fumeroles, can cause climate changes, asphyxiates animals, no damage to property, can contaminate groundwater supply
Describe Lahars
Mixture of rocks, ash and water, sets like concrete, flows down pre-existing valleys, destroys buildings and farmland, travels further than pyroclastic flow
Describe Blast
Due to build up and sudden release of pressure, triggers can be landslide or dome collapse, can be very dangerous depending on proximity
What are five monitoring techniques?
Ground deformation, Gas emissions, Seismic activity, Thermal anomalies, Gravity anomalies
How can ground deformation be used to monitor volcanoes?
Measure the angle of tilt of the land - increasing tilt could indicate inflation of a magma source
How can gas emissions be used to monitor volcanoes?
Changes in natural pattern could suggest magma moving to surface - prior to eruption gas levels should erupt unless the vent is blocked
How can seismic activity be used to monitor volcanoes?
Detects low magnitude earthquakes cause by the movement of magma towards the surface
How can thermal anomalies be used to monitor volcanoes?
As magma rises to the surface, the ground will get hotter
How can gravity anomalies be used to monitor volcanoes?
Dense rocks will increase the local gravity field and less dense rocks will reduce it - look for changes in normal value/spring length
Define Hazard Maps
A map that shows the potential hazards of a volcano, by looking at previous deposits and their extent
Who are hazard maps used by?
Town planners when considering where to build key buildings e.g. hospitals, schools
How do lahar channels manage/control volcanic hazards?
Changes and controls the direction of the lahar, and to remove larger debris to lessen the impact
How does strengthening rood manage/control volcanic hazards?
Stops them collapsing, to protect the building and the people in it
What are four ways of controlling lava flows?
Spray lava with water, Build a dam/wall, Drop concrete blocks into lava to cool and slow it, Explosives to change the direction of flow
What are four benefits of volcanoes?
Agriculture (fertile land due to ash), Tourism, Geothermal energy, Building products (e.g. gravel)
Define Isopach Maps
Maps that show the thickness of deposits
What can Isopach maps be used to determine?
Where the volcanic vent was and if the deposit was influenced by other external factors
