LEC.172 Geology Flashcards
Define igneous rock with an example
Form by cooling + solidification of molten rock and have quartz/feldspar/mica minerals, granite
Define sedimentary rock with an example
Form by hardening + cementation of layers of sediment deposited at Earth’s surface, sandstone
Define metamorphic rock with an example
Form from pre-existing rocks by transformation in solid state under intense heat + pressure, marble
What are the 4 principles of relative dating?
- Original horizontality
- Superposition (each layer of sedimentary rock in tectonically undisturbed sequence younger than one beneath it)
- Cross-cutting relationships (igneous intrusion must be younger than rocks it intrudes)
- Faunal succession (sedimentary strata contain fossils in a definite sequence)
When do solid solutions form + example?
When cations of similar sizes substitute for each other, olivine is a solid solution of Fe and Mg
Which is the only major silicate group to not show some degree of solid solution?
Quartz
What is natural glass and how is it formed?
Obsidian, formed when lava cools too quickly to form crystals (atoms randomly arranged in silica tetrahedra)
What are the different kinds of crystal structures that a chemical substance can form called + example?
Polymorphs, CaCO3 can form calcite or aragonite, pure C can form graphite or diamond
What are 5 diagnostic features of minerals?
- Habit (shape/morphology)
- Cleavage + fracture
- Specific gravity
- Colour
- Hardness
What are 2 ways of examining thin sections under a microscope?
- PPL (Plain Polarised Light)
- XPL (Cross Polarised Light)
Define magma and lava
Magma = molten rock beneath Earth’s surface
Lava = molten rock at Earth’s surface
What are the 3 types of pyroclast?
- Ash (<2 mm)
- Lapilli (2-64 mm)
- Volcanic bombs (>64 mm)
What are 4 components of ash?
- Pumice/scoria (fragments of vesicular lava)
- Lithics (rock fragments)
- Crystals (individual + broken)
- Glass shards (chilled magma with vesicle walls)
What does vesicular mean?
Gas trapped in melt at time of solidification (bubbles)
Define pyroclastic fall
Sedimentation of particles from a volcanic plume (forms pyroclastic fall deposit)
Define pyroclastic density current
Gravity flow of hot gases/particles which flow down flanks of volcano
Define lahar
Flow of mixture of water (from melting or rainfall) + volcanic material (forms lahar deposit)
Define debris avalanche
Flow of landslide material down flanks of volcano (forms debris avalanche deposit)
Define phreatic eruption
Steam eruption (no deposit)
Define kettle hole
Block of ice in lahar deposit has melted
Define batholiths
Solidified magma chambers
Define plutons
Smaller body of intrusive magma
What is the difference between sills and dykes (igneous intrusions)?
Sills = horizontal
Dykes = vertical
What 4 things are igneous rocks classified by?
- Magma type
- Silica content (% weight)
- Volcanic (<1 mm, fine-grained, extrusive)
- Plutonic (>5 mm, coarse-grained, intrusive)
What are the properties of ultramafic magma?
<45% silica
Plutonic rock: Peridotite
What are the properties of mafic magma?
45-52% silica
Volcanic: Basalt
Plutonic: Gabbro
What are the properties of intermediate magma?
52-65% silica
Volcanic: Andesite
Plutonic: Diorite
What are the properties of felsic magma?
> 65% silica
Volcanic: Rhyolite
Plutonic: Granite
What are the 2 types of chemically similar basaltic lava?
- Aa (blocky)
- Pahoehoe (ropy)
When are lava ‘pillows’ formed?
When basalt erupts into water and rapidly cools
From basalt –> andesite –> rhyolite, how do eruption temp and viscosity change?
Eruption temp decreases, viscosity increases
What is the magma composition of calderas and craters?
Mafic-felsic
What is the difference between a caldera and a crater?
Both topographic depressions, caldera diameter = >1 km, crater diameter = <1 km
How is a caldera formed and what is an example of a caldera?
- Magma fills magma chamber –> volcanic eruption
- Magma chamber partly depleted
- Summit collapses into chamber
E.g. Sollipulli, Chile
What is an example of a crater?
Poas, Costa Rica
What is the magma composition of composite volcanoes (stratovolcanoes) and an example?
Intermediate-felsic
E.g. Mount Rainier, Washington
What is the magma composition of lava domes, why are they dangerous, and an example?
Viscous intermediate-felsic
Slow extrusion of viscous lava so gases build up –> explosive eruption
E.g. Volcan de Colima, Mexico 2009
What is the magma composition of shield volcanoes and an example?
Low viscosity mafic
E.g. Mauna Loa, Hawaii
What are fissures, what viscosity is the lava at fissures, and an example?
Mafic lava rapidly flows away from fissures
Low viscosity
E.g. Kilaeua, Hawaii 1979
What are cinder cones, what is their magma composition, and an example?
Steep/conical/symmetrical formed by explosive eruptions
Mafic pyroclastic material
E.g. SP Crater, USA
What are 3 ways deformation can be studied?
- Field experiments
- Lab experiments
- Satellite observations
What is stress (deformation)?
Pressure
What are the 6 types of stress?
- Compression stress (shortens object)
- Tension stress (lengthens object)
- Normal stress (perpendicular to surface, either compression/tension)
- Shear stress (acts parallel to surface)
- Hydrostatic/uniform stress (uniform in all directions)
- Directed/differential stress (not uniform in all directions)
Define strain
Measure of the extent of deformation (change in shape/volume due to stress)
What do plastic and brittle deformation result in?
Plastic: folding e.g. folded dyke
Brittle: fracturing
What 3 things does stress depend on?
- Temperature and pressure
- Mineral/rock type
- Strain rate
At which temperatures and pressures do minerals/rocks deform most easily at?
Higher temperature and low pressure
Which minerals/rock types fault and fold?
Hard igneous/metamorphic rocks behave as brittle material = faulting, softer sedimentary rocks deform in a ductile manner = folding
How does strain rate affect whether a mineral/rock is faulted or folded?
Rapid deformation = brittle faulting, slow deformation = plastic folding
Define dip
Measure of maximum angular deviation of an inclined plane from the horizontal (perpendicular to strike direction)
What are the 8 types of folding?
- Monocline (step-like)
- Syncline/anticline
- Symmetrical (vertical axial plane + each limb dips at same angle)
- Asymmetrical (inclined axial plane + limbs dip at diff. angles)
- Overturned (1 limb rotated to be upside down)
- Recumbent (axial plane is horizontal)
- Non-plunging and plunging (inclined fold axis)
- Domes and basins
Define joint
Fracture along which little/no movement has occurred, perpendicular to fracture surface
Define fault
Fracture along which blocks on opposite sides of fracture plane have moved
What do GPS and LPS mean in terms of faulting/stress?
GPS = Greatest Principal Stress axis
LPS = Least Principal Stress axis
What are the 3 types of faults?
- Normal (GPS vertical, LPS horizontal)
- Reverse (GPS horizontal, LPS vertical)
- Transcurrent (GPS and LPS horizontal)
What do metamorphic rocks result from and in what state does this take place?
Transformation of other rocks beneath Earth’s surface, solid state
What are the 3 factors affecting transformation into metamorphic rocks?
- Temperature
- Pressure
- Fluid activity
How does temperature affect transformation into metamorphic rocks and what are the metamorphic minerals?
Increasing temp. increases rate that chemical reactions that produce new mineral assemblages, geothermometers
What is the increase in temperature with depth called and how does it vary with plate tectonic setting?
Geothermal gradient, continental plate extension = high, ancient continental plate = low
How does pressure affect transformation into metamorphic rocks, what are the metamorphic minerals, and at what rate does pressure increase at?
Increasing pressure increases rate that chemical reactions produce new mineral assemblages, geobarometers, 0.3-0.4 kilobars/km
What type of metamorphic rocks do uniform and differential stress produce?
Uniform stress: non-foliated
Differential stress: foliated
How does fluid activity affect transformation into metamorphic rocks?
Increase rate of chemical reactions, carry chemical substances, + change rock’s composition via metasomatism
Where does regional metamorphism occur and what does it result from?
Convergent plate boundaries (volcanic mountain belts) + cores of mountain chains formed by continental collision, results from high temp., high pressure, + extreme deformation in Earth’s crust
What increases from low to high grade metamorphic rocks and give examples of rocks from high to low grade?
Temp. and pressure, slate –> phyllite –> schist –> gneiss
What are metamorphic zones?
Zones based on distinctive silicate mineral assemblages
What are index minerals?
Characterise metamorphic rocks and are used to map metamorphic zones
What are the index minerals for clay-rich metamorphic rocks from low to high metamorphic grade?
Chlorite –> biotite –> garnet –> kyanite –> sillimanite
What is an isograd?
Line of equal metamorphic intensity (patterns of isograds follow faults + folds)
What are polymorphs?
The different structures formed when a chemical substance forms more than 1 kind of crystal structure
What are metamorphic facies and what do they provide clues to?
Groups of rocks characterised by specific mineral assemblage formed under the same broad T and P conditions from diff. parent rocks, plate tectonic processes responsible for metamorphism
When does contact metamorphism occur, what does it form, and what do mineral assemblages reflect?
When rock intruded by hot magma (high T and low P zones of metamorphism surround intrusion), hornfels and metamorphic aureoles, decreasing temp. with distance from intrusion
Where does dynamic metamorphism occur and what does it form?
Along fault zones (rocks subjected to high P), mylonites (formed by accumulation of high strain rates in ductile zones)
