Midterm 2 Flashcards
Veins
Veins are mineral deposits within
rock fractures in the country rock
that come from the magma
Pegmatites
Pegmatites are extremely coarse-grained veins that cut across finer-grained country rock • Happens when there is slow cooling of magma extra rich in water (dissolved in the magma) • The water allows elements to rapidly diffuse (move through) the magma to add to crystals so they grow very large -almost always felsic
Parts of a volcano
- Magma Chamber
- Flank Eruption
- Central Vent
- Crater
- Lava Flows
- Volcanic Debris
• Three main compositions of lava and
corresponding volcanic igneous rocks:
• Three main compositions of lava and corresponding volcanic igneous rocks: • Basaltic lava / basalt • Andesitic lava / andesite • Rhyolitic lava / rhyolite
Magma viscosity is controlled by 3
• Magma viscosity is controlled by temperature, composition,and gas content
• The higher the temperature of a magma or lava, the less
viscous it is e.g., as you heat up honey, it gets runnier – less
viscous
• Felsic lava has _____ silica content
and is more viscous than basaltic lava
higher
Basaltic Eruptions are the
Hawaiien Style, 10-100 m output, effusive
Pahoehoe vs Aa
Pahoehoe • Thin, glassy layer forms at surface of fluid lava • Layer is twisted and coiled as underlying lava is transported
• Aa • Lava degasses and forms bubbles, and becomes more viscous as it cools so the bubbles are trapped in the lava • Flows more slowly and solid layer breaks into rough, jagged blocks
pillow basalts/lava
• pillow-like blocks of basalt form when basaltic lava erupts under water • Outer skin of the pillow basalt cools fast and inner lava cools more slowly – outer skin is glassy, inner rock is crystalline
Andesitic Lavas
• Produced in volcanoes above subduction zones • Intermediate silica content • Lower temperatures • More viscous • Flow more slowly • Can produce explosive eruptions with large ash plumes – Vulcanian or Plinian eruptions • E.g., Mount St. Helens, erupted in 1980 (a Plinian eruption)
Rhyolitic Lavas
• Magma produced when large volumes of continental crust
are melted
• Highly viscous, can erupt at lower temperature (650-750˚C)
• High silica content
• Rich in potassium and sodium
• Typically flows 10x more slowly than basaltic lava and tends
to pile up in rounded deposits
• Gases are easily trapped causing large pressure increases
as the gasses expand
• Can produce most explosive of all volcanic eruptions!
vesicular textures caused by
• Gases trapped in lava during cooling produces vesicular
textures (bubbles)
tephra
• Tephra includes all pyroclastic debris - airborne rock and
volcanic dust ejected during a volcanic eruption
central vent volcanoes vs Large scale volcanic terrains
Central Vent Volcanoes • central vent • summit crater • flank eruptions • fissure eruptions
Large-scale Volcanic Terrains • no central vent • network of source material • extend over a large area • E.g. Mid-Atlantic Ridge
Stratovolcanoes
Stratovolcanoes (aka Composite Volcanoes)
• Form around vents that eject lava and pyroclasts
• Alternating layers form cone-shaped volcanoes, steep sided
in comparison with shield volcanoes
• Lava solidifies in core
and radiating dikes
• Commonly found
above subduction
zones – andesitic
composition
ex: cotopaxi
volcanic dome
Volcanic Dome
• Mounds that form in vents when viscous lava erupts slowly
• Associated with andesitic and rhyolitic magmas
• Remember: higher silica content = higher viscosity
• Plug vents and trap gases leading to pressure increases
within vent
shield volcanoes
Shield Volcanoes
• Mafic, low silica, low gas magma originates in the mantle
• Basaltic lava results in “Aa” and “Pahoehoe”
• Low viscosity creates broad, gentle slopes
• Lava tubes are common
ex: Kilauea, Hawaii
Cinder cone
Cinder Cones
• formed from fountains of basaltic lava
• commonly on the flanks of shield volcanoes
• composed of pyroclastic debris from a single vent
• Usually maximum of 300 m high
• short-lived features - sources often cut off after short period of
time (weeks to years)
T OR F
Large-scale volcanic terrains lack a central vent
T
Large Igneous Province3s
Large Igneous Provinces
• Large volumes of mafic intrusive and extrusive igneous rock
• created by processes other than seafloor spreading
• No central vent
• Fissure eruption that produced the Siberian Traps occurred
at time of the Permian mass extinction ~251 Ma
Fissure Eruptions in Large Igneous
Provinces
- Have produce the largest eruptions in Earth’s history
* Magma ejected from near vertical fractures in lithosphere
Flood Basalts
• Basaltic lava erupting from volcanic fissures that spread
over large areas of flat terrain
• Have occurred on continental scales, creating large
plateaus and mountain ranges
• Large Igneous Provinces
• Eruption ~16 Ma buried large portions of what are now
the states of Washington, Oregon and Idaho
• Form the Columbia Plateau, covers area of ~160,000 km2
eight types of volcanoes
• Cinder cone • Stratovolcano • Rhyolite caldera complex • Shield volcano • Maar vents and diatremes • Monogenetic field • Mid-ocean ridge • Large igneous province
Monogenetic field
• Poorly understood • Multiple maar vents and cinder cones • Erupt at different times • usually grow laterally from single magma source • Form fields of smaller vents and cones instead of mountains
ex:San Francisco Volcanic Field, Arizona
Wells Gray-Clearwater Volcanic field, BC
Kinds of volcanoes in Canada?
- Monogenetic field
2. Hospot
Most volcanoes are associated with:
• Spreading centres – spreading centre volcanism • Subduction zones – arc volcanism (island arcs and volcanic arcs) • Hotspots - intraplate volcanism -• Chains of volcanic islands form as oceanic lithosphere is transported over hot spots
volatiles
Volcanoes emit volatiles in addition to pyroclasts and lava • Volatiles are chemical compounds with low boiling points • Volatiles include: • Water vapor (H2O) – accounts for 75 to 90 % of volatiles • Carbon dioxide (CO2 ) • Sulfur dioxide (SO2 ) • Nitrogen (N2 ) • Hydrogen sulfide (H2S) • Volatiles may be emitted for centuries following initial eruption
Aerosols
Aerosols (tiny particles of dust or water)
intercept sunlight and the layer nearest the
Earth (the troposphere) cools
• Sulfuric acid, silicate dust (ash)
• Chlorine can also enhance ozone depletion
ex:Example: Mount Pinatubo
Volcanic Hazards
- Eruption clouds
- Lahars
- Flank collapse
- Caldera collapse
- Toxic gases
Deadliest volcano was RUIZ
Pyroclastic flows
Pyroclastic Flows
• Hot volcanic ash and gases ejected in cloud that moves
down the volcano’s side at high speed
• Solid particles are lifted up by hot gases – limited friction,
move very quickly
flank collapse
• Volcanoes constructed of layers of lava and ash
• If sides of volcano become too steep, weak ash layers may
cause flank to collapse
• Material can be very destructive
caldera collapse
• Potentially one of the most destructive natural phenomena
on Earth – have not occurred during recorded history
• Faulting leads to formation of secondary vents
• Caldera becomes insufficiently supported
• Collapse may trigger catastrophic eruption
Mt. Saint Helens
Mount St. Helens, USA • Active stratovolcano, last erupted in 1980 • 400 m of peak collapsed or exploded • 62 km2 of valley filled by a debris avalanche • ~150,000 m3 pyroclastic material deposited by lahars • 57 people killed
Mount Tambora, Indonesia
• Active stratovolcano, last erupted in 1967 • 1815 eruption was largest in recorded history • Ejecta volume of ~160 km3 • Emitted large volumes of sulfur dioxide and carbon dioxide • Created caldera measuring >7 km across and > 500 m deep • Caused global temperature decrease and widespread famine in 1816 – 1817
Physical Weathering
Physical Weathering
• fragmentation of rock without chemical change
• due to pressure release, abrasion, freeze-thaw, hydraulic action, growth of salt
crystals, and other physical means
• aided by presence of bedding planes, rock joints and other types of fractures
Exfoliation
• Jointing of granitic rock • Large flat or curved sheets of rock detach and fall
ventifacts
rocks abraded,
pitted, etched, grooved or
polished by wind-driven
particles
Biological Weathering
• Can contribute to physical weathering and chemical weathering • Activity of microorganisms can produce microscopic fractures in rocks • Root wedging can expand fractures • Burrowing animals may also promote fracturing
Chemical Weathering
• Reaction of minerals with water and air
• May promote mineral dissolution and/or formation of new minerals
• Oxygen (O
2) and carbon dioxide (CO
2) have a major influence on
weathering reactions
• Include hydrolysis,
oxidation and dissolution
Hydrolisis
chemical breakdown of a compound
due to reaction with water
Hydrolysis: water reacts with rocks
• Alter means to change in some way. In the case of
minerals, they can become a different mineral
through alteration, and water plays an important
role in alteration.
• For example: feldspars, and several other silicate
minerals, alter to form clay minerals
Weathering rates are
_____ when rainwater
pH is lower
higher
• Amount of CO2 in atmosphere is small (400 ppm) • concentration in rainwater is quite low (0.6 mg/L) • pH of rainwater ~5.6, some variation globally
• Feldspar weathering illustrates three main effects of
chemical weathering of silicate minerals
• Leaching – cations and silica are dissolved away
• Hydration – water is added to minerals
• Neutralization – solution (e.g., rainwater) is made less
acidic
What is the difference
between weathering
and erosion?
Weathering: chemical and
physical breakdown of rocks and
minerals.
Erosion: transport of rock and
mineral particles (sediments)
from one location to another by
wind, water, or ice.
Uplift-Weathering Hypothesis
• Global rate of chemical weathering dependent on availability of fresh rock • Mountain chains at convergent boundaries enhance weathering • Orogenesis – mountain building • As new silicate-rich crust is exposed to weathering, atmospheric CO2 is consumed and the climate cools
carbonate
produced from chemical weathering and ends up in oceans where it is used
oxidation
Oxidation: the role of oxygen • Oxygen can change the form of metal cations in rocks and minerals (and man-made materials) • Can cause new minerals to form • Example: • Steel will rust when exposed to the atmosphere and water – iron minerals form
tailings are produced
• Tailings are produced by separation of economic
from non-economic (gangue) minerals
• Sulfide minerals weather in the
presence of oxygen and water generate ___ and release ___
- Generate acid
* Release sulfate and metals
chemical stability
• Tendency for a mineral to retain its composition during
weathering
• accounts for observed differences in mineral weathering
rates
• Stability is dependent upon environmental conditions
• also on mineral properties!
• Determined by two principle mineral characteristics:
1. Solubility
2. Dissolution rate
Solubility
• The solubility of a mineral is the amount of that mineral you
can dissolve in water before the solution is saturated
• Point at which mineral will no longer dissolve
• Influenced by pressure, temperature, and pH
• Minerals with higher solubility are less stable and more
susceptible to weathering
• Examples (for water):
• Halite exhibits very
high solubility (~350 g/L)
• Quartz exhibits low
solubility (~0.008 g/L)
Dissolution Rate
• Amount of a mineral that dissolves in an unsaturated
solution in a given time
• Less stable minerals tend to dissolve more quickly
• Composition and bonding influence dissolution
Widespread dissolution of
carbonate rock leads to the
development of:
Karst Topography
Weathering of Clay Minerals
- Clay minerals are often produced by weathering
- They are phyllosilicates
- Last group of silicates to break down during weathering
• Physical weathering dominates in regions of... • Chemical weathering dominates in regions of ...
• Physical weathering dominates in regions of low temperature and low rainfall • Chemical weathering dominates in regions of high temperature and high rainfall
slaking
Slaking - alternating
wetting and drying
Sedimentary Rock
• Most of Earth’s surface is covered with layers of loose sediment • >75% of the land surface is Sedimentary Rock
There are three common types of
sediments:
Clastic Sediments
• Weathered and eroded pieces of rocks and minerals
• physical and chemical weathering of common silicate-bearing rocks
• range in size from boulders to sand, silt and clay
• Weathering intensity dictates mineralogy of sediments
Chemical and Biogenic Sediments
• Dissolved ions accumulate in water due to chemical weathering
• Chemical and biological reactions precipitate minerals from these
dissolved ions
chemical sediments
- Mineral precipitation due to evaporation, forms chemical sediments
- Seawater, other waters with high salt concentrations
- Common minerals in chemical sediments: halite, calcite, gypsum
biogenic sediments
Biogenic Sediments
Biomineralization
• Direct mechanism: organisms use dissolved ions or
molecules in water to produce shells or skeletons
• Indirect mechanism: minerals precipitate due to
environmental conditions created by organisms
Sedimentary Basins
• Depressions where sediments accumulate
• Often regions of long-term subsidence
• Depressions form when an area of crust subsides (sinks)
relative to surrounding crust
Trench Basin
• Trench basins form along subduction zones
• Sediments accumulate in the trench, form an
accretionary prism
• Sediments come from eroded volcanic arcs or coastal mountains
• E.g., basin trench off of Vancouver Island
Forearc Basin
• Form between subduction zone and volcanic arc
• Likely caused by warping and buckling of the crust at the edge
of the overriding plate as it interacts with the subducting plate
• E.g., the Strait of Georgia in B.C.
Foreland (flexural) basins
• Caused by crustal deformation at convergent plate boundaries • Mass of crustal thickening creates topographic loading - flexes the lithosphere under the mountains • The basin is a wedge-shaped depression parallel to the mountain belt • Fills with sediment eroded from mountain range
terrigenous
• Sediments eroded from land (terrigenous)
• Sediments and sedimentary rocks
are generally characterized by
bedding or stratification (layers) • Range in thickness from < 1 cm to several meters • Differentiated by rock or mineral type and particle size
Cross-Bedding
• Near-horizontal sedimentary units that are
internally composed of inclined beds
• Can be inclined as much as 35° from horizontal
graded bedding
smaller particals on top of larger
Bioturbation Structures
• Remnants of burrows and tunnels excavated by marine organisms in muds and sands • Examples: clams, worms, shrimp, etc. • Commonly occur as cylindrical tubes that may extend across bedding planes • Infilled and preserved in sedimentary rock • Often characterized by differing mineralogy
Burial
• Clastic sediments become trapped following deposition in sedimentary basins • New layers of sediment accumulate over older layers of sediment • Older sediments subjected to: • Increasing temperature • Increasing pressure • Chemical and biological reactions
Diagenesis
• Sediments or sedimentary rock changed to different
sedimentary rock
• occurs at temperatures and pressures lower than those
required to produce metamorphic rocks
Oil and Gas
Oil and Gas • Oil and gas are generated during diagenesis of sediments that contain organic matter • Coal derived from plant material • Oil and gas derived from diatoms (single-celled plants) • Subsidence and burial over time increases temperatures • Oil forms between 60 and 150°C • Natural gas forms at higher temp.
What is Metamorphism?
• Re-crystallization that alters the mineral composition and texture of parent rocks (protolith) • Caused by major increased in temperature and pressure • Occurs in the shallow to deep crust • Alteration continues until the rocks reach equilibrium with the new conditions
The protolith
is the parent rock that is altered by
metamorphism
• Can be sedimentary, igneous, or metamorphic rock
• the composition of the protolith is an important
factor in the mineralogy of the resulting
metamorphic rock
4 Principle factors driving Metamorphism
- Heat
- Pressure
- Fluids
- Time
Types of pressures and stress involved in
metamorphism 3
- Confining pressure (lithostatic pressure)
- Directed pressure (differential stress)
- Shear stress
Directed pressure
Directed Pressure
• recrystallized minerals exhibit
parallel alignment of textural
and structural features
Metasomatism
• Change in rock chemistry due to fluids adding or removing
chemical constituents
Metamorphic index minerals
- Produced either exclusively or often by metamorphism
- Provide an indication of metamorphic grade
- Form at limited range of temperature/pressure conditions
Foliated Rocks
- Classified according to four principle criteria:
- Metamorphic grade
- Grain (crystal) size
- Type of foliation
- Degree of banding
Porphyroblasts
• Metamorphic minerals can grow to large crystals
surrounded by much finer-grained matrix
• Crystal growth due to recrystallization of rock matrix
at high temperature and pressure
ex: Garnet
Non-Foliated Rocks (Granoblastic)
• contain crystals with equi-dimensional shapes • Form due to confining pressure • directed pressure produces foliation • Often associated with contact metamorphism
Metamorphic facies
• Facies is a combination of metamorphic grade (P & T
conditions of metamorphism) and mineral assemblage
