Midterm 2 Flashcards
(82 cards)
1
Q
Veins
A
Veins are mineral deposits within
rock fractures in the country rock
that come from the magma
2
Q
Pegmatites
A
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
3
Q
Parts of a volcano
A
- Magma Chamber
- Flank Eruption
- Central Vent
- Crater
- Lava Flows
- Volcanic Debris
4
Q
• Three main compositions of lava and
corresponding volcanic igneous rocks:
A
• Three main compositions of lava and corresponding volcanic igneous rocks: • Basaltic lava / basalt • Andesitic lava / andesite • Rhyolitic lava / rhyolite
5
Q
Magma viscosity is controlled by 3
A
• 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
6
Q
• Felsic lava has _____ silica content
and is more viscous than basaltic lava
A
higher
7
Q
Basaltic Eruptions are the
A
Hawaiien Style, 10-100 m output, effusive
8
Q
Pahoehoe vs Aa
A
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
9
Q
pillow basalts/lava
A
• 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
10
Q
Andesitic Lavas
A
• 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)
11
Q
Rhyolitic Lavas
A
• 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!
12
Q
vesicular textures caused by
A
• Gases trapped in lava during cooling produces vesicular
textures (bubbles)
13
Q
tephra
A
• Tephra includes all pyroclastic debris - airborne rock and
volcanic dust ejected during a volcanic eruption
14
Q
central vent volcanoes vs Large scale volcanic terrains
A
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
15
Q
Stratovolcanoes
A
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
16
Q
volcanic dome
A
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
17
Q
shield volcanoes
A
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
18
Q
Cinder cone
A
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)
19
Q
T OR F
Large-scale volcanic terrains lack a central vent
A
T
20
Q
Large Igneous Province3s
A
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
21
Q
Fissure Eruptions in Large Igneous
Provinces
A
- Have produce the largest eruptions in Earth’s history
* Magma ejected from near vertical fractures in lithosphere
22
Q
Flood Basalts
A
• 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
23
Q
eight types of volcanoes
A
• Cinder cone • Stratovolcano • Rhyolite caldera complex • Shield volcano • Maar vents and diatremes • Monogenetic field • Mid-ocean ridge • Large igneous province
24
Q
Monogenetic field
A
• 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
25
Kinds of volcanoes in Canada?
1. Monogenetic field
| 2. Hospot
26
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
```
27
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
```
28
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
29
Volcanic Hazards
* Eruption clouds
* Lahars
* Flank collapse
* Caldera collapse
* Toxic gases
Deadliest volcano was RUIZ
30
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
31
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
32
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
33
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
```
34
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
```
35
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
36
Exfoliation
```
• Jointing of granitic rock
• Large flat or
curved
sheets of
rock detach
and fall
```
37
ventifacts
rocks abraded,
pitted, etched, grooved or
polished by wind-driven
particles
38
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
```
39
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
40
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
41
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
```
42
• 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
43
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.
44
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
```
45
carbonate
produced from chemical weathering and ends up in oceans where it is used
46
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
```
47
tailings are produced
• Tailings are produced by separation of economic
| from non-economic (gangue) minerals
48
• Sulfide minerals weather in the
| presence of oxygen and water generate ___ and release ___
* Generate acid
| * Release sulfate and metals
49
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
50
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)
51
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
52
Widespread dissolution of
carbonate rock leads to the
development of:
Karst Topography
53
Weathering of Clay Minerals
* Clay minerals are often produced by weathering
* They are phyllosilicates
* Last group of silicates to break down during weathering
54
```
• 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
```
55
slaking
Slaking - alternating
| wetting and drying
56
Sedimentary Rock
```
• Most of Earth’s surface is
covered with layers of
loose sediment
• >75% of the land surface is
Sedimentary Rock
```
57
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
58
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
59
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
60
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
61
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
62
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.
63
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
```
64
terrigenous
• Sediments eroded from land (terrigenous)
65
• 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
```
66
Cross-Bedding
• Near-horizontal sedimentary units that are
internally composed of inclined beds
• Can be inclined as much as 35° from horizontal
67
graded bedding
smaller particals on top of larger
68
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
```
69
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
```
70
Diagenesis
• Sediments or sedimentary rock changed to different
sedimentary rock
• occurs at temperatures and pressures lower than those
required to produce metamorphic rocks
71
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.
```
72
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
```
73
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
74
4 Principle factors driving Metamorphism
* Heat
* Pressure
* Fluids
* Time
75
Types of pressures and stress involved in
| metamorphism 3
* Confining pressure (lithostatic pressure)
* Directed pressure (differential stress)
* Shear stress
76
Directed pressure
Directed Pressure
• recrystallized minerals exhibit
parallel alignment of textural
and structural features
77
Metasomatism
• Change in rock chemistry due to fluids adding or removing
| chemical constituents
78
Metamorphic index minerals
* Produced either exclusively or often by metamorphism
* Provide an indication of metamorphic grade
* Form at limited range of temperature/pressure conditions
79
Foliated Rocks
* Classified according to four principle criteria:
* Metamorphic grade
* Grain (crystal) size
* Type of foliation
* Degree of banding
80
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
81
Non-Foliated Rocks (Granoblastic)
```
• contain crystals with equi-dimensional
shapes
• Form due to confining pressure
• directed pressure produces foliation
• Often associated with contact
metamorphism
```
82
Metamorphic facies
• Facies is a combination of metamorphic grade (P & T
| conditions of metamorphism) and mineral assemblage
