Exam 2 Flashcards
(196 cards)
1
Q
What kind of rocks can become metamorphic (look at rock cycle)
A
any!
last rock type in the story
2
Q
Meta
A
Greek for after or beyond
3
Q
Morph
A
change
4
Q
Metamorphic
A
a parent rock is changed by heat and pressure to a new rock type
5
Q
2 types of metamorphism
A
- contact
- regional
6
Q
Where does contact metamorphism happen?
A
- Magma/lava touches rock
- Just heat, no pressure
7
Q
Where does regional metamorphism happen?
A
- Compression due to plate collisions or burial
- Both heat and pressure
8
Q
Where does metamorphism occur on Earth?
A
anywhere where heat and pressure can be applied to a rock without melting it
9
Q
Accretionary Wedge
A
ocean sediments that were deposited in trench and get compressed/metamorphosed by converging plates
associated with regional metamorphism
10
Q
Contact Metamorphism in Reality
A
a dark “layer” sandwiched between gray layers of rock
The black layer is actually an igneous intrusion…a body of magma that squeezed its way through the surrounding rocks. The name of this kind of intrusion is a sill.
The gray layers are limestones (sedimentary rocks).
The white halo here is marble, a metamorphic rock produced by heating limestones. The whiter appearance is the bake zone
11
Q
One of the common locations of regional metamorphism
A
trench
12
Q
Regional Metamorphism in Reality
A
Sedimentary rocks are folded and their texture changed by pressure (with some heat)
These are rocks from an ancient accretionary wedge
13
Q
Regional Metamorphism
A
Metamorphism occurs at the core of large mountain ranges (like the Himalayas) where rocks get buried and squeezed, increasing their heat and pressure.
Also occurs where two large continental plates collide
14
Q
Protoliths
A
Common Parent Rocks
15
Q
Parent rocks
A
original rock before metamorphism
16
Q
Common crust rocks that get metamorphosed…
A
limestone, shale, sandstone, granite
17
Q
What Happens during Metamorphism?
A
Depending on the type of minerals present and the intensity and combination of heat and pressure…
In the order of increasing temp. and press.
1. Rocks become more dense.
2. Existing minerals grow larger (recrystallization).
3. Minerals become stretched (shear) and compressed and line up in one direction (foliation).
4. Minerals separate by composition (banding)
5. Brand new minerals may form (neo-crystallization)
a rock does not always go through all of these affects
18
Q
The most common rock in the ocean
A
shale (sedimentary)
19
Q
Minerals in a Shale
A
- clay-sized clay minerals and are deposited in a low energy environment (deeper ocean)
- Clasts in shale had a long journey from their original continental crust source (granite)
Quartz & clay is all that’s left after chemically-weathering granite
20
Q
Where is shale commonly metamorphosed?
A
at an accretionary wedge, here, the shales are subject to low to high pressure with some heat
21
Q
Steps - low grade, medium grade, high grade
A
- Apply Some Pressure and Some Heat
- Apply More Heat and More Pressure
- Apply Intense Heat and Pressure (not enough to melt)
22
Q
Clay Minerals in Shale
A
microscopic sheet-silicate minerals
Micas (biotite, muscovite) are also sheet-silicates and are chemically related to clay.
23
Q
Example of low grade metamorphism
A
Slate
{clay minerals compress, air pockets (pores) go away, and the platy clay minerals line up in one direction (foliation)}
24
Q
Foliation
A
a metamorphic rock texture caused by directed pressure that causes minerals (typically platey and elongate clay and mica minerals) to align
{Equant minerals (square/round) do not show foliation as well (e.g., feldspar, quartz, calcite), but they do get squished!}
randomly oriented minerals: igneous or sedimentary
preferentially oriented material: metamorphic
25
Example of medium grade metamorphism
Schist
{- Clay minerals grow in size and become larger Mica (Biotite and Muscovite crystals.
- Mica becomes foliated.
- Note, the rock becomes shiny now because Mica is shiny}
26
Diffusion - a metamorphic process
- Atoms can migrate through the rock in a solid state (e.g., by diffusion) during temp and pressure changes
- Atoms re-combine to form unique, metamorphic minerals that are more stable at these high temps and pressure
27
Example of high grade metamorphism
Gneiss
{- Quartz grows and separates from the foliated mica.
- Some mica crystals change into feldspars and also separate from the mica (white/pink mineral).
- Separation of minerals here is called banding}
28
Banding
separation of minerals, shown like stripes
29
Compare the chemistry of gneiss, granite, and shale...
identical
(aside from the presence of water in clays in the shale)
30
What does limestone represent in metamorphism?
low grade
comprised of microscopic versions of the equant mineral calcite
--> chemical sedimentary rock of calcite and fossils + heat and pressure = marble, metamorphic rock
30
Characteristics of marble (after being metamorphosed from limestone)
Large calcite crystals, no cement, no fossils, no obvious foliation despite pressure, recrystallization occurred
31
What is the parthenon in Greece made out of
marble - beauty
however does not age well
32
What does sandstone represent in metamorphism?
comprised of sand-size quartz crystals. The crystals are often round
--> clastic sed. rock of quartz + heat and pressure = Quartzite
(metamorphic rock)
33
Characteristics of quartzite, after being metamorphosed
Larger quartz crystals (they grew) and no cement, lacks nay original pore spaces
34
Metamorphic Identification Chart (just 5 rocks!)
slate, schist, gneiss, quartzite, marble
35
What is Structural Geology?
The study of the 3 dimensional shape and distribution of large bodies of rock
36
What is the most basic structure that rocks beneath the surface can take?
Sedimentary rocks form originally in horizontal layers
ex. The Grand Canyon
37
Law of Original Horizontality
sedimentary rocks were originally deposited in horizontal layers (strata)
Why? water slows down as it enters a basin and sediment floating in suspension is deposited on the seafloor
38
Law of Superposition
the bottom layer in a stack of layers is the oldest layer, top layer is the youngest
39
Strata
rock layers
40
Structural Deformation
- Sedimentary layers stay horizontal until something happens.
- Deformation of sed. layers occurs due to stress (force) applied to a rock.
- Deformation: change in the orientation, pattern, width, length, thickness, (anything really) of a rock unit/layer
41
Different forms of stress, force
- compressional
- extensional
42
Compressional stress
At the convergence of two plates
ex. bunching up a carpet
43
Extensional stress
At the divergence of two plates
ex. pulling apart taffy/candy bar
44
Relative Tectonic Forces: Shear stress
Where rocks tear without vertical motion (Transform plate boundaries)
can cause masses of rock to slip
45
What structures form in sedimentary rocks due to stress?
deformation, also known as strain
Strain: the permanent result of stress in a rock
ex. - Tilted layers
- Folded layers (ductile behavior)
- Faults (brittle behavior)
- Metamorphic Foliation (pressure leads to mineral alignment and “squishing”)
46
Dip (tilting)
direction and angle at which the rocks tilt from original horizontal position
47
Tilted Sedimentary Layers
Tilting of layers can occur due to any relative tectonic motion (convergence or divergence).
48
Folding
- Typically caused by compression of sedimentary layers.
- Often associated with a more ductile-style of deformation.
- Ductile behavior = bending without breaking
49
What causes rocks to bend
Rocks may behave like a flexible plastic substance (meaning they are ductile and will permanently bend) when…
- Compressional stress is applied slowly and/or…
- When rocks are still warm/hot (buried at depth).
- Folding often occurs during mountain building at convergent margins
50
What causes rocks to break
When enough force is applied even something flexible can break.
We call this brittle behavior.
1. Rocks behave like a brittle solid when stress is…
- Applied quickly and/or
- When the rock is cold.
2. This behavior results in fracturing & faulting of rocks.
3. Faults are associated with any kind of plate boundary
51
Folds are classified into two main types:
anticline
syncline
52
Anticline
Fold where the strata dips away from the hinge
(makes an a frame)
striped pattern has the oldest rock layer in the middle of the fold/hinge
53
Syncline
Fold where the strata dips towards the hinge
(makes a v frame)
striped pattern has the youngest rock layer in the middle of the fold/hinge
54
Folds and Erosion
- Folding occurs during mountain building under compressional stress.
- Because the folds get pushed up into the air (uplift), they are subject to rapid erosion.
- Mountains erode very quickly (on a geologic scale = millions of years).
55
Appalachians
- 300 Myr Ago
Appalachian orogeny: continental-continental convergent
Orogeny = mountain building event
Mountainous area in PA with examples of folded rock {Notice the zig-zag pattern in the landscape (the dark patches are forested hills)}
56
Fault:
a fracture in rocks along which there has been movement
commonly associated with plate tectonics
{You can find a fault if you find offset layers of sedimentary rock.}
57
How Do Faults Form?
Three possibilities...
1. Compression (squeezing) of the rocks
- If rocks behave like a brittle solid you get a crack/fault during folding.
- Results in tall mountains.
2. Extension (pulling apart) of the rocks
- This forms cracks too.
- Cracking in this way forms a valley/rift instead of a mountain.
3. Transform (Shear) motion
- When rocks slide against each other.
- No compression (uplift) or extension (collapse).
58
How can we the the kind of relative motion (compression, extension, transform) that has occurred?
By the fault type
59
How do you determine the type of fault
must first identify how the land has moved around the fault
through the hanging and foot wall
60
Hanging wall
the block of rock that lies above the inclined fault
(overhanging)
61
Foot wall
the block of rock that lies below the inclined fault
(shape of a foot)
62
How do we identify fault types
by which relative direction the hanging wall moves (up, down, or side to side)
63
Normal fault
the hanging wall moves down relative to the footwall
caused by extension!
64
Extension
This kind of faulting occurs when the land spreads open (extension/divergence)
This creates a valley, not a mountain range
causes normal faults!
65
Reverse fault
the hanging wall moves up relative to the footwall
caused by compression!
66
Reverse fault
the hanging wall moves up relative to the footwall
caused by compression!
67
Compression
This kind of faulting occurs when the crust compresses/converges
Folded and faulted mountains are constructed by this process
causes reverse faults!
68
Thrust fault
type of low angle reverse fault with a dip of < 45°
Most common form of reverse fault associated with mountain building.
Steeper, reverse faults are rare
69
Which faults often occur together?
Folds and reverse (also known as thrust)
70
Mountain ranges that form due to plate convergence…
fold and thrust belts
71
Transform/Strike Slip (shear) fault
No vertical movement. Walls slide against each other (lateral movement)
72
Types of transform faults, two...
left lateral
right lateral
73
How to tell apart left from right lateral
If you are standing birds eye, looking across the fault the land appears to have moved to the left relative to where you are standing, and this will appear the same on the other side
vice versa for right
74
Famous transform fault example
San Andreas
75
The Geologic Timescale
divided into time blocks
The structure of the geologic time scale:
Eons – The largest subdivision of time (100s to 1000s Ma).
Eras – Subdivisions of an eon (65 to 100s Ma).
Periods – Subdivisions of an era (2 to 70 Ma).
Epochs – Subdivisions of a period.
Age – Subdivisions of epochs.
76
Goal of Today (relative age & geological age)
To be able to decipher the geologic history of Earth by interpreting a sequence of rocks (structures, layers, etc.).
77
Relative age
the comparative timing of events (oldest to youngest)
78
Absolute age
the actual chronology or dates of events (500 My years old)
79
Outcrop of Bedrock example
Letchworth Park
80
Deformation (relative age & geologic time)
tilting, folding, and faulting of layered rocks occurs after the rocks were deposited horizontally
Deformation is caused by Plate Tectonics.
Layers deposited first, then deformation happens
81
Principle of Cross Cutting Relations
Events or features that cross another rock are younger than the rock that they cross.
82
Dike
Vertical pipe of magma, or sheet, is younger than the rock it squeezes through
Extending off larger bodies
83
Igneous Intrusions (plutons):
any igneous body that intrudes or cross-cuts through pre-existing rock (ex. dike, sill, batholith)
84
Batholith
large magma bodies that feed other small bodies
85
Sills
horizontal sheet of magma that intrudes between sedimentary layers
86
Contact metamorphism
a principle of cross-cutting relations
an indicator of age
look for the bake zone
87
Principle of Inclusions
If a rock body contains fragments of other rock bodies then the fragments must be older
88
Unconformity
A surface between rock layers that represents missing time.
- No rocks were deposited during that time…or…
- Layers of rock were eroded away.
- Uplift of land out of water creates this gap in time (no rocks deposited + erosion).
89
How was the Geologic Timeline Built?
by relative and absolute age dating
90
Relative age dating
Sequence of events from oldest to youngest
91
Absolute age dating
Actual age of those events (in years)
92
How do we know where we are in the relative timescale at any given location?
1. The principle of fossil succession (relative age technique).
2. Correlation (matching rock layers between different locations).
3. Radiometric (Numerical) dating (absolute age technique).
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Fossil Succession
- Species evolve, exist for a time, and then go extinct (think, the dinosaurs).
- Fossils succeed one another in a known order in the rock record.
- Example: Humans exist in the rock record after T-Rex.
- A geologic time period (ex. Cambrian, Devonian, etc.) is defined by its fossil content and is typically (not always) named for the location for which the rocks were first/best described.
94
Stratigraphic column
(how sedimentary layers are often represented)
diagram of a vertical sequence of sedimentary rocks
95
Fossil range
first and last appearance is noted on the column on the right
- each fossil has a unique range
- overlapping ranges provide distinctive time markers
96
The Reality of Fossil Succession
Gradual Path (A Tree) of Evolutionary Change
A single species of life typically appears and evolves into other species gradually.
97
Correlation (relative age & geologic time)
matching up of rock layers across the Earth using fossils
98
Index Fossils
a fossil from an organism that occurred on Earth for a short period of time but existed over a large area of the planet
- Useful for matching layers (correlation) across the planet
- If you can find the index fossil, you are more confident of where you are in the sequence of relative time
(like a marker bed)
99
Geologic Timescale - the characteristics
- Constructed from incomplete stratigraphic sections across the globe.
- It is based on fossil occurrences, extinctions, and correlation.
- It is, at its heart, based on relative age.
100
Absolute (Numerical) age
based on radioactive decay of atoms in minerals
- Radioactive decay proceeds at a known, fixed rate.
- Radioactive elements act as internal clocks.
- Radioactive elements are naturally occurring.
101
Elements Come From...
- Nuclear Fusion inside of stars
- Supernova
these produce other less common versions of each atom which are either stable or unstable (radioactive)
102
Isotopes
(other versions of carbon)
atoms that have the same number of protons but different number of neutrons
103
Carbon Isotopes are either...
1. Stable - never breaks down (in nature)
or
2. Unstable - breaks down to something stable = radioactive (not in nature)
104
Carbon Isotopes
- Organisms contain lots of carbon – replenish carbon during their lifetime.
- When they die, unstable carbon-14 atoms in their body begins to decay (radioactive decay).
- The unstable parent isotope (14C) decays to a stable daughter element (14N).
105
Half-Life
the time it takes for half of the parent atoms to break down to the daughter atoms
(radioactive decay occurs at a regular pace, like a clock)
106
Radiometric Dating
The age of a mineral can be determined by…
- Measuring the ratio of parent to daughter isotopes.
- Calculating the amount of time by using the known half-life.
107
Dating Rocks
- Rocks don’t have a lot of Carbon (mostly Si, O, Al, K, Na, Fe, Mg)
- Half-life of 14C is short (thousands), rocks are old (millions)
- By the time we measure a rock all 14C is gone.
- We don’t use Carbon dating for rocks.
- Rare radioactive elements in rock: Uranium (U), Strontium (Sr), Potassium (K), Argon (Ar)
108
Best rocks to date when using radiometric dating:
Igneous rocks – crystallizes as one rock relatively quickly, trapping radioactive elements in the minerals/crystals
109
Worst rocks to date when using radiometric dating:
Sed rocks – contain clasts from different rocks which gives multiple ages
Meta rocks – heat and pressure resets age
110
Best igneous rocks to date when using radiometric dating:
Extrusive (volcanic) igneous rocks form instantly
- Volcanic ash beds cover vast areas and are the best (time marker bed)!
- You can find ash layers from one eruption all across the world! – helps to correlate rocks with an absolute time marker!
111
Dating the Geologic Column...
Sediments can be bracketed by absolute dates (defines major boundaries in the geologic column)
112
Methods for deterring the Age of the Earth
Before radioactivity-based dating methods…
- 20 Ma – From Earth cooling.
- 90 Ma –Ocean salinization.
(Assumed oceans were initially freshwater.)
(Measured the mass of dissolved material in rivers.)
- Uniformitarianism and evolution indicated an Earth older than ~100 Ma.
113
The Age of the Earth
- The oldest rocks on Earth’s surface date to 3.96 Ga.
- Zircons (most stable mineral) in ancient sandstones date to 4.1-4.4 Ga.
- Age of Earth is 4.57 Ga based on correlation with…
(Meteorites (asteroids and other planets))
(Moon rocks)
(Models for age of Sun)
114
What is a river/stream?
Conduit for flowing water from atmosphere to ocean. Produce runoff, travels from high elevations to low elevations.
115
Runoff
Water flow over the surface
116
Streamflow
Water flowing in streams/rivers
Begins on a hill following precipitation as moving sheetwash
117
Sheetwash
thin surface layer of water
118
How do rivers form?
The head of a river begins high up the mountains or the highlands.
Water moves down the steepest slope.
Erodes substrate once enough water accumulates.
Erosion creates a smalls stream!
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Rills
baby channels, channelized flow
120
Watershed/drainage basin
The area of convergent slopes that captures water
(Once a channel is created, it funnels subsequent flow)
Area confined within drainage divides that feeds runoff to a single point downhill
*landscape funnel!
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Drainage Divide
Highest elevation in a drainage network that separate different watersheds.
122
Largest watershed in US
Mississippi-Missouri
123
Which watershed do we live in?
the Atlantic Ocean Watershed here in Geneseo
(but western NY does contain a sliver of the Mississippi watershed, fed by the Alleghany River)
124
Tributaries
New channels form on the new slopes from flow routing
125
Flow routing
Rivers carve valleys in soil and bedrock.
That creates new sloping surfaces for other rivers to form.
(New channels form on the new slopes, there is also a main trunk)
126
Steps of drainage evolution:
1. A landscape is uplifted an exposed to weather/rainfall.
2. These early channels funnel water into them. As water flows into them, new tributary channels form on their flanks.
3. Water flowing off the red areas into this channel system cause the channels to not only get wider and deeper, but longer. The heads of the streams (black arrows) “migrate” uphill.
4. The rivers get even wider, deeper, and longer. What once started as a broad flat plateau in Step 1 has now become a “ridge” of elevated terrain dissected by rivers. This is how landscapes evolve on Earth in the presence of rainfall.
127
Dendritic pattern
a tree branch-like pattern where smaller streams (tributaries) connect to form larger rivers
(Most common river pattern)
128
Other drainage patterns that geology influences...
- trellis
- rectangular
- radical
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Example of Structurally Controlled Drainage
River following a fault in the Earth
130
Ways in which streams erode a landscape?
- vertical erosion of streams
- lateral erosion of streams
- headward migration of streams
131
Vertical Erosion of Streams
- a river removes rock from the bed
- the valley is deepened
132
Lateral Erosion of Streams
- a river removes rock along the banks
- the valley is widened
133
Headward Migration of Streams
- loose rock debris is brought down by the overland flow behind the river source
- the river source extends backwards
- the valley is increased in length
134
Headward Erosion
This view represents a stream’s topographic profile.
Streams will erode all material in their way to reach the base level (to reach equilibrium with the sea).
135
Equilibrium State of Rivers
The river will speed up here, causing erosion to be high at this spot in the profile of the river.
This leads to headward erosion…
136
Largely known example of headward erosion...
Niagara Falls
137
What changes occur from source to sink in a drainage network?
- channel size
- discharge
- sediment load
- clast size w distance
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Effects of channel size
The number of streams that contribute water increases downstream - the streams become larger
139
Effects of discharge
As the river travels downstream it gains water and velocity, increasing its discharge (volume / second)
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Effects of sediment load
As the river travels downstream it gains more sediment from contributing streams.
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Effects of clast size (w distance)
Large clasts are deposited early on, near the highlands. Clast size decreases as distance increases.
142
How do the types and shapes of rivers change with distance, moving downstream?
Linear gully --> braided river --> meandering river
143
The Three Channel Types
straight/linear
braided
meandering
144
The path of a river
All rivers want to reach a base level.
Base level: the lowest topographic level (elevation) to which a river flows. (Ultimate base level for all rivers = ocean).
145
Straight rivers
High elevation and steep slopes causes intense vertical erosion.
Rivers become confined to the bedrock that they cut into.
Rivers can’t move laterally so they are straight (linear) in map view
146
Slot canyon
river cut vertically into rock and can’t migrate laterally
147
Alluvial Fans
Fan-shaped, poorly-sorted deposit of sediment.
Alluvial fans are dominated by braided streams.
At the base of a mountain…(only relevant for steep terrains).
Coarser sediments are rapidly deposited at slope break (flow decelerates) forming a large pile of sediment.
148
Deposition at the base of mountains of braided rivers/streams
rapid deposition of large cobble, pebble, and sand-sized clasts (think conglomerate rock)
149
Braided river
Sediment bars: deposits of sediment in the channel. Sediment gets in the way of the water – the channel switches (avulsion)
High rate of deposition leads to braiding, because it just dumps t so quickly
150
Meandering rivers
Form on lowest slopes, near base level.
Channels transport sand, silt, and clay now.
Rivers migrate laterally on low slopes (no vertical erosion).
River banks are more cohesive than braided (vegetation & clay in banks) but banks can be eroded.
Steady, lateral migration (less chaotic/rapid migration compared to braided).
Muddy waters = suspended silt and clay
151
Meander formation/evolution
All it takes is a single instability (a weak point in the bank) and sinuosity/curviness can develop.
Once that spot starts to erode, the bank will begin to migrate (to the left at the first red dot). This migration is due to erosion along this bank. The water flowing through here turns towards that bank and away from the inner (right in this diagram) bank. The water that slams into the outer (left in this diagram) bank erodes that bank. The water then bounces off the eroding bank and gets re-directed towards the opposite bank further downstream. Here, it will begin eroding this bank too! This develops a unique sinuous pattern in the river (curvy pattern).
152
Cut bank
bank of a river meander that experiences the highest flow velocity and the most erosion (outside curve)
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Point bar
bank of a river meander that experiences the lowest flow velocity and the most deposition (inside curve)
154
Evolution of an oxbow lake
As the cut bank erodes through time, the entire meander migrates and becomes more and more sinuous. The bend becomes more curvy until it eventually becomes too curvy and the river bumps into itself. The river will then cut off the meander bend forming an abandoned oxbow lake. The river becomes more straight at that point, but the meandering process then starts all over again.
155
The Floodplain
A meandering river migrates within a broad valley (like ours below campus).
Flooding of the river deposits silt and clay onto the floor of this valley.
This generates a floodplain…a flat surface comprised of river sediments.
156
Where in the US is experiencing a massive decade long drought?
Out west
157
Sierra Nevada Snow Pack
March is the end of winter, and these mountains should be nearly 100% covered in snowpack, they are not barely at all
158
Why does the drought out west matter?
If you consider California alone, it is one of the worlds largest economies. The population of California is also high and is dependent on the water supply from the Sierra Nevada Mountains specifically.
159
California Aqueduct
This aqueduct is one of the most famous in the world. It transports snow melt from the Sierra Nevada Mountains all the way south to the LA basin.
160
How does water get into the ground?
First, rocks or sediment must have holes (pores), depends on porosity, those open spaces can hold water (or gas/oil).
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When it doesn’t rain or snow, where do you turn? For irrigation?
Groundwater
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Infiltration
The ability of precipitation to be absorbed by the ground. The resultant “groundwater” can remain near the surface in something called “the water table” or infiltrate deep into the bedrock.
(notice the small pipes or conduits of dye that are infiltrating deeper into the soil. Eventually, the moisture will collect at depth in a zone called the “zone of saturation” or the “water table”)
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Porosity
The measure of open space in a rock or soil.
163
Are igneous rocks porous?
Not usually, they have interlocking crystals (formed in magma/lava)
No gaps/holes between the crystals = not porous
164
Which igneous rocks are porous?
Vesicular Basalt and Pumice
(extrusive igneous rocks may contain (now empty) gas bubbles (vesicles))
165
Are metamorphic rocks porous?
No, heat & pressure closes pore spaces, re-crystallization, foliation, and growth of new minerals
Metamorphic = not porous
166
Are sedimentary rocks porous?
Yes (some more than others)
Clastic sedimentary rocks are made of cemented particles
Cement is not perfect, spaces between clasts
167
Well-sorted clasts:
all the same size, lots of space between clasts (more porous)
168
Poorly sorted clasts:
many different sizes, smaller particles fill the spaces between larger particles (less porous)
169
Well rounded clasts:
lots of space between clasts
(more porous)
170
Angular clasts:
still some space but some fit together like a puzzle, generally less porous
171
Limestone and porosity
(sedimentary) extremely porous
- Groundwater is often slightly acidic.
- This creates holes and caves in the Limestone.
- Holes and caves in limestone are called karst.
(When a limestone cave collapses, it creates sinkholes)
172
Karst
landforms related to dissolution of carbonate by groundwater
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What is the reason that porous rocks don’t always allow water into them?
Permeability - If the pores in a rock aren’t connected, water can’t get through
174
Permeability
The ability of water to pass through a rock (pore spaces are connected)
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Vesicular basalt's permeability...
Porous but not permeable.
Bubbles in the basalt are large, but they are not connected.
All igneous rocks are impermeable
176
Shale's permeability...
Like a stack of tiles.
Shale is made of very small (micron size) clasts of clay.
Water would have to pass through millions of microscopic spaces to get through the rock.
Shale = impermeable
Water may pass through fractures/cracks (joints) in shale.
177
Permeability and Grain Size
Finer grains = too many pore spaces for water to pass through.
Path of water through shale is too tortuous (complex)
178
Sandstone's porosity & permeability...
(sedimentary)
Well sorted and often rounded clasts (only sand).
Very porous and very permeable.
179
The Water Table
The level below the surface at which the sediment/soil/rock is saturated with groundwater.
180
Wetlands
water table is right at the surface
181
Water Table Topography
Water table mimics the topography.
Flows from higher to lower elevations .
182
Hydraulic head (groundwater pressure)
The weight of water and rock is greater over the other point
Groundwater will flow from high to low pressure
183
The Water Table and the Seasons
The wettest time of year in our area is Spring. Spring rains and snowmelt contribute water to the water table. The level of the water table therefore rises (you may have noticed that the ground is fairly soggy out there in April and May). However, as the growing season commences, and as the late dryer summer season begins, the water table lowers. The growing plants pull a significant volume of water from the moist unsaturated zone and the shallow water table and it is not replenished as quickly in the late summer. The water table is therefore lowest in the late summer/early autumn season.
184
Groundwater Flow
occurs on a variety of scales:
- Local, Intermediate, Regional.
- Groundwater can remain trapped at depth for thousands of years.
185
Aquifer
Sediment or rock that transmits water easily.
186
Aquitard
Sediment or rock that hinders water flow.
187
Tapping Groundwater
Wells are holes drilled into the saturated zone.
- Well hole is a zone of low pressure.
- Water flows into the well.
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Gradient
change in water elevation between two points (distance between two points)
189
Groundwater isolines
Like contour lines.
Represent elevation of the water table.
Water flows from high elevation to low elevation.
190
Groundwater Pumping
Sucking out too much water draws down the water table locally.
Cone of depression develops around the well.
This could suck water away from other nearby people.
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Groundwater Problems
Natural resource
- 30% of all freshwater on Earth is in the ground
- Threatened
Depletion, pollution, contamination
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Groundwater Depletion
Severe water table decline
- Dewater streams and lakes
- Subsidence (collapse from pores after sucking so much) ex. The Leaning Tower of Pisa, Italy &
The San Joaquin Valley, Calif
- Irrigation
193
Groundwater Pollution
Remember: groundwater flows downhill.
Need to think about uphill contaminants.
194
Groundwater Salt Contamination
Salt water intrusion
The freshwater table can mix with the saltwater table. Overpumping of freshwater can cause the wells to also take in saltwater which is harmful to humans if ingested in large quantities.
