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MN Rocks & Waters Exam 2
Terms in this set (65)
Time since last major glaciation is considered "modern" human history.
- Animal domestication
Why care about time
Helps us predict future conditions, and determine how humans have influenced current (and future) conditions.
Relative Age Dating
Ages of rocks can be dated relative to one another.
- Stratigraphy is the study of layering (strata = layers) of sedimentary rocks.
Relative Age Dating Principles
2. Original Horizontality
4. Cross-cutting relationship
In undisturbed strata, the oldest sed. rocks will be on the bottom and the youngest rocks on top.
- Strata are originally deposited in horizontal sheets.
- If strata are no longer horizontal, then post-depositional deformation has occurred (tilting or folding).
If pieces of rock A are included into rock B, then A must be older than B.
If a feature (igneous intrusion, fault, or unconformity) cuts across a rock unit or another feature, then the cutting feature must be younger than the ones it cuts across.
Periods of erosion or non-deposition. Represents "missing time" in the depositional record.
- Disconformity: occurs along the bedding planes between two sedimentary units.
- Angular unconformity: occurs at an angle to lower sedimentary units. (typically follows a tectonic event).
Erosional unconformity with Meta. or intrusive Ig. rocks below a Sed. unit.
preserved remains of a plant or animal.
- Records the evolution of life.
- Because of evolution, the types of fossils (fossil assemblage) present changes. These assemblages allow further dating of sed. rocks.
Geological Time Scale
Goes back 4.6 billion years (beginning of earth).
-Eon (largest unit of time)
Unstable atoms spontaneously decay.
- Lose particles and energy from nucleus.
- Become a different isotope or element.
* The number of atoms that decay is proportional to the number that were started with.
* Only certain elements are radioactive (non-radioactive elements are stable).
Unstable isotope that will spontaneously decay.
The product of decay.
Time it takes for half of the radioactive elements to decay.
- Useful for dating rocks or geologic events of different ages.
Causes of climate change
Climate is a result of complex interactions:
- Sun (solar energy)
Long term trends of climate change
- Started warm
- First glaciers ~ 2.4 Ga
- Currently in glacial cycle
-- Cool period
-- Past 35 Ma (entire existence of humans)
Climate and Life
- Life affects composition of atmosphere (affects climate).
- Life caused ~ 80 million years of glaciation in Carboniferous/Permian.
- Land plants had taken over (used up CO2 in atmos. during photosynthesis) (reduction in greenhouse gas caused global cooling & sent Earth into a glacial cycle).
- Passing open/closed by shifting continents
- Open ocean absorbs most solar radiation (low albedo)
- Land reflects much solar radiation
- Ice (white) reflects nearly all solar rad. (very high albedo)
- Solar radiation most intense at equator
Sea floor spreading
Increased seafloor spreading increases CO2 released from upwelling magma
- results in warming
- Short term (mo-yrs): ash and aerosol release > cooling
- Long term: increase greenhouse gas conc.
- Ocean can trap a lot of heat (acts as buffer).
- Conveyor belt circulation (brings warm water to poles, cold water back to lower latitudes)
Humans influenced climate change
- CO2 and methane levels began to artificially rise ~5-8 ka
- Offset reduction in solar radiation
Dunes and mud cracks
- May indicate past arid environment
Glacial Landforms (indicate colder climate)
- U-shaped valleys
- Glacial striations
- Kettle lakes
Ice forms in annual layers.
- Oxygen isotopes reflect climate.
- Ice cores show historical trends.
- Gas bubbles record atmospheric composition (greenhouse gases)
- Groundwater deposits minerals in annual layers
- Annual growth rings
Fossils (indication of climate change - not a duplicate term)
Type of organism indicates environmental climate.
(preserved in lake sediments)
- Pollen record indicates dominant plant species
Glaciers and Ice Ages
Glaciers: thick masses of ice (last all year & flow via gravity)
- Glacial coverage of Earth (present: ~10%, during ice ages: ~30%)
How glaciers form
- Snowfall accumulates and survives the following summer.
- Snow transforms to ice: snow is buried, compression reduces volume, snow turns into granular firn, firn becomes ice. 1's to 1000's yrs.
- Flow from high to low elevation in mountain settings.
fill mountain-top bowls
flow like rivers down valleys
Mountain ice caps
cover peaks and ridges
spread out at the end of a valley
- Vast ice sheets covering large land areas.
- Ice flows outward from thickest part of sheet.
- Two major ice remain on Earth: Greenland & Antartica
Movement of Glacial Ice
- Basal sliding: Signification quantities of meltwater forms at base of glacier. Water decreases friction, ice slides along substrates.
- Plastic deformation:
think silly putty.
- Flow rates vary widely (10 to 300 m per year).
- Rarely, glaciers may surge (20 to 110 m per day).
area of net snow addition
- colder temps prevent melting
- snow remains through summer
area of net loss
zones meet at the equilibrium (gain = loss)
- If accumulation = ablation, the glacial toe stays in the same place.
- If accumulation > ablation, the glacial toe advances.
- If accumulation < ablation, the glacial toe will retreat upslope.
Many types of sediment derive from glaciation.
drift is water sorted; unstratified drift is not sorted.
sediment dropped by glacial ice
- consists of all grain sizes (boulders to clay).
- Unmodified by water (unsorted, unstratified (ice-deposited)).
boulders dropped by glacial ice.
- Rocks are different from underlying bedrock.
- Often carried long distances in ice.
Sediment transported by meltwater
- muds are removed.
- dominated by sand and gravel.
Glacial lake sediment
Fine rock flour settles out of suspension in deep lakes.
- End moraines: form at stable toe of glacier.
- Terminal moraines: form at farthest edge of flow.
long, aligned hills.
- Asymmetric form: steep up-ice; tapered down-ice.
- Commonly occur as swarms aligned parallel to ice-flow.
- Long, sinuous ridges of sand and gravel.
- Form as meltwater channels within or below ice.
Laurentide ice sheet
Maximum of Laurentide ice sheet: 14,000 years ago.
- floods bring nutrients
- nutrients grow crops
- crops feed people
- floods destroy people/property
All of Earth's water = 1.39 billion km3
- only 3% of total water is fresh (not salty sea water)
- 69% ice (not available)
- 30% ground water (mostly available)
- 1% surface water (mostly available)
Movement of water from one reservoir to another
- Oceans, atmosphere, continents
- As: Liquid, solid, or vapor
Perennial v. Ephemeral streams
Perennial: East streams
Ephemeral: West streams
Watersheds (WS) can be nested inside one another
i.e. The Gilmore Creek WS (Lake Winona) is part of the larger Mississippi River WS
will adjust to changes in base level.
- local base level (dams, entering larger water body).
- global base level (sea level).
- Stage: arbitrary height or elevation of stream.
- Discharge: volumetric flow rate (m3/s). Measured by multiplying average velocity by cross-sectional area.
- Plotted on a hydrograph (tells us about watershed conditions).
- Many branching channels separated by sediment bars.
- Usu. wide and shallow w/ highly variable flows.
- Larger particle sizes.
- Steep gradient.
- Single thread that migrates back and forth across the floodplain.
- Flows are more consistent.
- Smaller particle sizes.
- Shallow gradient.
3. Point bars:
4. Cut bank:
Rivers are major economic assets:
- Farming (floodplains)
Many people live near rivers. Floods become more and more damaging as population increases.
Major MN rivers
3. St. Croix
St. Anthony Falls
- Provided hydro power and fueled industry and trade in the region.
- Saw mills in cities first then flour mills took over. Mills fueled growth.
- Shipping the product.
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