ESCI Test 2 Keypoints

Bowen's reaction series

-If pressure and/or temperature increase, chemical changes take place
� Minerals (particularly those at the low-temperature part of Bowen's reaction series) become
unstable and change their structure to one that is more stable - same ions (components) jus

Where does metamorphism occur?

1. occurs at convergent plate boundaries
2. occurs around the margins of magma bodies regardless of plate-tectonic settings
-magma hits cooler rocks as it moves in the subsurface and alters those surrounding rocks (contact metamorphism)
-primarily chemica

How do metamorphic rocks form?

igneous, sedimentary rocks, or metamorphic rocks are altered due to temperature (>200 �C) or high pressure (squeezed or sheared)

What do the changes of metamorphic rocks involve?

-chemical change
-at the atomic scale: reorganization of atoms to alter the existing mineral or to form new minerals that are stable at new pressure/temperature conditions
-at the microscopic to cm scale: reorientation of grains or crystals to form foliat

How do continents grow?

-continents consist of metamorphic belts sutured together
-oldest rocks are in the interiors of continents
-arrangement suggests that continental lithosphere grew through time

Appalachians

Make sure you understand 1.1 billion years of growth and fragmentation of the Appalachians ending with the modern plate configuration; many different orogeny came to determine present day Appalachian mountains

ages of continental crust

-In contrast to oceanic crust that is constantly recycled (oldest material is 164 million years old),
continents grow through time with oldest material in the middle (> 3 billion years old) and youngest
material added to the outside margins
-Continents gr

prognosis for Baja CA

it started on the North American plate,
became part of the Pacific plate where it is rifting away from North America and moving
northwestward toward the Aleutian (AK) subduction zone where it will be transferred back to the
North America plate

Vertical motions

a point in the lithosphere goes up or down relative to the Earth's center

Vertical tectonics

Plate-boundary interactions including spreading, sliding or colliding tend to produce vertical
motion concentrated at plate boundaries
-Vertical motions at plate boundaries when continents pull apart: A rift forms and the
crust drops down into a rift vall

Isostasy

term for the equilibrium or gravitational balance that the lithosphere seeks in response to the
volume of asthenosphere (or Low Velocity zone) displaced by lithospheric roots, such that the thicker the lithosphere, the deeper its roots and the higher it s

Isostatic equilibrium

A condition when the buoyancy force pushing lithosphere upward = gravitational force pulling lithosphere downward
-Both oceanic and continental crust float on the mantle lithosphere, and the lithosphere floats on the asthenosphere
-Most of the crust float

If a load is added to or removed from the lithosphere...

the lithosphere is effectively thickened or thinned
(and its density structure potentially changed), and it flexes upward or downward in response to the changing load
-In order for the lithosphere to flex or bend, some aesthenosphere is displaced to make

Loads are added to the lithosphere by

-Lithospheric-plate collisions
-Depositing sediment
-Adding volcanics deposits
-Advancing glacial ice sheets

Loads are removed from the lithosphere by

Melting of glacial ice sheets that produces glacial rebound or isostatic rebound
-As the load is removed, then the crust rebounds by flexing upward. The lithosphere goes back to its pre-glacial position
-The rate of isostatic rebound depends on how much w

two means of removing loads produce isostatic uplift but are not isostatic rebound, as they are not restoring the lithosphere to a pre-existing position

-Crustal extension that thins (in a horizontal plane) the lithosphere tectonically
-Erosion of the crust by surface processes, which are in turn controlled by climate -- Net effect is that
the surface elevation goes down, but a volume of rock moves up.

Earth's subsurface

underground or below the sea floor

Earth surface

interface between the lithosphere and the atmosphere/hydrosphere

Fundamental controls on landscape = Landscape is governed by the interplay between:

(1) lithosphere (plate-tectonic processes)
(2) the atmosphere/hydrosphere (climate)
(3) Homo sapiens

Weathering

-Weathering is the chemical or physical breakdown of rocks and minerals at or near the Earth's surface
-Chemical processes dominate in areas of high precipitation as most weathering reactions require
water
-Physical processes such as expansion/contraction

SEDIMENTARY ROCKS

-formed from lithification of sediments
-The most obvious characteristic of sedimentary rocks is layering or stratification
-Each layer is a bed or stratum. Strata are deposited initially on surfaces that are close to horizontal

2 major types of sedimentary rocks

1. Detrital sedimentary rocks = fragments of pre-existing igneous, sedimentary or metamorphic rock
2. Chemical/biochemical sedimentary rocks = precipitate from a supersaturated liquid

Detrital sedimentary rocks

classified on the basis of their grain size
-Conglomerate = gravel cemented together = largest grains (> 2 mm diameter)
-Sandstone = sand cemented together (1/16 - 2 mm)
-Mudstone/shale = mud cemented together = finest grains (<0.004 mm diameter)

Chemical sedimentary rocks

-Form from chemical and/or biochemical precipitation of minerals
-Mineral crystals settle out of the water because of their higher density and become separated
from the water to generate layered sedimentary deposits
-Inorganic chemical precipitation can o

history through sedimentary rocks

-One of the tasks of a geologist is to reconstruct the ancient processes, environment and climate during
deposition of ancient sediment using features preserved in sedimentary rocks
-river channels, mud cracks, cross-bedding

How do sedimentary rocks relate to plate tectonics?

-Most sediments (the largest volume) are deposited on continental shelves, with the widest shelves
associated with plate interiors in spreading (divergent) ocean basins
-If metamorphic rock is uplifted to the Earth's surface, it is subject to weathering,

If sedimentary rocks sit on the land surfaces of the Earth

they are then subject to weathering, become sediment, are transported and re-deposited, and get
"re-lithified" to form a new sedimentary rock

If sediment sits on ocean lithosphere

They ultimately go down a subduction zone to become metamorphic rock or magma

If sediments sit in a plate-collision zone

They are transformed by heat and pressure to form a metamorphic rock as they are deformed to
eventually becomes part of the continent

What is climate?

Climate is the average weather conditions at a given place over a period of years or decades up to thousands of years, which differs from:
-Weather, which is the much shorter term, day to day or hour to hour changes of the atmospheric condition

How do we know the climate changes?

1. direct measurements and written record
-Actual measurements of temperature, precipitation, wind direction or speed, etc
-Recorded glacial retreats and advances
-Recorded melting of permafrost
-Recorded desertification
-Written records indicating where

overall climate trends

- 1000 - 1320 A.D.: warming trend (= Medieval warm period)
- 1320- 1840's: global climate gradually became colder (= Little Ice Age)
- since 1870: steep increase in the mean annual surface temperature

example of climate changes: temperature

-globally averaged measurements of mean annual air temp at earth's surface, In the last 100 years, Earth's average surface temperature increased by about 0.8
�C (1.4 �F) with about two thirds of the increase occurring over just the last three decades
-If

What processes drive climate change?

1. astronomical cycles
2. volcanic eruptions
3. changes in ocean circulation
4. changes in the Earth's albedo

Astronomical cycles (also known as Milankovitch cycles)

The configuration between the Earth and the Sun is continuously changing, and these changes have an
impact on the amount of solar energy reaching any given point on the Earth's surface; i.e. have a climatic
effect.
-eccentricity, tilt of the Earth's axis,

Eccentricity

-100,000 year cycle (longest cycle)
-The Earth orbits around the sun (it takes 1 year), but the sun is never at the center of the orbital path
-At times of the year, the Earth is closer to the sun, and at other times, the Earth is farther away from
the su

Tilt of the Earth's axis

-The Earth's axis is tilted relative to the plane of its orbit
-Angle of that tilt changes through time going from a maximum tilt of 24.5� to a minimum tilt of 22.5�;
present tilt is 23.5�
-It takes 41,000 years to go from a maximum to a minimum and back

Precession of the Earth's axis

-23,000 year cycle (shortest cycle)
-When the Earth orbits the sun, the Earth does not come back to exactly to the same place one year
later, so the Earth is not precisely in the same position year after year when the Fall or Spring
equinox occurs (equino

Milankovitch cycle impact on climate

-All three cycles operate simultaneously such that when the curves are in phase they generate climatic
extremes
-If we consider the precession of the equinoxes + axis tilt + eccentricity together, at some point in time,
the Northern Hemisphere is tilted t

Volcanic eruptions

-Eruptions produce fine ash and volcanic aerosols that increase the opacity of the atmosphere by acting
as nuclei for cloud formation, clouds that reduce the penetration of solar radiation into the lower
atmosphere; Therefore eruptions act to cool the low

Changes in ocean circulation

-As water evaporates from the low-latitude Atlantic Ocean, it leaves behind saltier water. This warm but
denser higher-salinity water flows toward the North Pole where it cools, becomes yet denser and sinks
to the ocean bottom, thus drawing more warm wate

Changes in the Earth's albedo

Long-term effects on climate tend to be controlled by the Earth's albedo, which is the degree to which
the Earth reflects solar energy back into outer space

The "Greenhouse effect

-defined as "effect caused by certain gases whereby they transmit a large fraction of the shorter-wavelength solar radiation but strongly absorb longer-wavelength (infrared) radiation reflected by the Earth's surface"
-Is a natural effect; the planet's su

Anthropogenic (man-induced) effects on the Earth's albedo

If we (humans) change the concentration of any of the greenhouse gases, we are changing the
Earth's albedo (Albedo = Solar energy reflected back into outer space = Solar energy received -
greenhouse effect)

Greenhouse Gases

-Global measurements of CO2, and other greenhouse gases, taken for example in Hawaii far away
from industry and cities, show a steep increase in the atmosphere from 1958 to the present
-By combining the temperature and CO2 data sets, climate is showing a

role of geoscientists

decipher magnitudes and rates of climate change before significant human influence (pre-1750's), before written records were kept by most cultures

Methods to evaluate pre-historic past climate change

-Ice cores taken in modern glaciers, especially the interior of modern continental ice sheets
-Oxygen isotopes in glacial ice cores or sediment cores from the oceans or lakes

pre-historic past climate change: Ice cores taken in modern glaciers, especially the interior of modern continental ice sheets

-(e.g. cores from modern Greenland, Canadian Arctic Islands, Andes, Himalaya or Antarctica)
-Layers of glacier ice accumulate each year giving an annual record of atmospheric and climatic
conditions
-Ice cores are drilled down into glaciers to sample dept

CURRENT ice core results

Antarctic ice core indicate that MODERN CO2 LEVELS IN THE ATMOSPHERE ARE THE HIGHER
THAN THEY HAVE BEEN FOR > 800,000 YEARS
-Both CO2 and methane (CH4) have varied similarly with the glacial to interglacial cycles; high
concentrations of CO2 correlate wit

pre-historic past climate change: Oxygen isotopes in glacial ice cores or sediment cores from the oceans or lakes

-Any natural compound containing oxygen contains two stable isotopes of O: 16O (which is far more
abundant) and 18O, both of which occur in water that evaporates
-Method relies on the ratio between oxygen isotopes, i.e. both isotopes are measured

Glacial Ice Cores

-Measure the 18 O/ 16O ratio in the frozen H2O to infer atmospheric temperature at the time the snow
formed; this is a paleothermometer
-16O evaporates more readily than 18O. When the climate is cold enough to support glaciers, the
atmospheric water vapor

Greenland ice cores

indicate that compared to the rate of ancient change of the steep part of the global temperature of the last 30 years, which equals approximately 0.4 �C in the last 30 years, the natural system experienced pulses of much more rapid rates of change in the

Marine sediment or lake sediment cores

-Oxygen isotope ratio from fossilized skeletal remains, usually microscopic organisms are used to infer prehistoric temperatures
-When sea water evaporates, the lighter isotope 16O evaporates preferentially so that rain water or glacial
snow layers are ri

Anthropogenic greenhouse-gas production

if we compare the measured levels of CO2 and CH4 in the atmosphere, they are higher than any levels recorded geologically in the last 800,000 years-in the last 30 years, greenhouse gases became much larger than any other climate forcing mechanism

What is the percent of greenhouse gases that have remained in the atmosphere?

58% have stayed in the atmosphere in the last 45 years

Where did the 42% go?

The anthropogenic production of CO2 is tied to the natural Global Carbon Cycle
-Photosynthesis by green plants consumes CO2
-Vegetative biomass, peat and coal remove CO2 from the atmosphere and store carbon, making deforestation an important issue because

Lag effect

Greenhouse gases ALREADY INTRODUCED to the atmosphere as well as all natural factors
have produced a measured temperature increase of about 0.8 �C (= 1.4 �F in the Earth's average surface
temperature) over the last 100 years, with about two thirds of the

What are the implications of 2-3 �C of additional climate warming?

-Uneven response of local climate: not everywhere will warm on Earth; some regions will experience
colder and drier climates or colder and wetter climates, making prediction of local climate extremely
difficult. We use computer-generated Global Climate Mo

What is the basic control on weather and climate patterns?

atmospheric convection cells

atmospheric convection cells

-Results in latitudinal belts of deserts
-With climate change these belts shift or broaden and affect the potential for successful agriculture in the
adjacent areas
-SW MN, the Dakotas, Nebraska etc. sit on the edge of this belt, and only a slight decreas

What happens if the dry Polar-easterly belts shift or broaden?

The polar continental ice sheets enlarge
Best evidence for climate change in MN is a record of glaciation; our landscape has been sculpted by
glaciers

2 main types of glaciers

1. mountain glaciers
2. continental glaciers

mountain glaciers

(= alpine glaciers = valley glaciers) - occur in hilly or mountainous areas at any latitude
-usually flow down a pre-existing stream valley
-very efficient erosional agents
-Can glaciers occur at low latitudes near the equator? Yes, but at higher mountain

continental glaciers

(= polar ice caps or continental ice sheets)
-form on land; are dome shaped; cover large areas
-get very thick (3-4 km or 1-2 miles thick)
-are not confined by valleys
-flow outward toward their edges under the force of gravity
-modern examples: Antarctic

Past Continental-scale Glaciations

Evidence for past continental periods of glaciation comes from the same types of deposits and
features as observed in MN (moraines, eskers, kettle lakes, etc.) except that in most cases the
glacial deposits are now sedimentary rocks - in MN the glacial de

Periods of glaciation

More than one period of glaciation in Earth history
-800-600 million years ago - multiple ice ages
-315-245 million years ago - Permo-Carboniferous glaciation associated with Pangea (provided some evidence for continental drift)
We are not as aware of the

potential causes of glaciation

-Astronomical cycles, changes in ocean circulation, eruptions, changes in the Earth's albedo, etc
-- Probably not a single explanation for all ice advances
-Most theories require cooling of atmosphere, but they differ on the combination of mechanisms

budget of a glacier

-every glacier has an accumulation zone where snow accumulation > ablation
*accumulation = snowfall
*ablation = melting + sublimation +/- calving of icebergs
-every glacier has an ablation zone where ablation rate (by any of the 3 ablation processes) >!
a

What happens as snow and ice accumulate on land?

1) Isostasy - subsidence of the continental lithosphere due to ice load
2) Sea level falls as water is stored in the ice sheets
3) Glaciers erode the landscape and deposit sediment

glacial isostasy

-ice build-up is usually slow
� builds up gradually for 100,000 years (period of eccentricity)
� the weight of the ice on land depresses the continents and the continent slowly subsides as soft
material in the asthenosphere flows out
� at glacial maximum

Sea level falls as water is stored in the ice sheets

-At last maximum extent of continental glaciers, sea level was approximate 100 m lower than at present
exposing most of the continental shelves above sea level
When glaciers retreat:
-water is returned to sea, so sea level rises
-as sea level rose, the ma

Glaciers erode the landscape and deposit sediment

basal sliding; glacial erosion; transportation; deposition of sediment

Basal sliding

most glaciers have water at the base, due to warm land surface, pressure melting or
groundwater; lubricates the base of the ice and enhances the glacier's flow and its capacity to
erode

Glacial erosion

-generates some of the most spectacular landscape on Earth
-results in large-scale redistribution of rock mass over the Earths surface

Transportation

once sediment is incorporated into the ice, a glacier will transport nearly everything from clay-sized dust to house-sized boulders

Glacial fluvial (river) deposits

-glacial debris deposited by surface melt water from the glacier; called outwash
-large amounts of debris are carried away from the glacier's margin by melt water
-Such deposits fill pre-glacial valleys as in the Twin Cities

Large glacial meltwater lakes

-for example, Lake Agassiz in NW Minnesota, where the Red River now flows
-These lakes and rivers drained all the way to the Mississippi Delta in Louisiana
-More locally, these lakes drained down our river valleys (Minnesota and St. Croix
Rivers) and resu

Moraines

-Till is deposited directly by the ice; consists of!poorly sorted clay-sized rock fragments to boulder
-Moraines are till deposits from the base and margin of the ice that mark the extent of the glacial
advance
-If ice margin melts back and stabilizes at

Why are lakes important???

- 1% of Earth's continental land surface, but in MN lakes represent a much higher % of the landscape
Lakes provide detailed high-resolution record of climate at one locality (in contrast, the ocean provides a slower, lower-resolution record with effects m

Origins of lakes

1) Meteor impact craters
2) Collapse of volcanic cones to form crater lakes
3) Moraine-dammed glacial lake (occurred in MN)
4) Kettle lakes in depressions formed by melting of isolated ice blocks during melting and retreat of
glaciers (occur in MN -- Note

Playa lake

any type of lake, but ones in which the climate causes
them to intermittently dry up completely or partially for a time interval of one season to years
-No examples in MN, but several in N.D. and Manitoba
-Evaporation of the water results in deposition of

what are the long term prognosis of lakes

they fill with sediment

Sediment cores from lake bottoms

Provide detailed high-resolution record of climate at one locality
-Layers of sediment in lakes provide a yearly record of the climate
-Infer temperature and precipitation from the fossil pollen from plants growing in the vicinity of the
lake
-Very import

Where does water in lakes come from?

Lakes that have no outlet stream(s) are controlled by the balance between precipitation rates, evaporation
rates, inflow streams, and groundwater flux in and out of the lake. In many such lakes, groundwater flux
controls the lake level !$

Water IN (to a lake)

-precipitation directly on lake surface or runoff through soil
-surface streams and tributaries contribute water from the drainage basin
-ground water influx through the bottom/sides of the lake

Water OUT (of a lake)

-evaporation
-outlet streams
-ground water percolation--infiltration out through the sediment at the lake bottom and sides

Stratification (layering) of the water in lakes -- 2 types

1) Temperature stratification
2) Chemical stratification

Temperature Stratification

warmer water >4�C (>39� F) constitutes a less dense surface layer overlying a cold layer with temperatures closer to 4�C (39� F)
-Nutrients (NO3, PO4) from streams enhance organic productivity; organic debris settles, oxidation (rotting) consumes oxygen;

chemical stratification

(common in arid/warm regions) - evaporation leaves denser, saltier brine behind that settles to form the lower water layer, which often shows O2 depletion similar to the case above

Stratification (layering) of the water in lakes FACTS

-Many lakes are both chemically and thermally stratified as evaporation rates increase as temperature
increases; typical in tropical or low-latitude regions that lack seasons
-Lake must be deep enough for layering to form below the depth to which waves ca

Seasonal Cycles in a permanent lake: Summer

stratification as described above (layering)
-generates layer of black organic-rich clay
-or in many lakes, the influx from surface tributaries is high enough to keep the water mixed
and sediment is delivered to the lake bottom
-after overturn, generates

Seasonal Cycles in a permanent lake: Autumn

as upper water layer cools (Autumn), it goes through the critical temperature of 4�C (39�
F) at which water is densest - lake overturns due to density instability (heavy water overlying light
water) bringing oxygen and organic debris to the lake's bottom

Seasonal Cycles in a permanent lake:Winter

stratification forms as surface ice floats, the upper layer is <4 �C (<39 �F) and therefore lighter, and the lower layer is approximately 4� C (39 �F)
-generates very thin deposit dark-colored clay layer as detrital sediment does not reach the lake due to

Seasonal Cycles in a permanent lake:Spring

as surface water layer warms in Spring to 4� C (39 �F) - turnover occurs again with 2nd influx of oxygen to lower layer and mixing of the water column
-after overturn, generates layer of light-colored silt from steady rainout of sediment suspended in a
mi

seasonal cycles in an ephemeral or playa lake: rainy period (monsoon season)

no stratification, low-salinity phase; algae and plankton bloom in the warm surface layer
-generates thin layer of dark-colored organic-rich clay

seasonal cycles in an ephemeral or playa lake: dry season

Thermally and/or chemically-stratified water column
-generates white salt crystals or light-colored limey (carbonate) silt due to evaporation and concentration of mineral salts in the water, which sink to the bottom