HG study guide #2

• The potential to fit the continents together like puzzle pieces had been noted before (as early as 1596!), but…

• Alfred Wegener (1912, 1915) marshaled many lines of evidence that the continents had moved

• Many Permian-Triassic aged fossils are found across the southern continents

• Easy to explain if they were once joined (Gondwana)

• Distribution of Carboniferous-Permian Glacial Deposits 

• glaciers will be in cold areas where it will be ice all year round

• continents were located closer together and centered around the South Pole

• Distribution of older rocks matches up too

1. Divergent

2. Transform

3. Convergent

• pulling apart

• faulting/rift forming leads to volcanic and earthquake activity

• continental rift: faulting forms a rift valley and parallel valleys along it

• sliding past

• Can be ocean-ocean or continental

• Plates slide past on strike-slip faults

• Lithosphere isn’t created or destroyed

• Earthquakes common

• one plate pushed under another at subduction zone

• Ocean-ocean: colder and denser plate subducts, oceanic trench forms

  • Volcanic arc/orogeny on plate that isn’t subducted

• Continent-ocean: oceanic plate subducts

  • Volcanic arc/orogeny on continental plate

• Continent-continent: neither plate wants to subduct

  • Lithosphere shortens via crumbling and thickening

  • Leads to faulting and folding

• Wegener had no plausible explanation for how the continents moved – he proposed that they plowed across the ocean floor

• Most geologists rejected his theory for this reason

• low density

• felsic (feldspar-rich)

• underlies continental shelves as well as land

• high density

• mafic (iron-rich)

• very high density

• ultramafic (very iron-rich)

• = Moho (Mohorovicic discontinuity)

• marks an increase in rock density

• = crust and upper mantle

• firmly attaches, forming a rigid layer

• = slushy, partially molten layer below the lithosphere

• sinking blocks of crust

• earthquakes are going to happen when there is pressure, variety of depths

• sed rocks = most likely to have layers

• strike line = horizontal line

• tick mark records that it is going downhill

• number = magnitude of dip

1. Concave up

2. Layers dip toward each other

3. Youngest rock in the middle

1. Concave down

2. Layers dip away from each other

3. Oldest rock in the middle

• landslide risk

• oil exploration

• mineral resources

• groundwater resources

• Axial plane – boundary between two plates

• Syncline and anticline

• (saltwater) Oceans ~ 97.5%

• (freshwater)

• Ice caps and glaciers ~79%

• Groundwater ~20%

• Water in lakes ~52%

• Water in rivers

• Water in living organisms

• Water vapor in atmosphere

• Water in soil ~38%

• Evaporation, precipitation, groundwater, pools (reservoirs) and fluxes, evapotranspiration

• Water enters the atmosphere (air) by:

• Evaporation: conversion to water vapor from bodies of surface water (lakes, oceans, etc.)

• Evapotranspiration:

  • Water vapor is lost to the air by plants

  • Delivers groundwater into the atmosphere

• water enters plants through roots

• flows up vascular (tubelike) tissues

• leafe stomata:

  • Openings for gas exchange

  • water loss through these openings

  • draw additional water up the vascular tissues

• this process delivers water from soil (groundwater) to atmosphere (air)

• drying up of an area due to lack of plants

• no mechanism (plants) to get groundwater into atmosphere —> area dries up

• determines Earth’s climate over very long timescales

• volcanism releases carbon dioxide

• rock weathering absorbs (fixes) carbon dioxide

• Hydrogen Sulfide

• Methane (carbon gas — greenhouse gas)

• Carbon Dioxide (carbon gas — greenhouse gas)

• Water Vapor

• Atmospheric carbon enters biological systems via photosynthesis

• CO2+H2O => C6H12)6(sugar)+O2

• Carbon is then repurposed into a wide variety of organic molecule

  • fats/oils (lipids)

  • Proteins

  • Nucleic acids (DNA & RNA)

• ex.) “Blue-Green Algae” (photosynthesizing bacteria)

• ex.) Tree photosynthesizing

• Ancient Organic Material

• Generally the result of photosynthesis

• Contain many high-energy bonds → burn readily

• Major contributor to climate change

• Include:

  • Coal

  • Oil

  • Natural Gas

• Coal:

  • Accumulated plant matter

  • Start as peat bogs

  • Anoxic conditions prevent decay

• Oil:

  • Accumulation of algai biomass

  • Accumulate at bottoms of oceans and large lakes

• Organisms live and die

• Organic material is buried

• Heat and pressure alter molecule structure (not unlike metamorphic rock formation)

• Rocks exposed to air and water are susceptible to chemical weathering

• Produces:

  • Clastic sediments

  • Dissolved ions (including Ca+)

• Ca+ & CO2 can combine to form CaCO3 → limestone

• Can be caused by an excess of nitrogenous nutrients

• Results in anoxia

• Healthy Water living organisms: fresh water and natural balance between water organisms, oxygen, algae and nutrients: enough sunlight for water plants

• Eutrophication dead zone: explosive algal bloom due to excess nutrients coming from nitrogen (N) and phosphorus (P) rich pollutants; oxygen (O2) consumption by algae, reduced by sunlight, breakdown of natural chemical cycles

• Nitrate and Phosphate fertilizers

• Concentrated animal feeding operations

• Direct sewage and industrial waste discharge into water

• Improperly managed aquaculture

• Natural events

  • Stream discharge

  • flood events

• Flows from high pressure to low pressure

• Hot air rises, cold air sinks

• Locations that produce hot air, produce low pressure zones

• Warms air rises

• Dry air descends, warms, and becomes even drier

• Warm air rises, cools and loses moisture

• Dry belts at ~30º N/S and at the poles

• Can be altered by local geography

• Gases like O2 and O3 absorb Ultraviolet radiations

• Blocked from Earth’s Energy Budget

• O2 and O3 let through visible and some shortwave radiation

• Greenhouse gases also let through visible and shortwave radiation

• Earth absorbs visible and shortwave radiation

• Earth re-radiates near-and IR radiation

• Greenhouse gases absorb and re-radiate and near-and IR radiation

• reflectiveness of a surface

• controls how much energy is absorbed (as heat) vs reflected

• land vs water

• forest

• deserts

• glacial ice

• Availability of ice

• Bare rock vs foliage cover

• Information accessible in the present that indirectly indicates conditions in the past

• How do we get temperature info from the distant past? → PROXY DATA:

• Tree ring thickness and density

• Coral reef annual layers

• Lake sediment – snowmelt and pollen

• δ¹⁸O in mollusk shells and ice cores

• Various metrics agree that current average temperatures are higher than they have ever been in the past 1000 years.

• Rocks specific to climate:

  • E.g. Glacial deposits

  • E.g. Bauxite, only forms in tropical climates

• Fossil organisms present

• Geochemical data

  • Stable isotopes

  • Chemically equivalent, but have different masses

• some organisms live in specific environments. Their fossils indicate climate

• Cycads and crocodilians prefer tropical climates today but lived closer to the poles in the past – therefore, mid-to-high latitudes must have had warmer climates than today

• cold-adapted floras have more jagged leaves; warm-adapted ones have more smooth leaves.

• The percentage of smooth leaves in a fossil flora can be used to determine temperature.

• plants respire through openings called stomata

• In woody plants, the number of stomata increases as CO2 decreases

• Low CO2 correlates with low temperatures, so more stomata suggest low temperatures too

• at low temperatures, heavy isotopes move slower

  • this is fractionation

• Forms of an element that have a different number of neutrons than usual

• Atomic mass is different but chemical properties are the same

• Each radioisotope has a characteristic

• Half Life: the time it takes for half the atoms to decay

• On a linear scale, decay follows a hollow curve

• On a logarithmic scale, decay follows a straight line

• Short half-life good for young samples

• Long half-life good for old samples

Why?

• Must have enough parent isotope left to measure accurately

• Enough time must have past for some daughter isotope to accumulate

• In radiometric dating, we measure the ratio between parent and daughter isotope

  • There is no daughter isotope to begin with

• Above the blocking temperature, the daughter isotope diffuses out of a mineral

• Below the blocking tem, the daughter isotope collects

• Thus, a radiometric age is the age since the mineral last cooled below the blocking temp.

  • Can represent time since cooling from a magma or cooling after metamorphism

• Blocking temp. depends on the mineral and isotope

• Procedure: date igneous rocks and determine relative age of associated sedimentary rocks using:

  • Principle of Superposition

  • Principle of Cross-Cutting Relationships

  • Principle of Inclusions

• Radiometric dating does not tell you when a sedimentary rock was deposited

• Linear Magnetic Anomalies: can reverse time by “deleting” stripes of opposite polarity from oceanic ridges

• Paleomagnetism: magnetic grains are vertical at poles and horizontal at equator (measures paleolatitude)

• Paleobiogeography: can indicate paleolatitude (based on climate) and proximity of continents

• Paleoclimatology

• Regional Geologic and Tectonic History: matching up geologic features to find paleolongitude 

• Some rocks are only deposited in certain climatic conditions

• arid

• swamp

• high precipitation

• warm water

• Cenozoic and Mesozoic — Excellent

  • Magnetic seafloor anomalies allow precise reconstructions

• Paleozoic — Good

  • lots of paleomagnetic and geologic data

• Neoproterzoic — topic of current research

  • data are more sparse, but they are being collected

  • there are competing interpretations

• Earlier in the Precambrian — much less known

  • much less rock on which to work

• Cratons

  • large under-formed portions of continents

  • primarily Precambrian

• Precambrian Shield

  • craton exposed at surface

  • Canadian Shield exposed by glaciation

• continental crust formed during Archean

• high heat flow early in Earth’s history required small continents

• thinner sections of crust

• the Canadian Shield (northeastern Canada)

• stars cluster in galaxies

  • organized in disks

• Milky Way

  • our galaxy of stars

• Expanding universe

  • galaxies

    • redshift

  • originally concentrated into a single point

• Big Bang

  • ~13.7 billion years ago (age of universe)

• Galactic matter in concentrated

• stars form

  • our sun

• supernova

  • exploding star

• solar nebula

  • dense rotational cloud

• Fragments of larger rocky bodies that have undergone collision and broken into pieces

  • Provide important information concerning age of Earth

• Stony meteorites

  • Rocky composition

• Iron meteorites

  • Metallic composition

• Stony-iron meteorites

  • Mixture of rocky and metallic

  • Proxy for core composition

• Most date around 4.6 billion years ago

• 4.6 Ba

• Earth materials differentiated

  • dense at center

  • less dense silicates rose to surface

    • magma ocean

  • cooled to form crust

• meteorite impacts increased concentrations of some elements in upper Earth

• Early Atmosphere

  • Degassing from volcanic emission

  • CH4 and NH3 abundant

  • Little O2

    • No photosynthesis

• Earth’s oceans

  • Volcanic emissions cooled, condensed

  • Salts

    • Carried to sea by rivers and introduced at ridges

    • Approximately constant through time

• small Archean fragments

  • high heat flow limited continental thickness

• Zircon crystals

  • 4.1-4.2 B years old

  • weathered from felsic rocks

• Canadian Shield

  • 3.8-4.0 B years ago

• Greenstone belts

  • Weakly metamorphosed

  • Previously ancient sedimentary rocks

  • Abundant chlorite

    • Green color

  • Nested in high-grade felsic metamorphic rocks

• Greenstone belts contain igneous rocks

  • Volcanics contain pillow basalts

    • Underwater extrusion

  • Formation of sediments in deep water

    • Greywackes, mudstones, iron formations, volcanic sediments

• Continental accretion

  • Deep water sediments accreted to continent

  • Marine sediments form wedge between continental masses

• Smaller and simpler

• No organelles

• Many different metabolisms, including anaerobic

• Bacteria and Archaea

• Larger, more complex

• Organelles, incl. nucleus

• Generally require oxygen

• Some lineages evolved multicellularity

• Some think on the ocean floor, where hot, nutrient-rich waters are expelled

• Mid-ocean ridges, hydrothermal vents

• Not many sedimentary rocks older than ~3.5 Ga (=billions of years ago)

• Pretty good evidence there was life by at least 3.5-3.2 Ga, although people argue about the fossils and the exact date

• Hematite tubes: Possible microbial sheathes from 3.77-4.28 Ga, Quebec. Would have lived in a hydrothermal setting

• Other scientists dispute both the age of the rocks and the biological nature of the structures.

• ~580 Ma

• Fossils of non-mineralized “seaweeds” are relatively rare

• There were animals, but most of them…

  • Did not belong to modern phyla (e.g., echinoderms, arthropods)

  • Did not have legs, fins, etc.

  • Did not have obvious sense organs

  • Did not have obvious body openings (probably fed by absorption)

  • Did not have mineralized skeletons

  • Did not burrow into the sediment, at least deeply

• Almost all modern phyla with skeletons appeared

• Animals with legs, fins, body openings, sense organs, and skeletons were abundant

• Marine ecosystems are relatively modern: herbivory, predation, suspension feedings, etc. were present

• Burrowing and bioturbation increased

• Laurentia (N. America) rifts from Gondwana

• Laurentia coastlines become passive margins all the way around

• Sedimentation accommodated by:

  • Sea level rise

  • Thermal subsidence

• Sediment Sources:

  • Silicate weathering from continent

  • Carbonates precipitating from water

  • Biogenic carbonates

• Sea level low at beginning of Cambrian

• Rises over course of the period

• Laurentia (N. America) is almost entirely drowned by the end of the period

• Dominated by terrigenous sands

  • Mature silicates

    • Almost solely quartz

    • Rounded, well-sorted grains

  • Evidence for aeolian (wind-based) weathering

• Carbonates on shelf edges, far from terrestrial sources

• Toward end of Cambrian period, continent flooded

  • Way less terrigenous sediment

  • Widespread carbonate deposition

    • Tropical temperatures

    • Agitated water → carbonate precipitation

    • Rise of skeletal carbonate

• Continents above sea level

• NO PLANTS

• ex.) Still from the opening sequence of the movie Prometheus. This sequence depicts erosion and transportation of sediment on a landscape without plants.

• Minerals for structure and protections

  • calcite/Aragonite

  • Apatite

  • Silica

• Ediacaran trace fossils: simple horizontal burrows

• Cambrian trace fossils: some are more complex, branching and/or probing vertically into the sediment

ex.) Treptichnus pedum: defined the base of the Cambrian

• Ediacaran/Cambrian boundary type section

  • Fortune Head, SE Newfoundland

• Microbial mats cover seafloor

• Only shallow horizontal burrows

• Animals can attach to mat

• Bioturbated “mixed layer” replaces mats

• More burrowing, some vertical burrowing

• Sediment surface less stable

• Started at high sea level with extensive epicontinental seas

• Subduction zone forms on east coast of Laurentia

• Taconic orogeny at the end of the Ordovician

  • Island arc accreted to Laurentia

• Graptolites: colonial filter feeders

  • Good index fossils, wide geographic range b/c some floated closer to surface

• Corals: functioned as reef-builders

  • Rugose Corals

  • Tabulate Corals

  • Ecosystem engineers

  • Extinct now

• Cephalopods: nautiloids (curved walls) and ammonoids (zigzag walls)