HG study guide #2

Continental Drift

• 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

Boundary Types

1. Divergent

2. Transform

3. Convergent

Divergent

• pulling apart

• faulting/rift forming leads to volcanic and earthquake activity

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

Transform

• sliding past

• Can be ocean-ocean or continental

• Plates slide past on strike-slip faults

• Lithosphere isn’t created or destroyed

• Earthquakes common

Convergent

• 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’s “Continental Drift”

• 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

Continental Crust

• low density

• felsic (feldspar-rich)

• underlies continental shelves as well as land

Oceanic Crust

• high density

• mafic (iron-rich)

Mantle

• very high density

• ultramafic (very iron-rich)

Crust-Mantle Boundary

• = Moho (Mohorovicic discontinuity)

• marks an increase in rock density

Lithosphere

• = crust and upper mantle

• firmly attaches, forming a rigid layer

Asthenosphere

• = slushy, partially molten layer below the lithosphere

Most Earthquakes and Volcanoes Occur at Plate Boundaries

• sinking blocks of crust

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

Strike and Dip

• sed rocks = most likely to have layers

• strike line = horizontal line

• tick mark records that it is going downhill

• number = magnitude of dip

Syncline

1. Concave up

2. Layers dip toward each other

3. Youngest rock in the middle

Anticline

1. Concave down

2. Layers dip away from each other

3. Oldest rock in the middle

Strike and Dip? Why should we care?

• landslide risk

• oil exploration

• mineral resources

• groundwater resources

Overturned folds

• Axial plane – boundary between two plates

• Syncline and anticline

Relative Size of Different Water Reservoirs

• (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%

Water Cycle

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

Evaporation and 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

Evapotranspiration

• 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)

Desertification

• drying up of an area due to lack of plants

Deforestation

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

Carbon Cycle - Earth’s “Thermostat”

• determines Earth’s climate over very long timescales

• volcanism releases carbon dioxide

• rock weathering absorbs (fixes) carbon dioxide

Carbon Cycle - Volcanic Outgassing

• Hydrogen Sulfide

• Methane (carbon gas — greenhouse gas)

• Carbon Dioxide (carbon gas — greenhouse gas)

• Water Vapor

Biological Carbon

• 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

Fossil Fuels

• 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

Future Coal

• Coal:

  • Accumulated plant matter

  • Start as peat bogs

  • Anoxic conditions prevent decay

Future Oil

• Oil:

  • Accumulation of algai biomass

  • Accumulate at bottoms of oceans and large lakes

Fossil Fuel Formation

• Organisms live and die

• Organic material is buried

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

Why Continental Weathering Draws Down CO2

• 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

Eutrophication

• 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

Human Causes of Eutrophication

• 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

Rules for air flow

• Flows from high pressure to low pressure

• Hot air rises, cold air sinks

• Locations that produce hot air, produce low pressure zones

Global air circulation and precipitation

• 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

Greenhouse Gases

• 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

Albedo

• reflectiveness of a surface

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

Different Surfaces with Different Albedos

• land vs water

• forest

• deserts

• glacial ice

Continental Position Influences Albedo

• Availability of ice

• Bare rock vs foliage cover

Proxy Data

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

Climate Proxy Data: Recent 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.

Proxies for Condition of the More Distant Past

• 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

Fossils: Proxy Data

• 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

Lead Margin Analysis: Proxy Data

• 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.

Stomatal Density: Proxy Data

• 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

Oxygen Isotope Ratios

• at low temperatures, heavy isotopes move slower

  • this is fractionation

What are Isotopes?

• 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

When is a Particular Isotope Useful?

• 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

Blocking or Closure Temperature

• 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

Radiometric Dating Only puts Upper and Lower Boundaries on Date of Sedimentary Rocks

• 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

Reconstructing Paleogeography: Methods

• 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 

Paleoclimatology

• Some rocks are only deposited in certain climatic conditions

Evaporite

• arid

Coal

• swamp

• high precipitation

Most Carbonates

• warm water

Our knowledge of paleogeography

• 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

Precambrian Geology

• 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

Oldest Region of Crust

• the Canadian Shield (northeastern Canada)

Origin of the Universe

• 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

Origin of Earth and Moon

• 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

Origin of Continents

• 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

Prokaryote

• Smaller and simpler

• No organelles

• Many different metabolisms, including anaerobic

• Bacteria and Archaea

Eukaryote

• Larger, more complex

• Organelles, incl. nucleus

• Generally require oxygen

• Some lineages evolved multicellularity

When did life begin?

• 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.

Multicellular Algae

• ~580 Ma

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

Ediacaran Biota

• 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

In the Cambrian

• 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

Cambrian World — Geologic Environments and Events

• 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 in the Cambrian Period

• 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

Marine Sedimentation Around Laurentia

• 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

Why was there so much highly-processed terrigenous sediment?

• 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.

Mineralized Skeletons, what are they?

• 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

Ediacaran

• Microbial mats cover seafloor

• Only shallow horizontal burrows

• Animals can attach to mat

Later Cambrian

• Bioturbated “mixed layer” replaces mats

• More burrowing, some vertical burrowing

• Sediment surface less stable

Tectonic Changes to Laurentia

• 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

Prominent Ordovician Fauna

• 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)