Sections 9 and 10 - Plate Tectonics 

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

• Divergent: pulling apart

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

  • Faulting/rift forming leads to volcanic and earthquake activity

• 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 crumpling 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

Crust and Upper Mantle

• 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 and Upper Mantle

• Crust-mantle boundary = Moho (Mohorovicic discontinuity)

  ~ Marks an increase in rock density

• Lithosphere = crust + upper mantle

  ~ firmly attached, 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

Geologic Subsurface Structures

• 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

Section 11 - Biogeochemical Cycles

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

• Draws 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

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 molecules

• 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 algal 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)

Wildfires: Another Source of Atmospheric Carbon

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

Section 12 - Paleoclimate

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 & 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

Consider Also: Albedo

• 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 Conditions 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

Proxy Data

• Fossils – 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

Proxy Data

• Lead Margin Analysis – 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.

Proxy Data

• Stomatal density – 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

Proxy Data

• Oxygen isotope ratios – at low temperatures, heavy isotopes move slower. This is terms fractionation.

Section 13 - Geochronology 

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 dates 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

Section 14 - Paleogeography

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 & 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

Section 15 - Precambrian Geology ** need oldest regions of crust (where are they?)

Precambrian Geology

• Cratons

• Large under-formed portions of continents

• Primarily Precambrian

• Precambrian shield

• Craton exposed at surface

• Canadian Shield exposed by glaciation

Oldest Region of Crust: the Canadian Shield (northeastern Canada)

• Land bodies colliding with continents

• More continental crust through volcanic rocks – more felsic materials/minerals, lower melting point

Precambrian Geology

• Continental crust formed during Archean

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

• Thinner sections of crust

Origin of the Universe

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

• Provide important information concerning age of Earth

Origin of the Universe

• 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 the Universe

• Stars cluster in galaxies

• Organized in disks

• Milky Way

• Our galaxy of stars

• Expanding universe

• Galaxies move apart

• 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

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

Origin of Earth and Moon

• 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 old

Origin of Continents

• 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

Origin of Continents

• Continental accretion

• Deep water sediments accreted to continent

• Marine sediments form wedge between continental masses

Section 16 - Precambrian Life

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

Where did life begin?

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

• Mid-ocean ridges, hydrothermal vents

When did life begin?

• 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

Section 17 - Cambrian Period

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 an/or probing vertically into the sediment

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

• Ediacaran/Cambrian boundary type section

  • Fortune Head, SE Newfoundland

Increase in sediment disturbance during the early Paleozoic

• 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

Section 18 - Ordovician Period

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)