Fossils and Tectonics

list some tectonic settings + what they are

• Mid ocean ridge = new crust being formed

• Mantle plume = hot spot creating islands and volcanoes

• Seamounts and reefs

• Subduction zones = form at convergent boundaries -> subduction at deep sea trench causes volcanic arc to be made on overriding plate -> forms mountain range

• Passive margins = continental margins that used to be a site of active continental rifting

  • Two continental masses moving apart

  • No longer active

  • Source of energy resources like oil and gas

  • Rich sources of mineral deposits

what are mid ocean ridges

Sites where two tectonic plates move apart forming new oceanic crust by the passive outpouring of magma from the mantle

• Magma cools to form new rock

• Two oceanic tectonics

  • Only exposed at surface in Iceland

  • Rift systems in Iceland

  • Iceland lays between north American plate and Eurasian plate

• Form ocean mountain ranges

  • Higher elevation near mid ocean ridge

• Active volcanism

• Move away from mid ocean ridge

  • Oceanic elevation lowers and ocean floor deepens

  • Accumulation of sediment when old crust ages and subsides

• Mafic igneous rocks form

what are mantle plumes

Mantle upwelling of very hot mostly solid rock

what is the mantle plume hypothesis

1. Columns of really hot mantle rise from deep earth and develop heads, so when they reach earth's crust it causes a volcanic eruption

   1. Initiate continental break up, triggering mass extinction events, changing global climate due to volcanism

   2. E.g. in siberian traps causing mass extinction

2. After all the magma from the head is used up, the stem of the magma plume then is linked to the volcano

3. Lower volcanic activity

   1. Above sea level: Progressive volcanic island chains formed

      1. As the plate moves it creates a series of islands

      2. As original volcano moves away from magma plume it becomes extinct

      3. E.g. hawaii

      4. When volcano moves away from hotspot, it subsides as crust gets older -> eventually go below sea level

1. Below sea level: seamounts

2. Generally coral reefs form around these islands

what are subduction zones

When oceanic crust interacts with continental plate or two oceanic plates interacting

• Oceanic crust = denser = subducts under lighter continental crust

• Sedimentary rocks get scraped off subducting surface and form accretionary prism

• Rocks can be metamorphosed in subduction zone

• Partial melting causes igneous rocks to form and erupt at surface through volcanoes

• Mountain range forms with high elevations

  • Glaciated environments formed due to altitude

  • Flow of glaciers overtime creates wide valleys

    • Ice causes mechanical weathering

    • As ice falls, it transports debris with it

    • Rocks erode to become sediment

    • Tillites form from this sediment

  • Presence of tillites = the area was once glacial

what is Source to Sink process

Rivers transporting sediment from higher elevation (mountains) to lower elevations (deltas, oceans)

• Tectonism in orogens crushes rocks and create faults

• Faults exploited by weathering reactions -> crack that water, oxygen, etc… get into

• Erosion in the highlands from water and wind

• Sediment transported by river/fluvial networks into plains and oceans

what is the main driver of source to sink process

Rain shadows created due to presence of mountains

• Warm moist air comes from bodies of water

• Hits large mountain range and rises up

• When it reaches certain altitude -> condenses and causes rain

• Highly vegetated on the rainward side of mountain

  • Good for agriculture

• Rainshadow = Dry, arid region on other side of mountain

  • Desert regions

  • Since all rain is used up on one side

E.g. in andes

transportation vs sediment grain size

• Higher elevation = steeper gradients and coarser grain materials

• Lower elevation = smaller grains

  • E.g. transition to plains and flats

  • Sand

• Close to river delta = smallest grains

  • Silt and clay

what are deltas

When river meets base level -> at ocean or large lakes

• Sink of source to sink system

  • E.g. Nile, Ganges river deltas

• Sediments pile up and accumulate

• High rates of subsidence from compaction

  • Sink due to weight of sediment compacted

• Shape of delta reflects amount of sediment being deposited into system

what are passive margins

Inactive margins along continents that were once the site of plate boundary processes

• Once site of continental rifting

• Once seafloor spreading was established, plate boundary becomes an ocean

• Contain large amounts of sediments deposited

• Lots of oil and gas here

how do passive margins form

begin as continental rift systems

→ Occur when divergent boundaries form withing continents due to

• Extensional forces

  • E.g. other plate boundaries around the area causing continent to be split apart

• Passive upwelling of mantle plumes

  • Uplift due to rise of plume, causes land to dome where the plume lands

  • Outward impact of plume opens up rifts

  • Spreading apart at rifts

  • Volcanic dominated margin

  • E.g. rift system in east africa

• Large amount of sediment produced

what is Continent-continent collision

two continental plates colliding

Neither continental crust subduct since they are both buoyant

• Forms a crumpled zone of rocks due to collision

• Extensive faulting to create high mountains

• Faulting occurs over a really large area

• Metamorphic rock due to extreme changes in pressure and temperature

  • Burial

  • Uplift

  • Range of metamorphic rocks formed

 

example of continental-continental collision and its impact

E.g. India colliding with Eurasia to form the Himalayas

• Large amounts of erosion

• Lots of sediment travel down Ganges river

• Large amounts of sediment accumulate at Bay of Bengal

what evidence we use for Plate tectonic reconstructions

Seafloor spreading

Palaeomagnetic data

how does seafloor spreading allow us to do plate tectonic reconstruction? when do we use it?

Generally record motion younger times

• Generate new crust

• New crust cools and traps direction of magnetic field at that time

• Since earth magnetic field reverses polarity

• Continual generation of new crust record each time field changes polarity

• Get a symmetrical pattern of magnetic anomalies on either side of mid ocean ridge

• Can determine the age of oceanic crust by looking at magnetic anomaly signatures

We can then move the mid ocean ridges together and move continents to where they were back in time based on which rock was being made

what are Magnetic anomaly identifications? how are they used?

Map out magnetic anomalies using signatures in rocks

• Assign an age to them using seafloor spreading and calibrating to geomagnetic timescale -> age constraints (time of motion)

  • Closer to mid ocean ridge = less deep = younger

  • Further away form mid ocean ridge = deeper = older

  • We know at which times the earth changed polarity so we compare them to the magnetic signature in the rock to see when it was made

    • Closer to mid ocean ridge = stronger magnetic anomalies

    • Since less sediment cover and absorb signal

• Fracture zones tell us what direction the seafloor was spreading at that time -> movement constraints

what is Palaeomagnetic data? why do we use it?

Generally record motion for older times

If we don't have constraints (age and motion) from seafloor spreading, we use this data instead

• Present day earth has magnetic fields

• Field lines are represented by series of arrows

  • Parallel to surface of earth at equator

  • Perpendicular to surface of earth at magnetic pole

  • Each latitude has a different angle between the surface and the earth's magnetic field

• By knowing this angle = we can determine latitude of this rock where it was formed

  • Since rocks trap information about magnetic field -> rock can be magnetised

  • Where they are formed is usually not where they end up