L3 - Plate Tectonics Flashcards
To measure the evolution of the earth, we divide it into geological timescales (based on rock/fossil records). What are the 4 eons and their dates?
Hadean (4540-4000)
Archean (4000-2500)
Proterozoic (2500-541)
Phanaerozoic (541-0)
What are the 3 eras of the Phanerozoic?
Paleozoic (541-252)
Mesozoic (252-66)
Cenzoic (66-0)
What are three primary types of rocks?
- Igneous “fire formed”: formed through the cooling and solidification of magma or lava (we can obtain absolute dates from igneous rocks)
- Sedimentary: formed from chemical precipitates or fragments of earlier formed rocks (provide insight of relative dates of when which events occurred)
- Metamorphic: formed by the application of heat and pressure to either igneous or sedimentary rocks
How do we date rocks?
Absolute dates: radiometric dating of igneous rocks
Relative dating: study the relationships between rocks (sedimentary)
What is radiometric dating?
- We study the radioactive decay of an isotope and back-calculate to when that original rock started
- We date the teeny-tiny microscopic minerals formed within the rocks
What is an isotope?
- All atoms of a precise mass for a given element
- Different number of neutrons gives for a different mass
- Ex: C12 (stable), C13 (stable), C14 (unstable)
- With radio-carbon dating, we can only go back 50 000 years
What are parent and daughter isotopes?
Parent Isotope: initial isotope, radioactive parent
Stable Daughter: the new element produced as a result of radioactive decay
What is the process of isotopic dating of rocks?
- When the rock forms, you assume it contains a certain amount of this unstable parent isotope and NONE of the daughter isotope
- Over time, as that rock ages, you will have less of the parent atoms and more of the daughter atoms
- THUS, the rock can be accurately dated by determining the ratio of parent to daughter atoms
**Isotopic signature
Why do we use isotopes as a dating indicator?
- Their half-life (time required for half of the original population of radioactive atoms to decay) is exactly known
- Therefore the relative concentrations of these isotopes within a rock or mineral can measure the age
What is the principle of superposition?
- Sedimentary rocks allow us to infer an age or the date of an event based on the sequence
- Bottom layers are the oldest, top layers are younger
How does relative dating work?
- The appearance and disappearance of fossils are a good indicator of time
- We can match rocks the fossils were found in to other rocks based on similar sequencing characteristics (even across very large distances)
Where do we find fossils?
- Most abundant in marine sedimentary rocks
- Generally not found in igneous or metamorphic rocks
What are challenges to dating with fossils?
- Different levels of accumulation depending on where the fossil got deposited
- Lack of sediment (e.g. a storm that removed sediment)
- Too much sediment (e.g. a river may dump a large amount of sediment into the sea)
**We can’t use the thickness of the rocks to estimate how much time has passed.
When was the Earth formed?
- The Earth formed about 4.54 BYA out of a solar nebula (swirling cloud made up of bits and pieces left over from old stars that have exploded)
- Gravity pulled in a bunch of dust and gas into this nebula and Earth became the third planet from the sun
What is the Goldilocks Zone?
- Habitable zone: certain distance from a star where the temperature is just right (not too hot, not too cold for liquid water to exist)
- This principle drives a lot of the search for life on exo-planets
What was Earth like during the Hadean Eon (4540-4000)?
- Initially molten - what Hell would look like
- Constant bombardment of asteroids and comets hitting the surface
- Formation of the moon
Generally, how was the planet formed?
- Constant heavy bombardments heated the Earth
- This heating was also increased by gravitational contraction
- Thus, we had the partial total melting of the Earth, creating a magma ocean
- The iron-rich fraction of this liquid was heavier and it SUNK to Earth’s centre - creating the CORE
What was the composition of the earth 4.4 billion years ago (Ga)?
- Iron Core + Mantle + Crust
What changes occurred on Earth by the end of the Hadean?
- the Earth was cooled down enough to form rocks and oceans
- Steam in the atmosphere cooled down and fell as rain to create oceans
- First continents begin to form
How do we know this?
- Oldest piece of Earth’s crust/solid land comes from Western Australia
- Conducted isotopic dating on Zircon crystals: 4.4 Billion years
- Thus, within the first 100-100M years of our planet, there was enough cooling to form crust
- ALSO: ratio of oxygen isotopes within this zircon crystal indicate that the crystal likely formed in a cool, wet process at the Earth surface
- Suggests: parts of the Earth may have been covered with liquid water (at 4.4 Ga years ago)
What was Earth like during the Archean eon (4000-2500)?
- Liquid water was prevalent (lots of evidence for this)
- First evidence of life is dated back to the Archean
- Onset of plate tectonics
What is plate tectonics?
- All of the crusts on the planet move around like floating slabs of rock (~96km thick) on top of hot, mushy rock in the mantle
- Earth is constantly RECYCLING itself: creating new land and sub-ducting itself back into the mantle
What is continental drift?
- First proposed by Alfred Wegener in 1915
- Suggested that the continents of the Earth moved around over geologic time relative to each other, thus appearing to have “drifted” across the ocean bed
- Explanation to why animals and plant fossils, and rock formations are found on different continents (that were separated by oceans)
If continental drift had not occurred, what are the alternative explanations?
- Species evolved independently on separate continents – contradiciting Darwin’s theory of evolution
- Species swam to other continents (Noah’s Arc)
What is the evidence against these alternative hypotheses?
- Mesosaurus, a freshwater reptile that lived between 286-258 MYA, are found solely in Southern Africa and Eastern South America
- It would have been physiologically impossible, based on the fossils, for it to have swum this distance
- Rocks across the ocean were the exact same age and fit together really nicely like puzzle pieces
When did the theory of continental drift emerge into tectonic plates?
- 1950s: Wegener suggested that the rotation of the Earth caused the continents to shift towards and a part from each other
- 1970s: Today we know that continents because they rest on tectonic plates that are always moving and interacting
What is the structure of the Earth?
- Core
- Rich in Iron
- Inner-core is solid and the outer-core is liquid (source of the Earth’s magnetic field) - Mantle (very very hot rocks)
- Rich in silicon and oxygen - Crust (cooler, stiffer rocks)
- Also, rich in silicon and oxygen
What are tectonic plates made of?
- Rigid, cool lithosphere
- Lithosphere: rocky crust of the ocean floor and continents down to the upper mantle
- They sit on top of the hot and mushy mantle
What are the two types of crust on Earth?
- Oceanic: much thinner (7km thick), its chemical composition makes it very dense (sinks under continental crust), younger
- Continental: much thicker (10-70km), buoyant, mostly old rocks
Plate tectonic characteristics on Earth
- Earth’s crust is divided in 12 major plates
- Earth’s structures like volcanoes, mountains, ocean trenches, are caused by plate interactions (collision, pulling apart, scraping)
- Tectonic: deformation of the crust as a consequence of plate interaction
What causes the plates to move?
- Tectonic plates sit on a mantle, and the mantle has convection currents that drive plate tectonics
- Conveyor belt = convection cells in mantle
Plate boundaries = Zones of contact between plates
- Divergent: moving away
- Convergent: moving towards one another
- Transform: moving against one another
What are divergent boundaries?
- The space created by plates moving away from each other can be filled with new crustal material sourced from molten magma from below
How do divergent boundaries produce rifts in continental crusts?
- If the rift opens wide enough, it will form the thin rocky floor of a NEW OCEAN
- E.g., East African Rift, Iceland being slowly torn apart by continental rifting
How do divergent boundaries produce mid-ocean ridges?
- Mid-ocean ridges: divergent boundaries between oceanic plates (MOST ACTIVE)
- As they move apart, small amounts of magma rise to the sea floor and add new crust - sea floor spreading
- Mid-ocean ridges systems form some of the most extensive mountain ranges on Earth, they’re just under water
- Mid-Atlantic ridge: separated the North American Tectonic Plates from the Eurasian ones (40 000 km in length)
What are Black smokers/hydrothermal vents?
- Sea floor hot springs: molten hot magma heats up the water and pushes it up through the vent - found along mid-ocean ridges
- Entirely different ecosystem of organisms that we never new about
- Organisms at these vents get their energy from the vent itself and not sunlight (because it is so deep-down)
- Potentially place where life started??
What are convergent boundaries?
- Plates colliding: a denser plate is sub-ducted underneath the less dense plate. The plate being forced underneath will eventually get melted and destroyed (and recycled)
How does continent-continent collision work?
Both continental crusts are too light/buoyant to be subducted so a continent collision occurs creating especially large mountain ranges
- E.g. Himalayas
How does continent-oceanic collision work?
Denser oceanic plate is subducted, often forming a mountain range on the continent
- E.g., the Andes
What are subduction zones?
- Occur when one or both of the tectonic plates are composed of oceanic crusts
- Leads to oceanic crust being recycled
How does ocean-ocean collision work?
- One oceanic plated is eventually subducted under the other
- Formation of deep trenches
- E.g., Mariana Trench - 11km deep, deepest place on Earth
What are transform boundaries?
- When plates rub against each other, huge stresses are set up that can cause portions of the rock to break = resulting in earthquakes
- Faults: places where plates break
- No destruction or new formation of crusts
Which boundaries are most prevalent on Earth?
- Convergent & Divergent
- Very small amount of transform boundaries
How fast do plates move?
- Really slow: a few cm per year (rate at which your fingernails grow)
How do plate tectonics affect the evolution of life?
- Plate tectonic movements affect geography and could in turn affect food supply, climate, and diversity of life – could create or destroy niches
- Formation of Grand Canyon: squirrels and other small mammals were separated and could not reproduce with each other across this new GEOGRAPHIC BARRIER
What is the Pangea?
- A supercontinent composed of earlier continents (Europe, North America, Souther America, Africa all attached)~335 MYA
- Began to break a part ~ 175 MYA
- Terrestrial organisms could migrate across all continents and were only limited by their biotic potential
Breaking of the Pangea = Promotion of Biodiversity
- Creation of physical barriers such as seas
- Species were separated and exposed to different physical (climate) and biotic (change in predators) conditions
- Each species would then begin to adapt very differently and new species emerge
- New habitats were also created due to a change in biotic and abiotic factors such as climate
What is weathering? How are plate tectonics related?
- The (slow process of) breakdown of minerals in rocks at or near the Earth’s surface – releasing nutrients
- Weathering is caused by chemical and physical interactions with air, water and living organisms
- All the nutrients found in oceans come from weathering and erosion of rocks
- When weathering and erosion rates are higher, we should also see an increase in nutrients supplied to the oceans
- Relationship between an increase in nutrients in the oceans due to plate tectonics and bursts of evolutionary change ??? TBD
How do plate tectonics influence habitability?
- Are plate tectonics necessary for life on Earth?
- They can help to regulate the planet’s temperature, recycles nutrients, created hydrothermal vents, created new niches?
- Earth is the only accepted planet known to have plate tectonics
