Plate Tectonics and Principles of Geology Flashcards
What is the continental drift hypothesis, who developed it?
Proposed in 1912 by Alfred Wegener > single supercontinent (Pangaea) broke up to form the modern continents
Ideas developed by Alexander Du Toit in “Our Wandering Continents” 1937
Pangaea 200 million years ago was just Laurasia (north) and Gondwana (south)
*Continental theory wasn’t accepted or talked about for a while but then came back into the conversation…
Explain the 4 key groups of evidence for the continental drift hypothesis
Continental fit:
- Alexander Du Toit showed evidence of when they began to break up
- found if you go down about 200m where erosion doesn’t happen, the continents fit together really well
Similarity of rock sequences and mountain ranges:
- ex. pennsylvanian to jurassic sequence of rock - similar sequence of marine, non-marine and glacial rocks → likely that these continents were close together because it’s a very specific environment in which these rocks form
Glacial evidence:
- glacial striations in southern half (Gondwana) though South pole, India, and Antartica → causes grooves - can be used to talk about the motion of ice
Fossil evidence
- Gondwana - fossils found across continents
- Mesosaurus, Cynognathus, and Lystrosaurus
- Increasing biodiversity because of continents moving into different climatic zones
What are some other theories for HOW the continents moved?
- centrifugal force caused by Earth’s rotation?
- precession of the earth (wobbling of axis)?
- ‘tidal argument’ based on the tidal attraction of sun and moon (continents floating on water)
What is the theory of seafloor spreading and who proposed it?
1962: Hess proposed the theory of seafloor spreading
> seafloor separates at oceanic ridges
> new crust is formed by upwelling magma
> as the magma cools, the newly formed oceanic crust moves laterally away from the ridge
> oldest rocks but up against the continents - newest rocks are by the ocean seafloor ridges
Explain Earth’s Magnetic field - what records the reversals of the magnetic field?
- our earth is dipolar > lines go from south to north
- Magnetic poles don’t line up with geographical poles - on same end but not exact same location
- “normal” positions = same north and south geographic and magnetic poles and are close together in proximity
- Have been times where this flips into the reversed position and the northern magnetic pole is by the geographic southern pole and vice versa
- Lava and iron minerals in the lava record the reversals of the magnetic field
How can we put together seafloor spreading and earth’s magnetic field to explain how continents move?
*putting it together → From the seafloor ridge outward, you have a repeating pattern of the reversals of the magnetic poles and earth’s polarity
> A positive magnetic anomaly results when Earth’s magnetic field at the time of oceanic crust formation was the same as today, thus yielding a stronger-than-normal positive magnetic signal
How has seafloor spreading been confirmed?
- the ocean crust has magnetic anomalies that are both parallel to and symmetric around ocean ridges
- evidence indicates new oceanic crust forms along the spreading ridges
- deep sea drilling confirms older sediments and fossils are buried deeper
- oceanic crust is youngest near spreading ridges; oldest is farthest away
Explain how plate tectonics is the unifying theory of geology?
- makes sense of seemingly disparate phenomena
- helps us understand dynamic forces like volcanoes, earthquakes, and mountain building as interrelated and repeating events
- it proposed that large segments of Earth’s outer part (lithospheric plates) move relative to one another > continents and ocean basins are part of a lithosphere-atomosphere-hydrosphere system that evolved together with Earth’s interior
What does the core of the earth consist of and what is it composed of?
consists of:
- a small, solid inner region
- a larger, liquid outer portion
composed of:
- iron and a small amount of nickel (denser materials)
What are the three parts of the mantle and what is it composed of?
The mantle surrounds the core and is divided into three parts:
> solid lower mantle
> partially molten asthenosphere that overall, behaves plastically and flows slowly
> solid upper mantle
Composed primarily of peridotite:
> rock made of the mineral olivine
Describe the characteristics of the crust
- relatively thin outer skin of the Earth
- upper part of the lithosphere (lithosphere includes the crust and the upper mantle)
- consists of thin oceanic and thick continental crusts
- the asthenosphere is a semi-solid layer of earth’s upper mantle that’s located beneath the lithosphere
The strength of the Earth’s magnetic field is strongest at the _____ and weakest at the _______
poles
equator
What drives plate tectonics?
- convection cells > to a lesser extent than we thought
- ridge push
- slab pull
*Hot magma comes up from the mantle - from the mid-ocean ridge, gravity pushes the sides down and out to the sides
(visualize luggage moved along by a conveyor belt until it’s dumped into the baggage cart in the same way that plates are moved by convection cells until they are subducted into Earth’s interior)
How has the conception of convection changed in modern times?
The conceptual models of convection cells have changed over time from the old model of a singular convection cell to the modern one with warmer and cooler areas
> convection cells are no longer thought to drive plate motion alone
Explain ridge push
Ridge push develops because of the gravitational energy associated with the topographic elevation of the mid-ocean ridge
Explain slab pull
Slab pull develops because old oceanic lithosphere is denser than the underlying asthenosphere, so it sinks
What are the 3 types of plate boundaries?
- Divergent boundaries
- Convergent boundaries
> oceanic-oceanic
> oceanic-continental
> continental-continental - Transform
Explain divergent boundaries
where do they most often occur?
- plates are spreading and new oceanic lithosphere is forming - most often found in ocean ridges
ex. mid-atlantic ridge - magma rising from the mantle, pushing through the crust and creating cracks - end up with volcanism on the surface
- As the rift stretches, the crust gets thinner and you get a rift
>Ex. east Africa rift (rift valley) - Eventually the rift opens wide enough to form a narrow sea (present day example = red sea) - might have coastal mountain range on either side
- Eventually we end up with an ocean - continental “seaboard” bc the coastal mountains are gone
Explain Convergent Boundaries
3 types:
Oceanic-oceanic > ocean trench
- one oceanic plate is subducted beneath another - on the non subducted plate, a volcanic island arc forms from the rising magma generated from the subducting plate (partial melting)
- ex. Japanese islands
Oceanic-continental > subduction zone
- an oceanic plate is subducted beneath a continental plate, an andesitic volcanic mountain range is formed on the continental plate as a result of rising magma (partial melting)
- ex. Andes in south america
Continental-continental > mountains form
- when two continental plates converge, neither is subducted because of their great thickness and low and equal densities - as the two plates collide, a mountain range is formed in the interior of a new and larger continent
- ex. Ex. Himalayas in central Asia resulting from the collisions between India and Asia
Convergent boundaries are the cause of deformation, volcanism, mountain building, metamorphism, earthquakes, valuable mineral deposits
Explain transform plate boundaries
- plates are moving parallel to each other / against each other
- Occur mostly along fractures in the seafloor known as transform faults - they change one type of motion between plates into another type of motions - and most of them connect two oceanic ridge segments
- can create tsunami or earthquakes
- ex. coast of California
How do plate tectonics play a role in the rock cycle
- convergent plate boundaries create mountains
- mountains erode, producing sediments
- sediments are transported, buried, and lithified
- earth’s internal heat and pressure melts materials, producing igneous rocks
- sedimentary and igneous rocks are altered forming metamorphic rocks
What is Uniformitarianism? What is Actualism?
Uniformitarianism states that the laws of nature have remained unchanged through time and that the processes observed today also operated in the past
> “the present is the key to the past” - Lyell
> this principle cam from James Hutton’s studies > some consider him the founder of modern geology
Actualism is the application of modern processes to ancient systems > it takes into account that the processes may not have always occurred at the same rate (the rates aren’t uniform)
What are some examples of actualism?
Can be seen in modern and ancient sedimentary structures
> modern ripples, PEI
> modern mudcracks, TX
> Precambrian ripples, Red Rock Canyon, AB
> Permian ripples in cross-section, TX
Permian mudracks, TX
Where do ripples form?
in desserts and beach environments
What is stratigraphy? How is it interpreted?
A branch of geology concerned with composition, origin, age relationships, and geographic extent of layered (stratified) rocks (layered rocks = stratified rocks)
> mostly deals with sedimentary rocks
Sequences of rocks are interpreted using principles outlined by Steno (1669)
> principle of superposition
> principle of original horizontality
> principle of original lateral continuity
Explain Steno’s principles
- Principle of Superposition:
- oldest strata are at the bottom in an undisturbed sequence of strata - Principle of Original Horizontality
- all strata are horizontal when they form
- strata happens first, then non horizontal strata - Principle of Original Lateral Continuity
- strata originally are unbroken flat expanses > can be interrupted by erosion
What is the principle of cross-cutting relationships?
(Hutton)
- intrusive igneous rocks or faults are always younger than the rock they invade
What is the principle of Inclusions & Components?
(Charles Lyell)
- rock fragments within another body of rock are older than the including body of rock (think about pancakes –> the chocolate chips are ‘completed’ in the batter before the pancake gets cooked so the chocolate chips are therefore older
What are unconformities? When do they occur?
- happens when there is a substantial interval of time when erosion occurred rather than deposition
Angular Unconformity:
- occurs when horizontal sedimentary rock layers are deposited on tilted and eroded layers > creates an angular discordance with the overlying horizontal layers
Disconformity:
- occurs between parallel layers of sedimentary rock
- more subtle - time between the two layers
Nonconformity:
- occurs when younger sedimentary rocks are deposited on top of an eroded surface of igneous or metamorphic rock > indicates that long-term erosion exposed the older rock at the surface
- two very different types of environments form the different rocks in the layers
