L4: Continental Drift Flashcards
What is some of Wegener’s evidence/arguments for the continental drift?
- The apparent similarity of the coastlines of Africa and South America
- Similar rocks on different continents
- Glaciation in places now located in the tropics
- Remains of tropical plants in the Antarctic
- Fossil remains of plants and animals that lived in restricted areas but are now widely separated geographically
- (came after his death) Polar wander curves
What did Holmes provide for Wegener?
A mechanism to describe continental drifts called “convection,” which is now used are the origin concept for sea floor spreadings
What did Harry Hess discover regarding sea floor spreading?
Hypothesized that the typography could be explained by the ocean crust splitting along the oceanic ridges
What is earth’s longest feature?
The mid-atlantic ridge
What is a ridge?
- A crack in the earth where magma is able to come up from the within
- May be the same height as a mountain, usually found on the seafloor.
What is paleomagnetism?
- The magnetism in rocks that was induced by the earth’s magnetic field at the time of their formation
- Acts as physical proof for continental drift
Why aren’t magmas magnetic?
- Most Fe bearing minerals are weakly magnetic at surface temperatures, while at high temps. They lose their magnetism
- However, as it cools below the ‘curve point’ the Fe bearing minerals align along the earth’s magnetic field
- Here they regain paleomagnetism unless they’re reheated
What is Magnetic Reversal?
- Observation that the earth’s magnetic north has changed throughout geological time
- Rocks crystallizing at a time when the earth’s magnetic field was in the same orientation as today are said to be normally magnetized
Magnetic orientation
Look @ diagram
What is one consequence of spreading the ocean crust?
Older crust is farther from the ridge and new lava filling the gap along the ridge becomes magnetized with the polarity of Earth’s current magnetic field
What are the magnetic lines of force and why are they useful to understand?
- Run N-S and vary in dip with the latitude
- Inclination are the earth’s magnetic field lines
What is suggested from the polar wander curve?
- Continents have rather shifted than the poles move
- Magnetized rocks of different ages on a single continent may point to very different apparent pole positions. This can make it appear as though the magnetic poles have meandered through time if position of continents assumed to stay the same/static
- Therefore the polar wander curve states that the poles have remained stationary and the continents have meandered through time
What is angular deviation?
- Difference between the poles
- By looking at angles, we can determine where the magnetic north was before
- Magnetic lines of force reveal continental change
What is/was Pangea?
Theory that the earth once existed as one giant continent
What are some intriguing features topographic maps show in regards to seafloor spreading?
- There’s ridges running N-S in both Atlantic and Pacific oceans
- Along the margins of continents there are trenches several km deep
- The continents are bounded by shallow water shells
The distribution of earthquakes and volcanoes are also concentrated in _______, which directly correspond to the edges of ______ ________.
- Belts
- Different plates
What is the internal structure of the earth?
- By physical properties
- By chemical composition
- Mono - mohorovicic discontinuity = crust/mantel boundary
Continental crust vs oceanic crust
- Continental crust: less dense rock, more AlSi
- Ocean: More dense rock, more FeMg. More dense than continental due to composition (basaltic minerals)
What do volcanoes outline
The basic continental plates. They are typically found on the edge of plates
How do we know that earth’s core is denser?
- Denser materials sink to the center
- Hot material rises, cold material sinks
Define convection
- The circular motion that happens when warmer air or liquid — which has faster moving molecules, making it less dense — rises, while cooler air or liquid drops down
- Convection currents within the earth move layers of magma
- Convection in the ocean creates currents.
Earth is trying to get rid of its heat (this is why we have continental plates) - As this stuff rises and gives it away, a conduction current is set up
- Molt material rises, and push plates apart. Then it cools cools cools, and sinks back down again.
What are the divisions of Earth by means of chemical composition? (triangle diagram
- Atmosphere: nitrogen, oxygen
- Hydrosphere: oxygen, hydrogen
- Crust: Oxygen, silicone, aluminum, iron, Mg
- Mantel: Oxygen, silicon, Mg, Fe
- Core: Iron and nickel
How do we know that we have nickel in the core?
Meteorites were created around the same time as earth and all have nickel present, therefore we can assume nickel is present in Earth’s core
Define convergent plate boundary.
Plates that are moving towards one another on a collision course
Define transform boundary.
Plates slide past one another
Define divergent plate boundary
Plates that are separating apart
What are the types of plate boundaries?
- convergent plate boundary.
- divergent plate boundary
- transform boundary
What do we and don’t we know about convection and plate movement?
- We don’t know why heat energy is brought up
- We do know that 1. The lithosphere has the energy of motion and 2. The source of this energy = earth’s internal heat
What 3 forces may play a role in moving the brittle slabs of lithosphere around?
- Rising magma of oceanic ridges may push plates away from each other
- Lithosphere breaks and, being relatively cool and dense, sinks through the less-dense asthenosphere, pulling the plate along w/ it
- The whole plate may be sliding downhill away from the spreading ridge because the spreading center stands high on seafloor
What is elastic deformation?
- When stress is placed on an object it undergoes deformation/strain
- If this occurs, the amount of deformation is proportional to the amount of stress applied
- Rocks may have elastically
- Think of an elastic band; can put lots of strain on it before it breaks
What is the difference between brittle and ductile?
- B: Rocks may be able to rupture before plastic deformation occurs
- D: May rupture after plastic deformation occurs
Molten = ________. Oceanic and continental crust = ___________.
- Ductile
- Brittle
What is plastic deformation?
- Occurs once the elastic limit of an element is reached during elastic deformation
- Changes in this stage are permanent
What is brittle behaviour a characteristic of?
Characteristic of most rocks at near-surface conditions with low confining pressures (ex. lithosphere)
How can we imagine that continents float around on the mantel?
Higher temperatures and pressures allow rocks to behave more plastically (ex. The asthenosphere)
What is the asthenosphere?
- The upper layer of the earth’s mantle, below the lithosphere, in which there is relatively low resistance to plastic flow and convection is thought to occur
- Part of the mantel underneath the lithosphere
What is the lithosphere?
- The rigid outer part of the earth, consisting of the crust and upper mantle.
- Part of the crust and the top part of the mantel
Define convection.
The process by which hot, less dense material rise upward, being replaced by cold, dense, downward flowing material to create a convection current
