Class Notes- Unit 6 Flashcards
What is the early evidence of Pangea?
1) Jig-saw puzzle fit of continents
2) Matching rocks across oceans (pre-breakup rocks)
3) Matching fossils across (pre-breakup living environments)
What are causes and/or controls either totally or partially of plate tectonics?
- earthquakes
- volcanism
- topography/scenery/mtn. belts
- climate
- organic evolution
- jig-saw puzzle fit of continents
What explains the causes and/or controls of plate tectonics?
-rock matching across D.P.B (some oceans)
-Paleomagnetic features
(apparent polar wandering, sea floor magnetic stripes)
What is the mid-late 20th C. evidence of Pangea?
Structure of the lithosphere
Includes: mid oceanic ridges; oceanic trenches; structure of continents; earthquakes; volcanoes; paleomagnetism; tectonic mountains
What is modern evidence of Pangea?
- Increasing detail of evidence from 20th C.
- Satelliet mapping
What is the rate of movement for a continent per year?
2 cm/year
Continent movement rate can vary from___.
0-10 cm/year
What did Wegner divide northern and southern continents into?
Laurasia and Gondwana
Matching rocks across Gondwana–___between___America and___.
- Archean
- South
- Africa
What were the two fossils studied that matched fossils across the continents?
- Mesosaurus (shallow-marine reptile coastal)
- Glossopteris (leafs)
What is the range of the mesosaurus?
Range 299-280 Ma
What is the range of the glossopteris?
299-251 Ma
What is significant out the Appalachians?
Rocks of this mountain belt run straight out to sea and then truncate (rock matching because these same rocks are found on the coast of the N.A, Greenland, Europe).
What are the 6 steps of apparent polar wandering to calculate the movement of continents?
- Measure magnetic vector in rock (lava especially)
- Calculate VGP (virtual geomagnetic pole)
- Determine the absolute age of the rock
- Do this for a number of different rock ages and plot all the VGPs on a map–for one continent
- In every case we see a line–succession–of VGPs. This line of “apparent polar wandering”
- Compare these lines for the various continents. We have now done this for every continent back to about 3.0 Ga for some.
Where do most of the worlds most active geology (volcanoes and earthquakes) take place?
occurs at divergent and converged plate countries.
What is relative plate movement?
Defines the plate boundaries
What is absolute plate movement based on today and in the past?
- today satellite GPS surveying
2. in the past…paleomagnetic apparent polar wandering
What are precambrian regions built up from?
Ancient orogens
What is the part of a continent that is geologically stable?
Ancient craton
Where do earthquakes occur?
Earthquakes occur at Plate Margins where rock breaks, either by being pulled apart or compressed “Earthquake Zones”
What is the Earthquake focus?
Spot within the earth where the maximum energy is released
What is the earthquake epicentre?
Spot on EArth’s surface directly above the focus
Where will the earthquake epicentre be with a vertical fault?
On the fault line
Where will the epicentre of an earthquake be when the fault line is not vertical?
not on the fault line
Where do Earthquakes and visible volcanoes mostly occur?
- occur mainly at places where plates collide (Convergent Plate Boundaries)
- but also where they separate (Divergent Plate Boundaries)
___but not___occur where plates slip sideways past each other (Transform Plate Boundary).
Earthquakes but not volcanoes
___but not major___occur inside some plates at “hot spots.”
Volcanoes but not major earthquakes
What do divergent plate boundaries result in the formation of?
- earthquake zones
- mid-ocean ridges and rises
- volcanism
- creation of new crust
- opens ocean basins
“Divergence” produces…?
Extension which can result in NORMAL Faults (also called “extensional faults”).
Where are normal faults produced?
Anywhere the EArth’s crust is being horizontally stretched.
What’s happening near the top of plume?
decompression melting at MOR
What is plume?
- A mantle plume is an upwelling of abnormally hot rock within the Earth’s mantle
- As the heads of mantle plumes can partly melt when they reach shallow depths, they are often invoked as the cause of volcanic hotspots, such as Hawaii or Iceland, and flood basalts such as the Deccan and Siberian traps. Some such volcanic regions lie far from tectonic plate boundaries, while others represent unusually large-volume volcanism near plate boundaries or in large igneous provinces.
What is partial melting?
Some rock components melt at lower temperature than others.
What kind of melting does decompression melting at MOR produce?
partial melting
With decompression melting at MOR, what happens as magma rises in the plume?
- At first it is hot (2000 degrees C) and a high pressure keeps it solid.
- As it rises it is still hot (1100 degrees C) but at a lower pressure so decompression melting – partial melting – takes place.
What is the distance between sea level and the asthenosphere?
100 km
On average, how long is a diverging MOR?
88 000 km long
What is the 4 step evolutionary sequence for continental breakup?
1) Mantle plume under pre-existing continent that begins to separate the continent
2) A rift valley forms as the two sections separate by graben filled with lava
3) Linear sea forms
4) Ocean forms
What is a graben?
a graben is a depressed block of the Earth’s crust bordered by parallel faults.
What 3 things were determined by the evolutionary sequence of continental breakup?
- how do rocks of sea floor form?
- where is the youngest sea floor rock located?
- where are lower and higher intensity bands and what do these bands represent?
1) rocks of sea floor form from lava decompression melting at plume–recycling rock from the mantle to the crust
2) Lava closer to ridge is younger, and lava farther from ridge is older
3) Sea floor rocks varied in intensity of magnetism in long strips. Lower intensity bands are places were magma has reverse polarity and high intensity where lava has normal magnetic polarity.
What happens as new basalt is added to the ocean floor at the MOR (divergence, the sea floor, and paleomagnetism)?
It becomes magnetized according to the earth’s field at that time. Divergence carries this basalt away to both sides of the MOR an new basalt takes it place.
What type of sea floor rock does the occasional field reversal produce?
The occasional field reversal produces bands of normal and reversed magnetic polarized sea floor rock. Much like a tape recorder.
What do the sea floor rocks record?
Long history of paleomagnetic reversal
What confirms plate divergence is taking place?
- Rock ages on sea floor (basalt lava record)
- Earth’s magnetic polarity
