Midterm Study Guide 2 Flashcards
Plate Tectonics
Who first noticed that the continents today look like they fit together pretty well?
Alfred Wegener
There are at least seven lines of evidence that the continents were once all together in one large supercontinent (Pangea)can you name four?
● 1 The good geometrical fit of the continents, particularly when the continental shelves are considered.
● 2 Geological evidence: similar rock types and structures were found on different continents that seem to fit (Fig. 3). These rocks (or structures) now on different continents were found to have formed at the same time.
● 3 The occurrence of identical fossils on widely separated land masses or continents.
● 4 Paleoclimatic evidence: Rocks that once formed by the movement of ice sheets (glacial
deposits) occur in present day warm regions as Africa and Australia, whereas rocks of the same age in present day cold regions as e.g. Pennsylvania, formed under tropical conditions. This observation led Wegener to conclude that Africa and Australia were at one time located near the South Pole, whereas parts of North America were near the equator at the same time, and have since drifted northwards to their present day positions.
There are at least seven lines of evidence that the continents were once all together in one large supercontinent (Pangea)—-can you name four?
- Fossil Evidence
- Ancient Climates
- Puzzle-like Fit of the continents
- Distinctive rock units on different continents match when put together
- Mountain ranges on different continents match when put together.
- Distinctive fossils occur on several different continents.
- Connection between glacial deposits.
What lines of evidence are available to probe the interior of the earth, how do we know, for example, that the earth has a solid core and liquid outer core?
- Inge Lehmann, who was studying how seismic waves bounce off the interior of the Earth. Instead of bouncing off a solid core, Lehmann observed that the liquid outer core caused the waves to reflect differently from how they bounced off the inner core.
- from studying seismic waves (e.g., earthquakes), P,S waves
- some material comes to the surface from great depth
- mineral physics experiments
- geochemical fluxes
- study meteorites
- study the moon
- spin of Earth
- Earth’s magnetic field
How does magnetic information indicate that plates are moving away from spreading centers?
● Seafloor Spreading
● magnetism: new rock is added to the plates at spreading centers as they split apart. As this
lava cools, it becomes magnetized, much like a magnetic tape does in your tape recorder. When the magnetic field of the earth changes, this change is recorded in the rocks. We can read this record by sensing the changes in the magnetic field from ships, and we see stripes of different magnetism parallel to the spreading centers, as expected if the plates are splitting apart there.
From Paleomagnetism - When magma turns into a rock, from liquid to solid,
Magnetic minerals in the rock all orient toward the North Pole.
Magnetic poles show periods of time when N is S and S is N = reversal. From this we can see the sea floor spreading.
Name the three types of plate boundary, describe the plate motion at each boundary.
● 1) Transformfault boundaries: plates slide horizontally past each other
● 2) Divergent boundaries: plates move apart/create new lithosphere
● 3.) Convergent boundaries: plates collide & one is pulled into the mantle and recycled
How is continental crust created?
● Continental crust is produced and (far less often) destroyed mostly by plate tectonic processes, especially at convergent plate boundaries.
● Created at divergent boundaries, destroyed/recycled at convergent boundaries.
Why does convergence usually result in the creation of mountains?
● Crust is destroyed and recycled back into the interior of the earth as one
● When an oceanic plate subducts, it lifts the continental plate that overrides it, forming a
mountain. When two continental plates collide, both refuse to subduct and so they form big
mountains.
Why does convergence also usually result in the creation of an ocean trench?
● Usually one plate (oceanic) is less buoyant than the other (continental) and so instead of colliding, it happily subducts under the other place. Even in an oceanic-oceanic convergence, one plate will subduct. But in a continental-continental convergence, neither subduct.
Los Angeles sits near what plate boundary? What other tectonic features are there in and around LA?
● Los Angeles sits near the San Andreas fault plate boundary. Since motion along the fault is sideways and not vertical, it will creep towards San Francisco at about 6 centimeters per year. In about ten million years, San Francisco and Los Angeles will be side by side. Sits near the plate boundary of the Pacific and North American plates.
How can you tell how fast plates move? Roughly, how fast do plates move?
Plates move a few centimeters a year about the rate of fingernails (enough to still cause
worry about building something on or near a fault). Scientists can measure how fast the plates move by using global positioning system (GPS) technology. Since using GPS, scientists have found out that plate motions are nearly continuously going on and not something that happens every now and then such as when there is an earthquake! In fact, scientists that study these motions have learned that much of the motion between plates occurs without producing earthquakes.
By what mechanism(s) do plates movewhat is the driving force of plate tectonics?
Mantle convection is the driving force. Mantle is heated from below (and within) and is
cooled at the top. The hot mantle goes up in lines at ridges and the cool goes down in lines at subduction zones. When the cool spreads out across the surface (before it’s subducted), that’s when plate motion occurs.
Why are continental rocks very old and ocean crustal rocks never get older than 250 million years old?
● Oceanic crusts are less buoyant and are always the first to be recycled/subducted when it comes to converging plates. Continental rocks always want to stay on top because of how light they are in comparison. It is easier to take down a chunk of oceanic plate.
How did shelly fossils become embedded in limestone rocks at the top of Mt. Everest?
● The limestone was once at the bottom of the ocean floor. When the continentalcontinental
plates collided with each other, the limestone was pushed towards the top thus leaving the remnants of plankton and sea fossils at the top of Mt. Everest.
Sample Question: Q. Radioactive isotopes are used to determine the age of a rock because:
XX(a) they represent different time periods
(b) their abundance is a measure of time
XX(c) certain isotopes were only created at certain times
XX(d) they have a short half life (e) none of the above
Sample Question: Q. How do we know the interior of the earth is hot?
(a) because we have volcanoes
(b) because we have hot springs
(c) because mine shafts get hot
(d) because we have spreading centers
***(e) all of the above
E
continental drift
The movement of the earth’s continents relative to each other by appearing to drift across the ocean bed. This concept was fully developed by Alfred Wegener in 1912. Plate tectonics builds on this concept.
convection
The slow creeping motion of Earth’s rocky mantle caused by convection currents carrying heat from the interior of the earth to the surface. It is the driving force that causes tectonic plates to move around the Earth’s surface.
plate tectonics theory
a. plates are moving around but also explains observation of seafloor spreading;
assumption is that all deformation is found at plate boundaries; motion of plates
creates Earthquakes
b. Earth’s lithosphere (outer rigid shell) is composed of several dozen plates that float
on a ductile mantle.
magnetometer
a. measuring instrument used to measure the strength and direction of magnetic fields
