Chapter 2: Plate Tectonics Slide Set 5 Flashcards
Associate the Earth’s layers with solid fluid, liquid fluid, gas fluid
Solid Fluid:
- The asthenosphere
- Mesosphere (mantle below lithosphere)
Liquid Fluid:
- The ocean, or the outer core
Gas Fluid:
- The atmosphere!
Describe Earth’s outer core in terms of magnetic field
‣ Liquid iron (a conductor)
‣ Convecting to dissipate heat (electric current)
‣ Generates magnetic field
‣ Shaped in part by Earth’s rotation
How does Earth’s Dynamo work?
A convecting conducting fluid supported by internal heat and planetary rotation could self-sustain a magnetic field over astronomical timescales
What is the Geomagnetic Axial Dipole hypothesis?
Geomagnetic Axial Dipole hypothesis (Hospers, 1954)
‣ Earth’s field is dipole- dominated
‣ If you average the field over long enough, only axial
dipole with remain
‣ Field strength remained constant for the past ~ 3 Gyr
What is Palaeo-magnetic Pole?
‣ Field recorded when rocks formed
‣ Different inclination at different latitudes
‣ Rocks of the same age record the same field,
i.e., they point to the same palaeo-pole
What is the difference between the Apparent-Polar Wander and the True Polar Wander?
Comparing APWs across each continent allows geologists separate tectonics motions from TPW
The (big) assumption for measuring TPW:
Over geological time scales, the magnetic pole follows the rotation pole
What is bathymetry?
- Depth variation on sea-floor.
- Measured by sonar
Sea-floor maps created by ships crossing the oceans. Bathymetric maps are now produced using satellite data
Many bathymetric features were discovered by sonar. Name the features
These include deep-sea trenches, oceanic islands, seamounts, and guyots, mid-ocean ridges, abyssal plains, fracture zones
A bathymetric profile across the Atlantic Ocean (from X to X’ on the map) illustrates that mid-ocean ridges are elevated above deeper ________.
abyssal plains
True/False
All mid-ocean ridges are roughly symmetrical—bathymetry on one side of the axis is nearly a mirror image of
bathymetry on the other side.
True
What are marine sediments composed of?
minerals eroded from the continents and shells of dead micro-organisms, thicken away from the mid-ocean ridge.
What are deep-ocean trenches?
The deep areas occur in elongate troughs that are now referred to as trenches. Trenches border volcanic arcs,
curving chains of active volcanoes.
What are seamount chains?
Numerous volcanic islands poke up from the ocean floor: for example, the Hawaiian Islands lie in the middle of the Pacific.
In addition to islands that rise above sea level, sonar has detected many seamounts (isolated sub-marine mountains), which were once volcanoes but no longer erupt.
Volcanic islands and seamounts typically occur in chains, but in contrast to the volcanic arcs that border deep-ocean trenches, only one island at the end of a seamount and island chain remains capable of erupting volcanically today.
What are fracture zones?
Surveys reveal that the ocean floor is diced up by narrow bands of vertical cracks and broken-up rock. These fracture zones lie roughly at right angles to mid-ocean ridges. The ridge axis typically steps sideways when it intersects with a fracture zone.
What is the rock composition of the oceanic crust?
Beneath its sediment cover, oceanic crust bedrock consists primarily of basalt—it does not display the great variety of rock types found on continents
Explain the concept of heat flow and what theory it led to?
Heat flow, the rate at which heat rises from the Earth’s interior up through the crust, is not the same everywhere in the oceans. Rather, more heat rises beneath mid-ocean ridges than elsewhere. This observation led researchers to speculate that hot magma might be rising into the crust just below the mid-ocean ridge axis
Today’s view of the ocean floor reveals the location of:
- Mid-ocean ridges
- Deep-ocean trenches
- Oceanic fracture zones
Explain Harry Hess’ concept of Sea-Floor Spreading (a mechanism to explain continental drift)
The ocean floor is formed at the ridges, drifts away from them on both sides and then plunges into the mantle at the trenches
The seafloor viewed as a conveyor belt for marine material (marine sediments and oceanic crust), explaining the absence of old oceanic rocks and the thinness of marine sediment.
Basalt, an extrusive igneous rock, is formed at ________
mid-ocean ridges
Explain the magnetism on the sea-floor
Magnetism in sea-floor rocks varies farther from Mid-Ocean Ridges.
- Stripes of positive (stronger) and negative (weaker)
magnetic intensity
- Recorded in sea-floor basalts
What is magnetic anomaly
difference between the expected strength of the Earth’s magnetic field at a certain location and the actual measured strength at that location
How does the new sea-floor form and what happens to the old ocean floor?
New studies of the sea floor led to the proposal of sea-floor spreading.
New sea floor forms at mid-ocean ridges and then moves away from the axis, so ocean basins can get wider with time.
Old ocean floor sinks back into the mantle by subduction. As ocean basins grow or shrink, continents drift.
What are the two main evidences of sea-floor spreading?
- the existence of orderly variations in the strength of the measured magnetic field over the sea floor, producing a pattern of stripes called marine magnetic anomalies
- the variation in sediment thickness on the ocean crust, as measured by drilling.
At any given location on the surface of the Earth, the magnetic field that you measure includes two parts:
- produced by the main dipole of the Earth generated by circulation of molten iron in the outer core
- produced by the magnetism of near-surface rock
A time when the Earth’s field flips from normal to reversed polarity, or vice versa, is called a _______
magnetic reversal
When the Earth has reversed polarity, the south magnetic pole lies near the north geographic pole, and the north magnetic pole lies near the south geographic pole. The time interval between successive reversals is called a ________
chron or epochs
Why do marine anomalies form and what did the discovery led to?
Marine magnetic anomalies form because reversals of the Earth’s magnetic polarity take place while sea-floor spreading occurs. The discovery of these anomalies, as well as documentation that the sea-floor gets older away from the ridge axis, proved that the sea-floor spreading hypothesis is correct.
Sea-floor spreading predicts that magnetic anomalies should be ________
mirror images across the mid-ocean ridge…
What are Subchrons or events?
short-term magnetic reversals (1000’s to 200,000 years).
What is the record of magnetic reversals can be used for?
Whereas the cause of reversals remains to be elucidated, the record of magnetic reversals can be used to date thermoremanent magnetization in geologic cross-sections.
What does the width of the magnetic anomaly stripes related to and how does it defer by the rate?
The width of the magnetic anomaly stripes is related to the spreading rate.
- Faster spreading = wide stripes
- Slower spreading = narrow stripes
Continental lithosphere vs. oceanic lithosphere
Explain the differences
Differ markedly in their thicknesses.
On average, continental lithosphere has a thickness of 150 km, whereas old oceanic lithosphere has a thickness of about 100 km. (new oceanic lithosphere at a mid-ocean
ridge is much thinner)
How do the geophysicists able to determine how fast the ocean opened up?
Using the magnetic stratigraphy established on land (dated lava flows), geophysicists were able to determine how fast the ocean opened up.
What are isochrons?
are contours of equivalent age of the seafloor.
They show the time that has elapsed and the amount of spreading that has occurred since the magnetized rocks were injected as lava into a mid-ocean rift.
What are plate tectonics and what do they explain?
- Earth’s outer shell is broken into rigid plates that move and interact in three types of plate boundaries.
- Plate tectonics explains the origin and distribution of earthquakes and volcanoes, mountain belts and seamount chains, mid-ocean ridges and deep- ocean trenches, and changes in past positions of continents and ocean basins.
What are continental margins?
Where land meets the ocean
What are the types of continental margins?
- Margins near plate boundaries are “active.”
- Margins far from plate boundaries are “passive.” “
Earthquakes common along active margins.
What is the significance of the passive plate boundaries?
- Passive-margin continental crust thins seaward.
- Traps eroded sediment.
- Develops into the continental shelf.
Lithosphere is fragmented into ~____ major tectonic plates.
12
What are plate boundaries?
- Locations on Earth where tectonic plates meet.
- Identified by concentrations of earthquakes.
- Associated with many other dynamic phenomena.
True/False
Plate interiors are almost earthquake-free
True
What are the types of plate boundaries?
Divergent boundary
Convergent boundary
Transform boundary
Explain the divergent plate boundaries and give an example on Earth
- tectonic plates move apart
- Lithosphere thickens away from the ridge axis.
- New lithosphere created at divergent boundary
- Also called: mid-ocean ridge, ridge.
EX: Iceland - Mid Atlantic Ridge
Explain the convergent plate boundaries
- tectonic plates move together
- The process of plate consumption is called subduction.
Convergent plate boundaries are also called
convergent margin, subduction zone, trench.
Convergent margins are the sites of lithospheric destruction. There are 3 distinct types:
- Oceanic-Oceanic
- Oceanic –Continental
- Continental-Continental
Explain the transform plate boundaries and give an example on Earth
- tectonic plates slide sideways.
- Plate material is neither created nor destroyed.
The San Andreas Fault
Transform plate boundaries are also called
transform fault, transform
