Chapter 3 Flashcards
(23 cards)
Explain what causes instability in the Earth’s magnetic field.
The liquid in the outer core moves similar to boiling water, but in slow motion.
This flow pattern causes changes in the flow pattern. This churning motion causes the field to twist and turn, causing the field to shift. After this period of instability, the field will resume a stable motion, but may actually be flipped by 180 degrees, reversing its polarity.
Explain the dynamo model (early and recent theory)
First model
- The magnetic field is believed to have originated in the outer core - It has been suggested that the rotation, as in a dynamo, involves cells of turbulence in hot metal liquid of the outer core, combined with the Coriolis effect
- Coriolis effect: the rotation of Earth on its axis deflects the path of other rotating material – such as ocean water
Simpler explanation: In recent years it was discovered that the inner core moves slightly faster that the rest of the planet, including the outer core.
What are some basic facts of Earth’s magnetic field?
- protects us from space radiation and solar storms - regenerates continuously
- its origin is one of the 5 greatest unsolved mysteries in physics, according to Einstein
- the magnetic poles are not the same as the geographic poles (poles of axis of rotation)
What was the explanation of volcanic ridges found in the Atlantic? Who proposed this? What does it imply?
Harry Hess, in 1960, proposed that the the ridges represented spreading centers where Earth’s crust was moving in opposite directions like conveyor belts, meaning that the atlantic ocean was spreading by about 2.5 cm a year. This implies that North/South America and Europe/Africa were in contact 180 million years ago.
What two lines of research made up the second major development in plate tectonic theory? Describe a major discovery made in these research processes.
Two lines of research:
- hunt for mineral natural resources
- map the geographical features of the ocean floors.
A huge volcanic ridge was found to extend from the for North to the far South. This was surrounded by various networks. They were also active, making the largest network anyone had ever seen.
What caused the second major development to the plate tectonic idea? What were the prompts that this brought?
After the second world war, countries needed to go underwater to detect sunken ships. Scientists found repetitive magnetic records underwater, but were unable to process this data as yet.
Who came up with the initial ideas of plate tectonics? What were these initial ideas?
Alfred Wegener He introduced the idea of continental drift, based on the fact that the jigsaw fit between continents was too good to be a coincidence. He fitted the continents together to form Pangea, the supercontinent that is belived to have drifted apart over time.
What is produced by plate tectonics?
- Continents
- Ocean basins
- Mountain ranges
What does the theory of plate tectonics help explain?
- For geological engineers, plate tectonics explains why and where significant deformation of Earth’s surface occurs.
- For economic earth scientists, it explains the type and location of many metallic mineral deposits
- It explains the global distribution of earthquake and volcanic hazards
Explain how rocks can act as a measurement of Earth’s magnetic field.
The Earth’s magnetic field orients the magnetite crystals found in basalt, a common molten rock, along the field lines.
Any magnetites in rocks that are NOT along the equator also point up/down from the ground’s plane, depending on their degree of latitude.
If we read the magnetic properties of basalt properly, we can tell where the lava eruption was relative to the north magnetic pole of the time.
How is paleomagnetism used as evidence for continental drift?
Earlier we learned that the disruption of the molten magnetites (“strorms”) may cause a reversal of polarity upon stabilization.
We also know that whenever a volcano erupts, the magma that hardens can show the magnetic field at the time of eruption. The fields’ orientations show age.
Rocks with alternating fields were found in symmetrical patterns across ridges, at equal distance from the ‘spreding centres’, indicating a continental drift.
What did the paleomagnetic evidence indicate about continental drift?
By measuring the age of the rock, and determining the distance from the ridge centre, we can estimate the rate of spreading. This is also measured through a GPS system that determined that continents shift a few centimetres a year.
Why were deep trenches found in the ocean? What did this indicate?
The deep trenches indicated earthquake foci, and sites of extreme compression. This indicated the possiblity of lithosphere returning to the mantle (however did not prove this).
Who made the seismograph? What was it used for/how did it work?
Hugo Benioff made the seismograph, with his co-worker, Charles Richter.
It was used to measure earthquake locations and examine the trenches that marked them.
During earthquakes, the seismograph detected a series of earthquake foci defining a thin zone sloped downward away from the continental plate side, implying that the ocean floor slab was descending.
What method was developed to make seismogrophy more precise, and what did this achieve?
Seismic tomogrophy was developed. It is a technique that uses the fact that seismic waves travel faster through colder material than warmer material to map out subduction zones.
Using this, we can define the cooler ocean slabs as they fall from Earth’s surface toward the core-mantle boundary, and create an image of the subduction.
What are the different types of plate boundaries / stress zones?
Tensional stress: a spreading centre (divergent boundary) leads to crust production.
Compression stress: a subduction zone (convergent boundary) leads to crust consumption.
Transform plate boundaries: one plate slips more or less horizontally past the other. This is sometimes called a “strike-slip fault”. No crust is producedd or consumed.
Why does convection considered a simple minded explanation for “dragging” down a plate into a subduction zone?
The lithosphere is brittle, and the asthenosphere is fluid, making it difficult for convection to flow continuously through each layer.
Since the inner core materials are 0.2 grams / cubic centimeter, gravity is expected to play a factor in this “pulling.”
Explain the theory that explains the presence of volcanoes on Hawaiian islands, despite the lack of a plate boundary. Explain its proofs as well as what it is.
There is a bottomless stationary blow-torch-like source of magma, over which the Pacific plate passes. This is
Proof:
- As you go further North of the big Hawaiian island, the volanic islands / seamounts are dead
- On the big island, there is increased volcanic activity toward the Southern boundary
- A new volcanic mound, Loihi is rapidly building up from the seafloor
- There exists a tightly defined plume, on top of which is called a hotspot, that is centred beneath the big island (under Loihi)
This is called a mantle plume.
What is an ultra low velocity zone? What characterizes one? What does it have to do with mantle plumes?
An Ultra Low Velocity Zone (discontinuous zone) is an area where seismic shock waves tend to lose velocity (because of high heat).
They are characterized by large reservoirs of magma.
It is believed that mantle plumes originate at the core-mantle boundary, from a discontinous zone.
How do mantle plumes work?
The magma in the mantle plume moves upward, changing between solid and liquid form at various stages, for a couple million years and pushes up against the lithosphere.
Upon contact with the lithosphere, the magma spread out, as this is a significant barrier. Once the magma reached a point of weakness, it breaks through as an active volcano. Plumes generally exist for 100-200 million years.
Explain plume activity under Africa and its implications.
A superplume is thought to exist beneath Africa, which would explain the large number of hot spots across the continent. The plume is throught to have spread out beneath the crust, and wherever the is weakness, we have a hot spot. This theory is thought to explain why Africa has a higher elevation than expected given its composition.
Why does the magma of a mantle plume try to rise through the crust?
It has a lower density (2.5 g/cm3) vs oceanic (3.0 g/cm3) and continental (2.7 g/cm3) crust. With oceanic crust, the magma almost always breaks through the lithosphere. With continental crust, it still tries to break through, and evenutally succeed.
Do plumes produce spreading centers? If so, how? Examples?
When a mantle flume breaks through the lithosphere, it may break the plate into 2-3 segments, dpending on how well it is centred. The failed rift is called an aulacogen.
95% percent of the force contributing to this is gravity, so once the slab is broken it is pulled by gravity into a subduction zone.
Example. A hot spot sits between the Red Sea, Gulf of Aden and East African Rift. The East African Rift failed to diverge, but the other two seas widen every year.
