Chapter 2

Did Wegener's idea of continental drift gain traction?

- 1915 he published Origins of Continents and Oceans that suggested that continents plowed through ocean basins because of the driving force of gravitational attraction and tidal forces from the sun and moon - he received lots of criticism because tidal gravitational attractions are too small to move continents so his mechanism defies laws of physics

Why was the idea of continental drift important?

It brought us to the theory accepted today which is that there are plate tectonics

What are intraplate features?

- volcanic islands within a plate - island chains - theory of plate tectonics doesnt seem to explain this

What are seamounts?

- volcanoes that are cone shaped on top

What are tablemounts/guyots?

- volcanoes that are flat on top

Who was Harry Hess?

- WWII submarine captain and geologist - left his depth recorder on while his ship was traveling the sea - recordings showed extensive mountain ridges near the centers of ocean basins and deep narrow trenches at the edges of ocean basins

What was the History of Ocean Basins?

- book published by Hess in 1962 - contained the idea of seafloor spreading and the associated circular movement of rock material in the mantle (convection cells)

Who are Frederick Vine and Drummond Matthews

- combined the patterns of magnetic sea floor stripes with the process of sea floor spreading to explain the pattern of alternating and symmetric stripes on the sea floor - said one was because of normal magnetic pole and the other was from reversed magnetic pole

What is a magnetic dip?

- degree to which a magnetite particle points into earth - directly related to latitude

What are ultra slow spreading centers?

- discovered along the southwest indian and arctic segments of the mid ocean ridge - spreading rates of less than 2 cm/yr - deep rift valley - volcanoes that occur in widely spaced intervals - spreading so slow that the earth's mantle is exposed on the ocean floor between the volcanoes

What is an oceanic rise?

- fast spreading - gently sloping - example is the east pacific rise between the pacific and nazca plates (avg spreading rate is 16.5 cm/yr)

What is the difference between fast spreading and slow spreading segments of the mid-ocean ridge?

- fast spreading produces broader and less rugged segments of the global mid ocean ridge system; rocks have less time to cool, contract, and sink in a process called subsidence; slope is less steep than slow - slow spreading produces segments that are larger and better developed

What are divergent plate boundaries?

- found along oceanic ridges where the new lithosphere is being added - two plates moving apart - oceanic-oceanic: new sea floor is created; tectonic process is sea floor is spreading; features are mid ocean ridge, volcanoes, young lava flows; examples are mid-atlantic ridge and east pacific rise - continental- continental: as a continent splits apart new seafloor is created; tectonic process is continental rifting; features are rift valley, volcanoes, and young lava flows; examples are east africa rift valleys, red sea, and gulf of california

What are transform plate boundaries?

- found where lithospheric plates slowly grind past one another - shallow but strong earthquakes in lithosphere - tectonic process is transform faulting - sea floor feature is fault - example of oceanic transform plate boundaries: mendocino fault, eltanin fault (between mid-ocean ridges) - example of continental transform plate boundaries: san andreas fault, alpine fault (new zealand)

what are the three stages of the development of a coral reef?

- fringing reefs: initially develop on the side of a landmass where temp, salinity and turbidity is suitable; associated with active volcanoes; not very thick or well developed; amount of living coral is relatively small; if sea level doesnt rise or land doesnt subside then the process stops here - barrier reef: linear or circular reefs separated from a landmass by a well-developed lagoon; grows upward; largest one is great barrier reef - atoll: encloses a lagoon but generally has many channels that allow for circulation with the ocean; buildup of crushed debris often form narrow islands that encircle the central lagoon and are big enough for human habitation; continue to grow after volcanoes are submerged

Who was Alfred Wegener?

- german meteorologist and geophysicist who was the first to propose the idea of mobile continents (1912) and called it continental drift - proposed that all of the continents used to fit together and named it Pangaea and the huge ocean Panthalassa (existed 540 to 300 million years ago) - ocean Panthalassa also included smaller seas such as Tethys Sea

What are transform faults?

- gives the mid ocean ridge a zig zag appearance - offsets oriented perpendicular to the mid ocean ridge - occurs between two segments of the mid ocean ridge - oceanic is ocean floor only - continental cuts across a continent

What is heat flow?

- heat from the earth's interior is released to the surface - very high at mid ocean ridges and low at subduction zones (expected because trenches are double the thickness of mid ocean ridge) - current models indicate the heat moves to the surface with magma in convection motion - heat is carried to mid ocean ridge spreading centers

What rocks are affected by earth's magnetic field?

- igneous rocks solidify from magma and contain magnetite - these particles (once cooled) are frozen into position which indicates the angle of earth's magnetic field at that place and time since they align themselves with the magnetic field since they are magnetic - magnetite is also found in sedimentary rocks and basalt

What is a magnetometer?

- instrument towed behind ocean vessel - measures the earth's magnetic field and how it was affected by sea floor rocks

How is an ocean basin formed?

- molten material rises to the surface causing an upwarping and thinning of the crust - volcanic activity produces vast quantities of high density basaltic rock - as plates begin to move apart, a linear rift valley is formed and volcanism continues - further splitting of the land (rifting) and more spreading cause the area to drop below sea level - then the rift valley floods with sea water and a young linear sea is formed - after millions of years of sea floor spreading , a full fledged ocean basin is created with a mid ocean ridge in the middle of the two landmasses

What is the Wilson Cycle?

- named after John Tuzo Wilson - uses the plate tectonic process to show the distinctive life cycle of an ocean basin during their formation, growth, and destruction over many millions of years

Where is the oldest oceanic crust? Where is the newest oceanic crust?

- oldest is furthest away from mid ocean ridge (farther you get the older it is) - newest is along the mid ocean ridge

What evidence supports plate tectonics?

- paleomagnetism - sea floor spreading/pattern

What is some evidence that supports the idea of continental drift?

- puzzle like fit of the continents - similar rock types, ages, and structures on different continents (where it is proposed they would have fit together) - occurrence of past glacial activity in areas that are now tropical (Africa, South America, India, Australia) - direction of glacial flow is consistent with the grooves found on many continents today - plant and animal fossils indicate different climate than today - animal fossils and plants fossils are found on different continents (ex Mesosaurus fossils have only been found in eastern south america and western africa) - modern organisms with similar ancestries

What are the forces that drive plate movement?

- slab pull: generated by the pull of weight of a plate as it sinks underneath an overlying plate, pulling the rest of the plate behind it in a similar fashion to how a heavy comforter often slides off the bed and onto the floor - slab suction: created as a subducting plate drags against the viscous mantle and causes the mantle to flow toward the subduction zone, thereby sucking in nearby plates much in the same way pulling a plug from a full bathtub draws floating objects toward the drain - there is a weak, partially molten layer at the base of a lithosphere that aids sliding and may reduce the force required for plate subduction

What are oceanic ridges?

- slow spreading - steep slopes - example is the mid atlantic between the south american and african plates (avg spreading rate is 2.5 cm/yr)

What are ocean trenches?

- where ocean floor is being destroyed - deepest parts of ocean floor - resemble a narrow crease of trough on a map of sea floor - largest earthquakes occur near these (caused by plate bending downward and slowly plunging back into earth's interior (process is called subduction))

What are convergent plate boundaries?

- where plates are moving together and one plate subducts beneath the other - plate movement is together and they collide - deep ocean trench - volcanic arc - oceanic-continental: old sea floor is destroyed; tectonic process is subduction; features are trench and volcanic arc on land; examples are peru-chile trench and andes mountains; denser plate is subducted - oceanic-oceanic: old sea floor is destroyed; tectonic process is subduction; features are trench and volcanic arc as islands; examples are mariana trench and aleutian islands - continental-continental: tectonic process is collision; feature is tall mountains; examples are himalaya mountains and alps - deep focus earthquakes - mineral structure changes associated

What is a terrane (exotic terrane)?

-fragments of crustal material broken off from one plate and accreted or sutured onto another -preserve their own distinctive geological history that is different from the surrounding area

How old is the oldest ocean floor?

180 million years old

When did the idea of plate tectonics/continental drift become more accepted?

1960s

How much sea floor is created each year by sea floor spreading?

20 cubic km

How old is the ocean?

4 billion years old

How many tectonic plates are there?

7 major ones with many smaller ones

When was the last time there was a magnetic reversal?

750,000 years ago

Where are most plate boundaries?

90% are on the sea floor

Are plates moving the same speed as they always have been?

Evidence shows they are moving slower today than they once were (move 2-12 cm per year now vs 19-30 cm a year a while ago)

Does the magnetic north pole always correlate with the geographic North Pole?

No

Does the process that regenerates the ocean floor also happen on land?

No

Has the magnetic polarity always been the same?

No, the magnetic polarity can switch causing magnetic north to be magnetic south - 184 major reversals have happened in the last 83 million years - reversal is highly irregular (ranges from 25,000 years to more than 30 million years) - average reversal is once every 450,000 years - average about 5,000 years to flip (1,000 to 25,000) - recorded in rocks by the magnetite

Where is the rate of sea floor spreading the greatest?

Pacific

Are plates being continuously pulled or pushed apart?

Plates are being continuously pulled apart

What is paleomagnetism?

The study of Earth's ancient magnetic field - interprets where rocks first formed

What is a rift valley?

a central downdropped linear depression

What is the mid ocean ridge?

a continuous underwater mountain range that winds through every ocean basin in the world and resembles the seam on a baseball - volcanic in origin - wraps one and a half times around the world - rises more than 2.5 km above surrounding deep ocean floor - rises above sea level in some places (ex. Iceland) - new ocean floor forms at the crest, or axis, of mid ocean ridge

What is a volcanic arc?

a convergent plate boundaries that creates a chain of volcanoes - arc shaped row of highly active volcanoes - generally parallels the trench and occurs above the subduction zone

Where is the spreading center located?

along the axis of the mid ocean ridge

What is an island arc?

an arc shaped row of volcanic islands - generated by oceanic-oceanic convergence

What are hotspots?

areas of intense volcanic activity that remain in more or less the same location over long periods of geological time and are unrelated to plate boundaries - ex. volcanism in Yellowstone and Hawaii

What ocean has the simplest and most symmetric pattern of age distribution?

atlantic (pacific is the least due to subduction zones)

What was the structure and distribution of coral reefs?

book published by Charles Darwin in 1842 that suggested origin of a coral reef depends on the sinking of volcanic islands

What is a nematath?

chain of extinct volcanoes that is progressively older as one travels away from a hotspot - hotspot track

What is continental acceleration?

continental material added to edges of continents through plate motion to create larger land masses

What is a continental arc?

created by andesitic volcanic eruptions and by the folding and uplifting associated with plate collision - generated from oceanic-continental convergance

How is the energy released during an earthquake related to the spreading rate?

for divergent plate boundaries they are very closely related - faster the sea floor spreads the less energy released

What are two of the youngest seas in the world?

gulf of california and read sea

Who was Sir Edward Bullard?

in 1960s, used a computer model to fit the continents together and noticed they fit together best when they were 2,000 m below sea level which is halfway between shoreline and deep ocean basin

Has the Earth's magnetic field remained steady over time?

it has been decreasing by about 5% every decade - may be an indication that the polarity is about to flip

What are magnetic anomalies?

lines of alternating bands of magnetic fields that are solidified into place as new crust is formed - pattern of north-south magnetism "stripes"

How is earthquake intensity measured?

measured on a scale called the seismic moment magnitude which reflects the energy released to create long-period seismic waves - for earthquakes in the rift valley of mid atlantic ridge reach a max magnitude of 6 - for earthquakes occuring along the axis of the fast spreading east pacific rise rarely exceeds 4.5

What is a subduction zone?

oceanic trench site of crust destruction - sloping area from the trench along the downward moving plate - subduction can generate deep ocean trenches

What layer of the earth are plate tectonics located?

part of the lithosphere, floats on the more fluid asthenosphere

What is the most convincing evidence that shows sea floor spreading?

pattern of alternating and symmetric stripes all over the sea floor

What is paleogeography?

study of historical changes of continental shapes and positions

What are earthquakes?

sudden releases of energy caused by fault movement or volcanic eruptions - occur at ocean trenches (indicative of energy released during subduction) - occur at ridges (indicative of energy released from sea floor spreading) - also occur due to plate tectonics hitting each other - earthquake activity mirrors tectonic plate boundaries

What do the deposits of ancient glaciation in lower latitude regions suggest?

there was a worldwide ice age OR some continents that are now in tropical areas were once located much closer to the poles - unlikely that the entire earth was covered in ice due to coal deposits from the same geological age (300 million years ago)

What is a mantle plume?

vertical tube shaped areas of hot molten rock that arise from deep within the mantle

What are plate boundaries?

where plates interact with each other - associated with mountain building, volcanic activity, and earthquakes - different types are divergent, convergent, and transform boundaries