Geology Ch 5

What is the average rate of seafloor spreading in modern oceans?

5 centimeters (2 inches) per year.

Continental-Continental Convergence

Continental crust is buoyant -Neither plate subducts during continent-continent collisions

Briefly describe the process of continental rifting. Name a location where it is occurring today.

Continental rifting is a divergent margin forming in continental crust. (when divergent boundaries develop within a continent) The landmass splits by divergent plate movements pulling the lithosphere apart, resulting in the formation of an ocean basin within the split crust.

Paleomagnetism

The study of the alignment of magnetic minerals in rock,specifically as it relates to the reversal of Earth's magnetic poles; also the magnetic properties that rock requires during formation

What new findings about the ocean floor did oceanographers discover after World War II?

The oceanic ridge system.

What is the age of the oldest sediments recovered using deep-ocean drilling? How do the ages of these sediments compare to the ages of the oldest continental rocks?

The oldest seafloor sediments recovered are 180 million years old. This is much much younger than the oldest continental rock, which exceeds 4 billion years in age.

Subduction

The process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary.

What was the first line of evidence that led early investigators to suspect that the continents were once connected?

The puzzle-piece fit of the continents—especially Africa and North and South America.

East African Rift

demonstrates initial stage of continental rifting

Name two plates that are growing in size. Name a plate that is shrinking in size.

e African and Antarctic plates are growing. The Pacific plate is shrinking.

Continental volcanic arcs

form in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent.

Transform plate boundaries

form when two plates slide horizontally past one another - Transform faults- No lithosphere is produced or destroyed- Connect spreading centers and offsets oceanic ridges

Convergent plate boundaries

occur when two plates move toward each other - Convergence rate is equal to seafloor spreading - Characteristics vary depending on subducting crust - Subduction zones

Convection

occurs as hot, less dense material rises and surface material cools and sinks

Oceanic lithosphere

varies in thickness • Thin at ridges, up to 100 km thick in deep-ocean basins • Mafic composition • More dense than continental lithosphere

Volcanoes in the Hawaiian Island-Emperor Seamount Chain increase in age with distance from the Big Island of Hawaii

- A cylinder of upwelling hot rock (mantle plume) is beneath Hawaii - A hot spot is an area of volcanism, high heat flow, and crustal uplift above a mantle plume - A hot-spot track formed as the Pacific Plate moved over the hot spot

Oceanic lithosphere + continental lithosphere = subduction of oceanic lithosphere

- Continental lithosphere is less dense - Water from descending oceanic crust triggers partial melting of asthenosphere at ~100 km - Molten material is less dense and rises • Continental volcanic arcs

Oceanographic exploration increased dramatically following World War II. They found:

- Discovery of global oceanic ridge system - Earthquakes at great depths in western Pacific ocean trenches - No oceanic crust older than 180 million years - Thin sediment accumulations in deep-ocean basins (These developments led to theory of plate tectonics)

Interaction between plates at plate boundaries

- Divergent boundaries (constructive margins) • Two plates move apart • Upwelling of hot material from mantle creates new seafloor - Convergent boundaries (destructive margins) • Two plates move together • Oceanic lithosphere descends and is reabsorbed into mantle • Two continental blocks create a mountain belt - Transform plate boundaries (conservative margins) • Two plates slide past each other• No lithosphere is created or destroyed

Deep ocean trenches

- Long, linear depressions - Result of subduction

Characteristics of convergent plate boundaries vary depending on type of crust being subducted

- Oceanic + continental - Oceanic + oceanic - Continental + continental (not too sure about this, was in the ppt)

Lithosphere is broken into irregular plates

- Plates move as rigid units relative to other plates - 7 major plates make up 94% of Earth

Glossopteris

- Seed fern - Africa, Australia, India, South America, and Antarctica

Mesosaurus

- Small Permian aquatic freshwater reptile - Found in eastern South America and western Afric

List the three types of plate boundaries and describe the relative motion along each.

-Convergent boundaries occur where plates are moving toward each other. These are called "destructive" margins because crust is destroyed during convergence. -Divergent boundaries occur where plates are moving away from each other. These are called "constructive" margins because new magma flows to fill the gap that forms between the plates and cools into new crust. -Transform boundaries are conservative because as the plates slide past each other, no crust is formed or destroyed.

List four features that characterize the oceanic ridge system.

-It covers 20 percent of Earth's surface. -The crest is commonly 2 to 3 kilometers higher than adjacent ocean basins. -It varies in width from 1000 to over 4000 kilometers. -There is a rift valley along the crest of some ridge segments.

continental drift

-The hypothesis that states that the continents once formed a single landmass, broke up, and drifted to their present locations - Single supercontinent of all of Earth's land: Pangaea

Describe slab pull and ridge push. Which of these forces appears to contribute more to plate motion?

1. Slab pull is subduction of cold, dense slabs of oceanic lithosphere acting as the driving force of plate motion. 2. Ridge push is a gravity-driven mechanism resulting from the elevated position of the oceanic ridge, causing slabs of lithosphere to "slide" down the flanks of the ridge.

List two characteristics that differentiate transform faults from the two other types of plate boundaries.

1. Transform faults have motion within the horizontal plane rather than the vertical plane. 2. Transform faults are conservative: no crust is created or destroyed. Both divergent and convergent plate boundaries have motion within the vertical plane and create or destroy crust, respectively.

Continental lithosphere

150-200 km thick • Felsic composition

oceanic ridge system

A system of continuous elevated zones on the floor of all major ocean basins and varying in width from 500 to 5000 km (300 to 3000 mi). The rifts at the crests of ridges represent divergent plater boundaries.

Describe how Fred Vine and D. H. Matthews related the seafloor-spreading hypothesis to magnetic reversals.

After researchers discovered alternating stripes of high- and low-intensity magnetism, Vine and Matthews suggested that the stripes of high-intensity magnetism are regions where the paleomagnetism of the ocean crust exhibits normal polarity, while low-intensity magnetism is due to reversed polarity. They reasoned that as magma solidifies along narrow rifts at an oceanic ridge, it is magnetized with the polarity of the existing magnetic field, and because of seafloor spreading, these strips are carried away in opposite directions.

What characteristic of a slab of oceanic lithosphere leads to the formation of deep- ocean trenches instead of shallow trenches?

As oceanic crust is subducting beneath another tectonic plate, it is sinking deep into the mantle. Subduction of these plates occurs at various angles, depending on how cool and dense the subducting slab is. In all cases, the sinking motion as one plate drags beneath the other creates a deep and linear "deep-ocean trench." The density of oceanic plates results in the formation of deep trenches as opposed to shallow trenches.

Briefly describe the view held by most geologists regarding the ocean basins and continents prior to the 1960s.

Before the 1960s, most geologists figured that the continents and oceans were unmoving, were fixed in their geographic locations, and had been so for essentially all of time.

What distinguishes a continental volcanic arc from a volcanic island arc?

Both continental volcanic arcs and volcanic island arcs are arcuate chains of volcanoes formed at subduction zones. However, continental volcanic arcs form at oceanic-continental subduction zones, and volcanic island arcs form at oceanic-oceanic subduction zones.

Early in the twentieth century, what was the prevailing view of how land animals migrated across vast expanses of open ocean?

Early views were far-fetched ideas about rafting, land bridges, and island stepping-stones to get from one continent, across an ocean, and onto another continent.

Briefly describe changes in the positions of the continents if we assume that the plate motions we see today continue 50 million years into the future.

In North America, the Baja Peninsula and the portion of Southern California that lies west of the San Andreas Fault will have slid past the North American plate. If Africa continues to collide with Eurasia, the result will be the closing the Mediterranean. Australia will be astride the equator and, along with New Guinea, will be on a collision course with Asia. Meanwhile, North and South America will begin to separate, while the Atlantic and Indian Oceans continue to grow at the expense of the Pacific Ocean.

fracture zones

Linear breaks in the seafloor - Fracture zones are inactive - Active faults occur between offset ridge segments

Compare and contrast Earth's lithosphere and asthenosphere.

Lithosphere: -outermost, cool, rigid, and strong outer layer of Earth's crust -varies in thickness and density depending on whether it forms oceanic crust (thinner, denser) or continental crust (thicker, less dense) -Responds to forces by bending or breaking Asthenosphere: -uppermost part of the Earth's mantle; it is hot and weak and flows like a very thick liquid. -hotter, weaker mantle below the lithosphere - Rocks are nearly melted at this temperature and pressure - Responds to forces by flowing- Moves independently from lithosphere

Explain why the discovery of the fossil remains of Mesosaurus in both South America and Africa, but nowhere else, supports the continental drift hypothesis.

Mesosaurus was a freshwater reptile, and migration between the continents as they are now would have been impossible; however, if they were connected (as in Pangea), then this animal could have easily moved between the two by freshwater river systems.

List the seven largest lithospheric plates.

North American South American Pacific African Eurasian Australian-Indian Antarctic

What group of geologists were the least receptive to the continental drift hypothesis? Why?

North American geologists were least receptive to the continental drift hypothesis because the vast majority of the evidence collected to support the hypothesis came from everywhere on Earth except North America.

Why does oceanic lithosphere subduct, while continental lithosphere does not?

Oceanic lithosphere is basaltic in composition, which is rich in iron and magnesium, making it dense. Continental lithosphere is granitic in composition, rich in silica, and not very dense. In the meeting of two tectonic plates, the denser one will be the one forced to sink and subduct beneath the other. Continental crust will always be the least dense in a meeting of two plates, so it will never subduct.

Sketch or describe how two plates move in relation to each other along divergent plate boundaries.

Plates at divergent boundaries are moving away from each other. They produce long and narrow fractures where they move apart. These fractures fill and seal up with newly formed crust by magma welling up from below.

Sketch or describe how two plates move in relation to each other along a transform plate boundary.

Plates slide horizontally past one another without the production or destruction of crust.

How do sedimentary cores from the ocean floor support the concept of seafloor spreading?

Sedimentary cores show patterns of magnetized rock thanks to the natural compass properties of magnetite. The cores from either side of the ocean ridge have patterns that match both in strata thickness and in paleomagnetism.

What geological processes are associated with the upward and downward circulation in the mantle?

Subducting oceanic plates drive the cold downward-moving portion of convective flow, while shallow upwelling of hot rock along the oceanic ridge and mantle plumes drive the upward movement of convective flow.

Explain why the rate of lithosphere production is roughly equal to the rate of lithosphere destruction.

Tectonic plate motion equals out in all directions—while some crust is being formed at divergent margins, crust elsewhere is also being destroyed by subduction at convergent margins.

What new ocean basin was created by the breakup of Pangaea?

The Atlantic Ocean basin was created through the breakup of Pangea.

Briefly describe how mountain belts such as the Himalayas form.

The Himalayas are the product of continental-continental collision. This is where two continental plates collide, and because neither is dense enough to sink into the mantle, they ram into each other, crumpling and buckling into faults and folds as the mountain belt is driven upward.

Assuming that hot spots remain fixed, in what direction was the Pacific plate moving while the Hawaiian Islands were forming?

The Pacific plate was moving northwest when the Hawaiian Islands were forming.

Briefly describe the two models proposed for mantle-plate convection.

The whole-mantle model, in which cold oceanic lithosphere sinks to nearly the core- mantle boundary and stirs the entire mantle The layer cake model, in which the driving forces are the same, but there are two zones of convection: a thin, dynamic layer in the upper mantle and a thick, larger, sluggish one located below

How did Wegener account for evidence of glaciers in portions of South America, Africa, and India when areas in North America, Europe, and Asia supported lush tropical swamps?

Wegener's Pangea put the southern parts of the land mass near the South Pole, a good place for glaciers, while the swampy parts of North America, Europe, and Asia were located near the equator.

Describe two aspects of Wegener's continental drift hypothesis were objectionable to most Earth scientists.

Wegener's suggestions of how the continents moved were problematic. He suggested that gravitational forces of the Moon and Sun were capable of gradually moving the continents and that larger and sturdier continents broke through thinner oceanic crust, much as icebreakers cut through ice.

Rift valley

a deep canyon along the crest of a ridge resulting from tensional forces

Seafloor spreading

process by which new seafloor is created along the ocean ridge system