chapter 10

Oceans cover approximately ________ of Earth's surface area?

70%

Where are most of the current active continental margins located on Earth today? Why? What is the relation of the age of rocks on the ocean floor to the location of these active margins?

Active continental margins are associated with trenches. They are formed along convergent plate boundaries where an oceanic plate is being subducted under continental lithosphere. Examples: Mariana Trench, Puerto Rico Trench. The relation between ocean floor ages and the trench is not straightforward. It depends on the distance to the nearest divergent boundary. For instance, the divergent boundary on the western edge of the small Juan de Fuca plate is producing very young basalts that are then "quickly" subducted in the trench on its east side, near the coasts of Washington and Oregon.

Briefly discuss the formation and structure of oceanic crust. What rock compositions are involved and how do these compositions change as a result of interaction with seawater?

Based on ophiolite complexes, the oceanic crust is made of four layers. Layer 1-sediment; Layer 2-pillow basalts; Layer 3-sheeted dike complex; Layer 4-gabbro. Mafic magma is primarily generated through decompression at the ocean ridges. The resulting rocks (pillow basalts vs. basaltic dikes vs. gabbro) depend on how and where the magma solidifies (temperature, pressure, intrusive, extrusive). The fractured lava allows seawater to penetrate to depths of 2-3 km. As it circulates through the hot crust, it heats up and produces hydrothermal metamorphism. Simultaneously the elements/minerals dissolve into the hot seawater. As the buoyant hot seawater flows back through the ocean floor into the sea, the dissolved elements/minerals will precipitate into smokers.

________ correctly formulated a basic theory concerning the formation of atolls in the 1800s.

Charles Darwin

________ proposed a hypothesis (that later became a theory) on the creation of atolls.

Charles Darwin

________ are not associated with a mid-ocean ridge.

Deep ocean trenches

List several areas on Earth where continental rifting is occurring today. Eventually, what will happen to these areas as rifting continues? Finally, how is continental rifting similar to seafloor spreading and how is it different?

Examples: East Africa Rift, Rio Grande Rift (southwest United States), Baikal Rift (south-central Siberia), and Rhine Valley (northwest Europe). As rifting continues, the system will evolve into a young ocean basin (e.g., Red Sea), then into a mature ocean with passive continental margins (e.g., Atlantic Ocean), then one with active continental margins (e.g. Pacific Ocean), and finally into a continental-continental convergence zone (e.g., Himalaya Mountain region). Continental rifting is similar to seafloor spreading in that new magma is being produced from decompression and some of it erupts on the surface. It is different in that it has interconnected rift valleys that are underlain by continental lithosphere.

Abyssal plains with sediments covering the seafloor igneous rocks are more extensive in the central Pacific basin than in the North Atlantic.

False

In the oceans, calcium carbonate is more soluble in warm, surface waters than in deeper, colder waters.

False

Seafloor hot springs occur mainly in oceanic, abyssal plains.

False

Submarine canyons form the deepest parts of the ocean basins.

False

________ continental margins typically exhibit wide, extensive, continental shelves.

Passive

________ is (are) not part of an ophiolite complex.

Residual, unmelted, crustal lithosphere

________ is characterized by the subduction of old dense lithosphere.

Spontaneous subduction or Mariana-type subduction zone

________ is the oceanward edge of a continental shelf.

The top of the continental slope

Geologically, what is the best way to explain the thousands of feet of coral limestone beneath most atolls?

The volcano never reached the surface, allowing a very thick cap of coral limestones to accumulate.

Which of the following is NOT true of deep ocean trenches?

They are geologically very stable.

An atoll is a low, coral reef island typically surrounding an interior lagoon.

True

Submerged, flat-topped seamounts are known as guyots.

True

The west coast of South America and the east coast of North America have very different continental margins.

True

A(n) ________ is the vast, relatively deep, flat, sediment-covered portion of the deep-ocean basin.

abyssal plain

Seamounts ________.

are volcanoes that form on the sea floor.

Where in the oceans are biological communities thriving without sunlight?

around seafloor, hot spring vents

A low lying, coral limestone island with a central lagoon is called a(n) ________.

atoll

The continental rise is located ________.

between an abyssal plain and continental slope

The ________ lies at the base of the continental slope.

continental rise

The gently sloping submerged surface extending from the shoreline toward the ocean is termed the ________.

continental shelf

Geologically, ________ are actually submerged parts of the continents.

continental shelves

Submerged, flat-topped volcanoes that rise from the deep ocean floor are called ________.

guyots

"Black smokers" are associated with ________.

metal-rich sulfide deposits, hot water, oceanic ridges

Submarine canyons found on the continental slope and rise have been created ________. -because of a plate plunging into the mantle -by rivers during the ice age -by faulting -none of these

none of these

A(n) ________ is a volcanic mountain, built up from the seafloor, that never reached the sea surface.

seamount

Water depths and seafloor topography are now routinely determined by ________.

ship-mounted echo sounders

An echo sounder operates by measuring the time required for a ________.

sound pulse to travel from a ship to the seafloor and back

Valleys that lead from the continental shelf into deeper waters are known as ________.

submarine canyons

Which one of the following is not connected in any way with submarine, hot spring vents?

thick turbidites on the continental rise

A(n) ________ marks the site where old, oceanic lithosphere begins its descent into a subduction zone.

trench

The Atlantic and Pacific basins have oceanic ridges; the Indian Ocean has no oceanic ridge.

False

The average height of the continents above sea level is greater than the average depth of the ocean basins below sea level.

False

The continental rise lies at the top of the continental slope.

False

Young oceanic lithosphere typically experiences what type of subduction and at what angle?

Forced Subduction; shallow angle

In the 1870s, the ________ was involved in the first far-reaching, comprehensive study of Earth's oceans.

H.M.S. Challenger

A(n) ________ is a low lying, coral reef island perched above a sunken, truncated volcano.

atoll