PM 13 The Ocean Floor

Features of continental margins The features of active and passive continental margins and the characteristics of ocean basins are evidence for existence, or absence of a plate boundary. Read the sentences below, and fill in the blanks to make them complete and correct. Match the words in the left column to the appropriate blanks in the sentences on the right. Make certain each sentence is complete before submitting your answer. Note that some terms will remain unused.

1. Oceanic crust spreads at mid-ocean ridges, creating new crust and slowly enlarging the ocean basin. 2. Submarine canyons are deep valleys at passive continental boundaries. They are likely carved out by turbidity currents, which occur when sediment-laden water rapidly moves down slope. 3. Melt from the subducting plate rises to the surface, erupts through the crust on the overriding plate and forms continental volcanic arcs on continents, or volcanic island arcs in the ocean. 4. Subduction zones surrounding the Pacific Ocean give rise to volcanoes. This area surrounding the Pacific Ocean is called the Ring of Fire.

The oceans cover approximately ________ percent of Earth's surface.

70

What is a table mount?

A seamount that grew above sea level, experienced wave erosion, and is now inactive.

________ continental margins occur where oceanic lithosphere is being subducted beneath the edge of a continent.

Active

Ocean basin size and continental margins Ocean basins change size over long periods. The type of tectonic activity is an important reason for this. - Oceans expand where new crust is being formed by tectonic activity. - Oceans shrink where crust is destroyed by a different type of tectonic activity. New ocean crust is formed at mid-ocean ridges that drive older crust further and further away from these ridges. - The Atlantic Ocean has a mid-ocean ridge that trends north-south and the Indian Ocean has one that trends east-west. - The north and central portion of the Pacific Ocean does not contain a spreading center. You can think of oceanic crust working as a conveyor belt: - Crust is formed at a mid-ocean ridge, travels for millions of years, then is destroyed at subduction plate boundaries. - Older oceanic crust subducts (dives) beneath continental crust at active continental margins where the subducted crust is destroyed. - Passive continental margins are not associated with a type of plate boundary because the crust along these margins does not change much. We can infer that subduction occurs at active continental margins that have deep ocean trenches. We can make this inference because trenches form where subducting plates meet overriding plates. Topographic maps are a useful tool to determine if a margin has a trench. The trench suggests that subduction is present, which indicates the margin is active. Compare continental margins and ocean basins of the Atlantic and Pacific oceans using the map below. Deep ocean trenches are delineated with red lines, which indicates the margin is active. Label these features correctly for margin type and ocean basin dynamics. Drag the appropriate items to their respective bins.

Active ContinentalMarginsActive ContinentalMargins Continental margin D Continental margin E Passive ContinentalMarginsPassive ContinentalMargins Continental margin A Continental margin B Continental margin C Shrinking Ocean BasinShrinking Ocean Basin Pacific Ocean Basin Expanding Ocean BasinExpanding Ocean Basin Atlantic Ocean Basin

How are active continental margins related to plate tectonics?

Active continental margins are located along convergent plate boundaries, where oceanic lithosphere subducts beneath the edge of the continent.

Select the correct description of a passive continental margin and the correct description of an active continental margin. Select one for each.

An active continental margin is located along a convergent plate boundary, has a deep-ocean trench where subduction takes place, and may have evidence of an accretionary wedge or of subduction erosion. A passive continental margin is tectonically inactive and features a continental shelf, a continental slope, and a continental rise (from land toward the sea).

What is a seamount?

An active volcano that occurs along the crest of the mid-ocean ridge.

Which of the following mountain ranges are examples of continental arcs?

Andes and Cascades

Excluding the Southern Ocean, name the four main ocean basins from the smallest surface area to the largest surface area.

Arctic Ocean, Indian Ocean, Atlantic Ocean, Pacific Ocean

The ________ Ocean has more extensive abyssal plains than the Pacific Ocean because it has fewer trenches to trap sediments moving down the continental slope.

Atlantic

What is the difference between a barrier reef and an atoll?

Barrier reefs surround an inactive volcano emerging above sea level, while atolls surround the previous location of a submerged volcano.

________ sediment consists of shells and hard parts of marine organisms.

Biogenous

The deepest place in the world's oceans is known as the ________.

Challenger Deep

Which of the following statements about coral reefs and seamounts is most accurate?

Coral reefs at the ocean surface grow away from seamounts over time.

How are deep-ocean trenches related to plate boundaries?

Deep-ocean trenches are sites of plate convergence where an oceanic plate subducts under another plate.

Which is the correct order of coral reef development near seamounts?

Fringing reef, barrier reef, atoll

The first systematic bathymetric measurements came on the voyage of ________.

HMS Challenger

Describe the three basic types of seafloor sediments, with one example of each. Select all that apply.

Hydrogenous sediments, which are derived from water, include manganese nodules. Terrigenous sediments, which are derived from land, include weathered grains from rocks. Biogenous sediments, which are derived from organisms, include calcareous ooze.

Contrast the distribution of land and water in the Northern Hemisphere and the Southern Hemisphere.

In the Northern Hemisphere, 61 percent of the surface is water whereas 39 percent of the surface is land. In contrast, the surface of the Southern Hemisphere is 81 percent water and 19 percent land.

Where do coral reefs exist with sea mounts?

In tropical latitudes

Why is manganese not mined on the seabed given that there are abundant manganese nodules in some parts of the ocean floor?

It is not economical to mine manganese nodules at present-day world prices for manganese and associated metals in nodules.

How does the area of Earth's surface covered by oceans compare with the area covered by land?

Nearly 71 percent of Earth's surface is covered by global oceans, whereas roughly 29 percent of Earth's surface is covered by land.

The ________ Ocean is largest.

Pacific

The lowest point on earth is in the ________ Ocean.

Pacific

Which ocean has the greatest average depth?

Pacific

Why do coral reefs at the ocean surface grow away from seamounts over time?

Seamounts subside as tectonic plates move. Coral reefs grow away from the seamount in order to stay near the sunlight.

Which of the following is true regarding shallow subduction angles?

Sediment and seamounts are scraped from the down-going plate to produce an accretionary wedge.

_______ sediments are those that are generated on land.

Terrigenous

Which of the following correctly describes the Atlantic and Pacific continental margins?

The Atlantic is a passive margin, and the Pacific is an active margin.

What would happen to Earth if ocean floor were created at divergent boundaries at a faster rate than it is destroyed at convergent boundaries?

The Earth would increase in volume.

Which of the following oceans is the largest and deepest on Earth?

The Pacific Ocean

How does the average depth of the oceans compare to the average elevation of the continents?

The average depth of the oceans is nearly four and a half times the average elevation of continents above sea level.

What is the relationship between the crust and lithosphere?

The crust is part of the lithosphere.

What is the primary reason for the elevated position of the oceanic ridge system?

The oceanic lithosphere at the ridge is hotter than the older lithosphere that is farther from the ridge. Because the hotter lithosphere at the ridge is less dense than the cooler lithosphere farther away, the lithosphere is more elevated at the ridge, sinking as it cools as it moves away from the ridge.

The 2010 Deepwater Horizon oil spill in the Gulf of Mexico was the largest environmental disaster in North American history. It was brought on in part by new technology allowing development of oil and gas reserves in deep water. Given the scope of the spill, why do companies take a risk of such a disaster given the scope of the litigation following the disaster?

The potential profit is so large from a major oil find that the companies take the risk, try to use all cautions to minimize risk, but sometimes fail.

How can satellites orbiting Earth determine features on the seafloor without being able to directly observe them beneath several kilometers of seawater?

The satellite measures the variation in the sea-surface elevation by bouncing microwaves off the water's surface, which mimics the shape of the seafloor.

Choose the true statement regarding the continental shelf.

The shelf represents the flooded portions of continents.

Why are seafloor sediments useful in studying past climates?

These sediments are composed of organisms that once lived near the surface of the sea. Because the number and types of these organisms change as the climate changes, the sea sediments they create provide a history of changes in climate.

Which statement best explains the formation of seamounts?

They represent volcanoes situated over mantle plumes that have since moved away from their source of magma and subsided.

Why are abyssal plains more extensive on the floor of the Atlantic than on the floor of the Pacific?

Unlike the floor of the Pacific Ocean, the floor of the Atlantic Ocean has very few trenches to act as traps for sediment carried down the continental slope.

African dust from the Sahara desert is observed across the Atlantic as far west as the Caribbean region where it produces a hazy sky condition during the annual dry season. You are a climate scientist and make a prediction that the Sahara was not a desert during glacial periods in the northern hemisphere. How could you test this hypothesis by looking at deep sea sediments from the Atlantic off west Africa?

You could look at the sediments to see if there were biogenic sediments interbedded with sediments with terrigeneous input corresponding to the dry, interglacial intervals.

What is a volcanic arc?

a row of volcanoes that forms on the overriding plate near a subduction zone

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

abyssal plain

"Black smokers" are associated with ________.

all of these

Where is the youngest ocean floor found?

along the crest of mid-ocean ridges

Seamounts ________.

are volcanoes that form on the ocean floor

Radiolarian chert is a common sedimentary rock accreted along active margins in the circum-pacific. It is produced in the deep sea by the accumulation of siliceous radiolarian shells. What kind of sediment is radiolarian chert?

biogenous sediment

Turbidity currents travel from the shelf through __________, creating deep sea fans.

canyons

What are the three major topographic provinces of the ocean floor?

continental margins oceanic ridges deep-ocean basins

Oil and gas resources are most common on the ________.

continental shelf

Moving off the east coast of North America, identify the sequence of environments you would encounter as you move deeper into the Atlantic Ocean.

continental shelf, continental slope, continental rise, abyssal plain

Which is not one of the three broad zones describing the ocean floor?

continental slope

Which of the following sedimentary rocks contains biogenous sediments?

coquina

Where do the sediments visible in black smokers come from?

crustal material

Plates move apart at __________ boundaries, move together at __________ boundaries, and move side-by-side at __________ boundaries.

divergent/convergent/transform

How do atolls form?

from fringing coral reefs around a volcano

Which scenario would generate hydrogenous sediments to create sedimentary rocks? Select all that apply.

hot water leaving the oceanic crust and coming in contact with cold seawater at black smokers evaporation of seawater

Manganese nodules are an example of ________.

hydrogenous sediment

Minerals that crystallize directly from seawater are examples of ________.

hydrogenous sediment

Much of the Gulf of Mexico is underlain by thick layers of salt that formed in a small ocean basin like the Red Sea, which formed from sea floor spreading. This salt is a good example of ________.

hydrogenous sediment

Gas hydrates mostly contain ________

methane

Which geologic features are associated with divergent boundaries?

mid-ocean ridges and continental rift valleys

Complete this statement: Divergence is to moving away as convergence is to _______.

moving toward

The best modern technique for obtaining high resolution bathymetry of the seafloor is ________.

multibeam bathymetry

Which geologic features are associated with convergent boundaries?

ocean trenches and volcanoes

Which of the following are hydrogenous sediments? Select all that apply.

salt silica calcium carbonate (CaCO3)

Most of our table salt comes from ________.

salt obtained by evaporating seawater in dry climates

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

seamount

Abyssal plains are very flat because ________.

sediments accumulate to cover the seafloor topography

If the subduction angle is steep, the overriding plate may undergo __________.

subduction erosion

Turbidites are ________ sediments.

terrigenous

Sailors in the 16th-18th century dreaded "rounding the horn" because of strong winds and heavy seas. Cape Horn is about latitude 55°S at the tip of South America. These sailors considered this a passage from the Atlantic to Pacific oceans, but this ocean is now often called ________.

the Southern Ocean

What is bathymetry?

the topography of the ocean floor and the measurement of ocean depths

Which coast represents an active continental margin?

the west coast of South America

Ocean ridges form because ________.

they are a broad seafloor highland formed by sea floor spreading

It is thought that submarine canyons on the continental slope have been generated by ________.

turbidity currents

Which process creates most submarine canyons?

turbidity currents