BYU Geography: Lesson 6

Where does about two-thirds of the earth's human population live?

About two-thirds of the population of the earth lives along coastal regions.

beaches

A beach is an expanse of sand or pebbles along a seashore that is washed by the tide and waves. Erosion often reduces beach width in winter when wind and wave action are more powerful and frequent. In summer, gentle wave action transports sediments that replenish the beach with sand. Most beach sand consists of light-colored quartz and feldspar sand grains—the result of weathering and erosion of rocks such as granite. Some beach sand comes directly from shoreline erosion, but much is created by the action of rivers flowing to the sea. Most beach sand contains fragments of smoothed and rounded shells from clams and other marine creatures. Tropical beaches often consist entirely of shell and coral fragments. Beaches in areas of volcanic activity can be black, where the sand was created by erosion of volcanic rock. There are several types of beaches found along the coasts of the United States. Common to northern California and Oregon is the narrow stretch of sand bounded by rolling surf and a rocky cliff mainland, called a spit. These beaches are located where waves break upon a coast of hard bedrock with little available sediment. The swash (wave uprush) and the return backwash carry pebble-size fragments ashore, while finer sand is washed to sea. Numerous beaches on the Atlantic Coast are of the spit type, such as New York's Coney Island where a narrow shoal extends seaward. This type of beach is created by wave action over a lengthy period. Beaches are a major attraction for coastal living. About two-thirds of the world's population lives within a narrow coastal belt that extends landward from the ocean's edge. In the United States, many large cities are located near or on an ocean shoreline or alongside one of the Great Lakes.

Describe abyssal plains.

Abyssal plains are among the smoothest surfaces on the planet, with less than five feet of vertical variation for every mile. These level plains are the result of a constant rain of sediments.

abyssal plains

Abyssal plains are found next to the continental slopes at depths greater than 9,000 to 10,000 feet. They are areas of near-freezing water temperatures where there are no seasons or sunlight. The abyssal plains are regarded as the true ocean floor. The few marine inhabitants found in the region survive only because they have adapted to the hostile environment of bitter cold and immense pressure. Abyssal plains are among the smoothest surfaces on the planet, with less than five feet of vertical variation for every mile. These level plains are the result of a constant rain of sediments. These sediments, ranging from the remains of large marine life to microscopic particles, drift slowly downward and fill in depressions on the irregular, rocky ocean floor. Often, coastal sediments are washed down the continental slope as turbidity currents. (A turbidity current is a downward flow of suspended mud-like sediments. The descent is caused by gravity.) Sediments from large rivers reach the ocean floor primarily by way of submarine canyons, which cut through the continental slopes.

What do all deserts have in common?

All deserts have low precipitation.

Pacific Ocean

By far the largest of the four oceans, the Pacific Ocean covers nearly one-third of the globe, or an area of approximately 64 million square miles. The land area of the United States, including Alaska and Hawaii, covers less than 4 million square miles. The Pacific Ocean is immense. All the continents could be placed into it and there would still be room left over. Even with the many topographic features found on the seafloor, such as plateaus, ridges, trenches, and seamounts, it has an average depth of 13,000 feet. The Pacific Ocean is approximately 11,000 miles wide at the equator. Not only is the Pacific Ocean the largest and deepest, it is probably the most violent of all oceans. The Pacific Ocean has typhoons in the equatorial regions, nearly 300 active volcanoes which vent steam and smoke on its borders, and tidal waves that are periodically unleashed. Unlike the basin floors of the Atlantic and Indian Oceans, the Pacific Ocean is characterized by the Central Pacific Trough. This feature extends from the Aleutian Islands southward to Antarctica and from Japan to the west coast of North America. The basin floors are not completely flat, and ridges and seamounts abound. Along with a number of deep ocean trenches, the Pacific Ocean has many flat-topped seamounts, or guyots. These are rarely found in other oceans.

Compare oceanic canyons on the west and east coasts of the US; how are they different and how are they similar?

Canyons found off the east and west coasts of the United States both have the classic "V" profile with steep walls and rock outcroppings. East Coast canyons begin at the very edge of the landward side of the continental slope and extend seaward in an almost straight line. West Coast submarine canyons are extremely rugged, twisted, and begin less than 1,000 feet offshore because of the exceedingly narrow continental shelf.

Arctic Ocean

Centered approximately on the North Pole, the Arctic Ocean is the smallest of the world's oceans, covering about 4,732,000 square miles. Maximum depth is 18,050 feet. The ocean is divided into two nearly equal basins: the Eurasia and the Amerasia. The Lomonosov Ridge extends from northeastern Greenland to central Siberia and separates the basins. The Arctic Ocean is surrounded by the landmasses of Eurasia, North America, and Greenland, and is unlike the other three oceans because of the perennial ice cover. The extent of sea ice is seasonal between 60°N and 75°N latitude, but above 75°N it is relatively permanent. Ice cover reduces energy exchange with the atmosphere, resulting in reduced precipitation and cold temperatures. A unique feature of the ice fields of the Arctic Ocean was the discovery in 1946 of large-sized ice islands. One of the first major ice islands covered an area of more than 300 square miles. In later years, ice islands were extensively tracked to determine current movement. They were also used as scientific research sites for the study of polar meteorology and oceanography.1

What are some ways that humans cause desertification?

Humans sometimes over-cultivate areas or engage in other damaging agricultural practices, such as placing too many domestic animals in an area, which results in overgrazing. Humans also remove vegetation for fuel or building material.

Which lake is the largest in the world?

Lake Superior is the largest lake in the world.

How did Lake Victoria form?

Lake Victoria is located in a depression caused by spreading plates near the Great Rift. More than half of the water comes from rain falling directly into the lake; the remainder comes from tributaries.

Lake Victoria

Lake Victoria, located in Africa, is the world's second largest freshwater lake. This lake was formed quite differently from the Great Lakes. Its formation is connected to the development of the Great Rift Valley that you learned about in lesson 5. As you will recall, a rift valley is formed when two tectonic plates pull away from one another. The lake formed in a low-lying area that developed as the land stretched. The Nile River runs through portions of the Great Rift Valley and receives much of its water from Lake Victoria. The lake gets more than half of its water from rain; the rest of its water comes from streams and rivers that empty into it.

What are two sources of lake water?

Lake water comes from tributaries, such as streams and rivers; it also comes from rain falling directly on the lakes.

types of lakes

Lakes are critical for the survival of many life forms, including human beings. These bodies of water are formed in many different ways; some are formed where glaciers scarred the earth, and others are formed in basins that are created by tectonic forces. Lakes can be salt or fresh water, deep or shallow. A lake by definition is a large inland body of standing water. So the salty Caspian Sea, the largest inland body of water in the world is actually considered a lake. Let's take a look at a few of the most prominent lakes on the earth.

What is usually the main force that removes water from basins that do not have an outlet?

Lakes that do not have an outlet typically empty by evaporation.

Rank the four oceans according to size. Which is the largest? Which is the smallest?

Largest to Smallest: 1) Pacific Ocean (largest by far) 2) Atlantic Ocean 3) Indian Ocean 4) Artic Ocean

How much of the earth's surface is covered by ocean water?

Over 70%

What are some things people are doing to prevent desertification?

Sand fences are being used to prevent further desertification in some areas. Trees and grasses are being planted along the borders of deserts to hold back the sand. Boulders and petroleum are being used to prevent sand from moving. Oases and farmlands are being protected by planting trees for windbreaks. In China, a 3500-mile-long tree wall is being planted to hold desert sands in place.

Which ocean do scientists know the most about?

Scientists know the most about the Atlantic Ocean.

Which oceanic features are formed by volcanic activity?

Seamounts and oceanic ridges are all formed through volcanic activity.

seamounts

Seamounts are isolated mountains rising from 3,000 to 10,000 feet above the surrounding seabed. Shaped like cones, they have a characteristic depression similar to a crater at the summit. Samplings gathered from over fifty seamounts found evidence that indicates they are of volcanic origin. Seamounts are found in all oceans, but are more numerous in the Pacific Ocean with over 2,000 identified. They are especially abundant in the Gulf of Alaska. The Cobb Seamount, discovered in 1950, is in a chain of seamounts that extends into the Gulf of Alaska. Located just 270 miles off the coast of Washington, it is one of the most thoroughly explored of all seamounts. Cobb rises from a depth of nearly 9,000 feet to within 124 feet of the surface. Because of relatively shallow depth and good lighting conditions, divers have explored and mapped most of the twenty-three acre flat top. The closest land feature similar to the seamount is a volcano that rises upwards from surrounding flatlands.

mid-ocean ridges

Several mid-ocean ridges are longer than the longest mountain ranges on Earth. They are tall as well, rising to 12,000 feet above the ocean floor, and their peaks penetrate the ocean's surface to form islands such as Iceland and the Azores in the Atlantic Ocean and the Galapagos Islands in the Pacific Ocean. Most of the ridges crest at a depth of about 8,000 feet and their width varies from 500 to 1,500 miles. Unlike typical continental mountain ranges that have a singular, pronounced line of peaks, oceanic ridges have two lines of peaks separated by a prominent depression known as a rift valley. This valley ranges from fifteen to thirty miles in width and cradles an active seismic belt.

How is beach sand formed?

Some beach sand comes directly from shoreline erosion, but much is created by the action of rivers flowing to the sea. Most beach sand contains fragments of smoothed and rounded shells of clams and other marine creatures. Tropical beaches often consist entirely of shell and coral fragments. Beaches in areas of volcanic activity can be black, where the sand was created by erosion of volcanic rock.

Why aren't all lakes freshwater lakes?

Some lakes collect minerals, such as salt, because the water evaporates instead of escaping through an outlet.

Why don't some lakes have an outlet?

Some lakes do not have enough water pouring into them to erode away the bank and create an outlet for the water to escape through.

What is the definition of a "sea"?

Technically, a sea is a subdivision of an ocean.

Which ocean has no areas located in the midlatitudes?

The Arctic Ocean

How is the Arctic Ocean different from the other three oceans?

The Arctic Ocean is mostly frozen.

In this lesson you learned that Antarctica is the world's largest desert. Think back to previous lessons to answer this question: why isn't the Arctic considered a desert?

The Arctic is not a desert because it is not a landmass; it is made of mostly of ice.

How far south does the Indian Ocean stretch?

The Indian Ocean stretches to Antarctica.

The Dead Sea and the Great Salt Lake

The Dead Sea and the Great Salt Lake both formed in basins. This means that like Lake Victoria, they formed in low lying areas of continents where precipitation and runoff from higher elevations, along with rain or snow, begin to puddle up and collect. Unlike Lake Victoria, the amount of water that fills the Dead Sea and the Great Salt Lake is relatively low, so the water pressure is not strong enough to cut outlets such as streams or rivers to drain the area. Evaporation is the main force that removes water from their basins. The water that runs into the basins brings a lot of minerals, including salt. Since minerals do not evaporate with the water, these lakes become salty. Both the Dead Sea and Great Salt Lake are very salty. The salt collects on the soil as the level of the lakes rise and fall. Because of the amount of salt in the water, objects floating in these water bodies experience extra buoyancy!

How do lakes such as the Dead Sea form?

The Dead Sea formed in a low-lying area from precipitation runoff.

The Great Lakes

The Great Lakes Basin was formed by glacier activity. The continental glaciers repeatedly moved over what is now the Great Lakes region as they thawed and melted. As the huge glaciers inched forward, they scoured the surface of the earth and leveled hills. Small valleys created by the river systems of the previous era were deepened and enlarged to form the basins for what are now the Great Lakes. The Great Lakes and their connecting channels form the largest surface freshwater system on Earth. If you stood on the moon, you could see the lakes. The Great Lakes, in their current state, are one of the youngest natural features on the North American continent. Covering more than 94,000 square miles and draining more than twice as much land, these freshwater seas hold an estimated six quadrillion gallons of water, or about one-fifth of the world's surface freshwater supply and nine-tenths of the US supply. Lake Superior is the largest of the Great Lakes, and if you are measuring surface area, is the largest freshwater lake in the world

How were the Great Lakes formed?

The Great Lakes were formed when glaciers dug gouges into the earth.

Why do heavy concentrations of minerals collect in the Great Salt Lake but not in Lake Superior?

The Great Salt Lake collects water from precipitation runoff. The runoff collects minerals and carries them to the lake. The water pools in the lake and eventually evaporates, leaving the minerals behind. Lake Superior is fed by many streams and rivers and receives large amounts of water, and the continual flow prevents mineral buildup from occurring.

Indian Ocean

The Indian Ocean is often thought of incorrectly as a tropical ocean. The Indian Ocean stretches southward to Antarctica. It is triangular and bordered by Africa, Asia, Antarctica, and Australia. Although it covers about 28.5 million square miles, it is smaller than the Atlantic Ocean and less than half the size of the Pacific Ocean. The maximum width is 6,200 miles between the southernmost portions of Africa and Australia. The Indian Ocean contains about 20 percent of the earth's water surface. Many island nations are found within the boundaries of this ocean: Madagascar, which is the world's fourth largest island, the Seychelles, the Maldives, Mauritius, and Sri Lanka. The average depth of the Indian Ocean is about 12,750 feet. The deepest is 24,440 feet in the Java Trench in the extreme northeast corner of the basin. The Indian Ocean, like the Atlantic Ocean, is divided by a mid-ocean ridge, the Mid-Indian Ocean Ridge, which separates the basin into nearly equal portions. The ocean's continental shelves are narrow, averaging 125 miles in width except off Australia's western coast where it broadens to 600 miles.

continental shelf

The continental shelf, the region from the coastline to the edge of the continental slope, covers about eight percent of the global seafloor area. The continental shelf is a national asset for most nations. It is a source of fish, both commercial and sport, and in some areas it is also a source of oil and natural gas. Shelves are not of uniform width. They vary considerably in size off the coasts of the United States. They are almost negligible along Southern California's coast and Florida's southeast coast. Off Florida's west coast, however, the shelf extends 200 miles into the Gulf of Mexico. The average width worldwide is about forty miles. Continental shelves look like the adjacent land. Coastal areas that are mountainous will have a continental shelf with similar features, which is the case off Southern California. Coastal areas dominated by low hills or plains are likely to have a shelf of nearly flat plains or low hills. This is observed along most of the US Atlantic and Gulf Coasts. Shelves off the mouths of larger rivers, such as the Mississippi, are usually broad with a large mud base of continental sediments. Continental shelves along the coasts of the United States cover an area of about 891,000 square miles. Alaska's continental shelf alone occupies over 591,000 square miles. The Atlantic Coast's continental shelf covers 140,000 square miles, about equal to the land mass areas of New York, New Jersey, Ohio, and Pennsylvania combined. The Gulf of Mexico's continental shelf covers 135,000 square miles, nearly the same area as Nebraska and Iowa combined. The continental shelf off the coasts of California, Oregon, and Washington covers 25,000 square miles.

continental slopes and canyons

The continental slope gradually rises from the abyssal plains, but it climbs as steeply as 45 degrees as it approaches land. In some areas, the slope is interrupted by broad wedges of sediment deposits called continental rises. Continental slopes are often gouged by deep valleys or submarine canyons, many with the same proportions as the Grand Canyon. While most canyons were originally formed during the last Ice Age, some are the result of earthquakes. Canyons found off the east and west coasts of the United States are similar in one respect: they both have the classic "V" profile with steep walls and rock outcroppings. East Coast canyons of the United States begin at the very edge of the landward side of the continental slope and extend to seaward in an almost straight line. The Hudson Canyon off Long Island, New York, is the best known and studied canyon on the East Coast. It was first identified in 1864. The canyon begins as a shallow valley crossing the continental shelf. The axis of the canyon then trails down the continental slope for a distance of about fifty miles. The greatest wall height is about 4,000 feet at a depth of 6,000 feet. West Coast submarine canyons are noticeably different from those on the East Coast. They are extremely rugged, twisted, and begin less than 1,000 feet offshore. This is due to the exceedingly narrow continental shelf. Monterey Canyon is the deepest, largest, and most thoroughly studied canyon on the West Coast. It starts near Moss Landing, California, at a depth of fifty feet. The canyon, with walls up to 6,000 feet, extends seaward for more than sixty miles. It terminates at a depth of about 10,000 feet.

Have the great deserts of the earth significantly grown or shrunk because of human activities?

The great deserts of the world seem to grow and shrink cyclically, regardless of human activity.

Atlantic Ocean

The hourglass-shaped Atlantic Ocean covers approximately 20 percent of the earth's surface and is the second largest of the four oceans. It extends from the North Pole southward for 10,000 miles to the Antarctic continent and covers 41 million square miles. Width of the Atlantic Ocean varies from 1,769 miles between Brazil and Liberia and approximately 3,000 miles between Norfolk, Virginia, and Gibraltar. More is known about the Atlantic Ocean than any other ocean because of heavy commercial and military ship traffic connecting Europe and North America. Average depth is 12,000 feet and the greatest depth is 28,374 feet in the Puerto Rico Trench. If Alaska's Mount McKinley (20,320 feet) was to rise from the floor of the Puerto Rico trench, its peak would still be about 1.5 miles below the surface of the Atlantic. The Mid-Atlantic Ridge divides the seafloor nearly through the center and stretches from the polar regions of the north to Antarctica in the south. The Mid-Atlantic Ridge was created when the super continent of Pangaea split apart 190 million years ago. The ridge lies about 10,000 feet below the water level except in a few areas where it surfaces as islands. This mountain range is as much as 500 miles wide. Rugged valleys extend outward from the ridge line to the abyssal plains. The Mid-Atlantic Ridge is a continuous feature of the basin floor with one exception. There is a significant break in the ridge near the equator at the Romanche furrow where the crest of the ridge dips 15,000 feet below the surface. This break in the mountain chain allows deep water to flow freely between the Atlantic Ocean's east and west sides. This unrestricted movement provides a thorough circulation of the ocean basin that has a pronounced effect on deep water currents, density, and temperature.

How are continental shelves similar to adjoining coastal areas?

The terrain of continental shelves is similar to adjoining coastal areas; for instance, if the coast is mountainous, the adjoining continental shelf is also mountainous.

What do the Atlantic Ocean and the Indian Ocean have in common that is different from the other two oceans?

They each contain 20 percent of the earth's water and they are both divided by a mid-ocean ridge.

Where are oceanic trenches found?

Trenches are found adjacent and parallel to continents and island chains.

trenches

Trenches are found adjacent and parallel to continents and island chains. At least twenty-two trenches have been identified, although not all are classified as major. Of this number, eighteen are in the Pacific Ocean, three in the Atlantic Ocean, and one (the Java Trench) in the Indian Ocean. Trenches are not uniform in depth or width, but the depths of major trenches exceed 18,000 feet and the widths vary from ten to twenty-two miles. The deepest point in the ocean is the Challenger Deep, which is 35,810 feet deep, in the Marianas Trench. The depths of many trenches are greater than the elevation of the world's highest mountain. Mount Everest (29,028 feet) would be less imposing if dropped into the Challenger Deep. In fact, the tip of Mount Everest would be farther down than the depth of the Grand Canyon.

What are the two main forces that cause erosion?

Wind and water are the two main forces that cause erosion.

What huge feature does the Pacific Ocean contain that is different from the other three oceans?

the Central Pacific Trough

Which lake system contains one-fifth of the world's freshwater supply?

the Great Lakes

Which portion of the oceans is most often used by humans for both recreation and commercial activity?

the continental shelf

What factors make the Pacific Ocean violent?

typhoons, volcanoes, earthquakes