Chapter 24 Earth’s Water
Water on Earth • Water is the most abundant chemical compound near the surface of the earth. It covers about 70% of the earth’s surface. • Over 97% of earth’s waters is stored in the earth’s oceans and contains high levels of dissolved salts, so this water is unfit for human and animal consumption and for most agricultural purposes. • Freshwater is what humans and animals can consume and what is fit for agriculture. About 2/3 of the earth’s freshwater is locked up in the ice caps of Greenland and the Antarctic and in glaciers, so only less than 1% of all the water found on the earth is available as freshwater. It is continuously being replenished by the hydrologic cycle. • About .04% is found as water vapor in the atmosphere.
Fig. 24.2 Estimates of the distribution of water found on earth’s surface.
Freshwater • Water evaporates from the oceans in the first step of the hydrologic cycle. It leaves dissolved salts behind. Precipitation is fresh water. • More water is evaporated from the ocean than is returned by precipitation. More water is precipitated over the land than evaporates. The difference is returned to the ocean by rivers and streams. • The gaseous water vapor is easily transported in the atmosphere. Precipitation is not even throughout the earth, some places receive a lot of water while other places receive almost none.
Freshwater • Not all precipitation ends up as part of the fresh water supply. • Much water evaporates again after precipitation. Eventually 70% of the rain eventually returns to the atmosphere. • The remaining amount either flows downhill across the surface of the land toward a lower place forming streams (this is called runoff) or soaks the land. About 30% is runoff. • The water that soaks the land becomes groundwater. It moves through sediments and rocks beneath the surface downhill and eventually flows into the streams. Only about 1% of the precipitation soaks into the ground.
Some of the precipitation soaks into the ground to become groundwater. Groundwater slowly moves underground, and some of it emerges in streambeds, Keeping the streams running during dry spells.
Surface Water • If streams of water formed by precipitation are followed it is found that they eventually merge with other streams until they merge to form a major river. • The land area drained by a stream is known as the stream’s drainage basin or watershed. • The watershed of a large river includes all the water sheds of the smaller streams that feed into the large river. • Two adjacent watersheds are separated by a line called a divide. A continental divide separates river systems that drain into opposite sides of the continent. The North American continental divide trends northwestward through the Rocky Mountains.
The approximate watersheds of the Columbia River, the Colorado River, and the Mississippi River.
Surface Water • Water moving downhill is sometimes stopped by a depression in a watershed, where water collects in a pond or a lake. • A pond or lake can occur naturally in a depression or it can be created by building a dam. • If these accumulations of water are used for water storage, flood control, or to generate electricity they are called a reservoir.
Seawater • More than 70% of the surface of the earth is covered by seawater, with an average depth of 12,500 ft. The land areas cover 30% with an average elevation of only about 2700 ft. • There is only one continuous ocean with parts that have been given names. • A sea is a smaller part of the ocean or an inland body of salty water.
The Ocean • The Pacific Ocean is the largest of the three principal ocean regions (70 mil mi2), and the deepest (2.4 mi). It is circled by active converging plate boundaries. It has been described as being circled by a “rim of fire”. • There are volcanoes, oceanic trenches, island arcs and earthquakes. • The Atlantic Ocean is second in size (41 mil mi2), and the shallowest (2.1 mi). • It is bounded by nearly parallel continental margins with a diverging plate boundary between. It lacks the trench and island arc features of the Pacific, but it does have islands, such as Iceland, that are part of the Mid-Atlantic Ridge at the plate boundary. • The shallow seas of the Atlantic, such as the Mediterranean, Caribbean, and the Gulf of Mexico contribute to its shallowness. • The Indian Ocean is the smallest (29 mil mi.) and about the same depth as the Pacific (2.4 mi).
Distribution of the oceans and major seas on the earth’s surface. There is really only one ocean.
Ions in Seawater • The dissolved materials of seawater are present in the form of ions because of the strong dissolving ability of water molecules. • Almost all of the chemical elements are present, but only six ions make up more than 99% of any given sample of seawater. • Sodium and chlorine are the most abundant ions. Sea salt is mostly sodium chloride but it also contains salts of the four metal ions (positive ions) combined with the different negative ions. • The amount of dissolved salts in seawater is called salinity. It is the mass of salts dissolved in 1 kg of seawater. This amount is referred to as parts per thousand and the symbol o/oo is used. • The average salinity of seawater is about 35 o/oo (this corresponds to 3.5 %). This number can increase by evaporation of the water and formation of sea ice. This number can decrease by heavy precipitation, the melting of ice, and the addition of freshwater by a large river.
Salinity is defined as the mass of salts dissolved in 1.0 kg (1000 g) of seawater. This corresponds to 35 0/00 (35 parts per thousand)
Movement of Seawater • Two types of motion on the surface of the ocean: waves that have been produced by some disturbance, and currents, which move water from one place to another. • An ocean wave is a moving disturbance that travels across the surface of the ocean. • Each wave has series of ridges or mounds of water called crests. They are followed by depressions called a troughs. • An ocean wave can be described by: • The wave height, the vertical distance between the top of the crest and the bottom of the next trough. • The wavelength, which is the horizontal distance between two successive crests. • The wave period, which is the time required for two successive crests of the wave to pass a given point.
Ocean Waves • The characteristics of an ocean wave formed by the wind depend on 3 factors: • the wind speed • The length of time that the wind blows • The fetch, which is the distance the wind blows across the open ocean. • The larger waves are produced by strong winds that blow for a longer time over a long fetch. • The wind doesn’t blow in just one direction, and shifting winds produce a chaotic pattern of waves of many different heights and wavelengths. This is especially true during a storm. • The waves formed during a storm travel long distances and eventually become regular groups of long-wavelength waves with a low wave height called a swell.
Waves • Swells don’t transport water across the open ocean. The only of an object on the ocean is up and down unless it is moved along by a wind or by a current. It is a circular path with a diameter equal to the wave height. • Below the surface the circular paths become progressively smaller with increasing depth and soon there is not circular movement whatsoever.
Waves • As a swell moves from the deep ocean to the shore the waves pass over the shallower water depths. • Eventually the circular motion of the water particles begins to reach the ocean bottom and as the water particles move across the ocean bottom near the shore the friction results in the waves moving slower and the wave height increasing. • The bottom of the wave will then move slower than the top due to the friction at the bottom and the top will become a breaker. The zone where the breakers occur is called the surf. • Some of the energy brought by the surf erodes the shoreline, breaking up rock masses into the sands that are carried by local currents back to the ocean.
As a pattern of swell approaches a gently sloping beach the friction between the circular motion of the water particles and the bottom slows the wave, and the wave front becomes steeper and steeper. When the depth is about one and one third times the wave height, the wave breaks forward, moving water towards the beach.
Waves • When a wave breaks it tosses water toward the shore, where the water begins to accumulate. • This buildup of water tends to move away in currents, or streams, as the water returns to a lower level. • Some of the water might return directly to the sea by moving beneath the breakers and creating an undertow. • Other parts of the accumulated water might be pushed along by the waves and produce a longshore current that moves parallel to the shore in the surf zone. • It will eventually find a lower place or a channel that is deeper than the adjacent bottom. It will then produce a rip current, a strong stream of water that bursts out against the waves and returns water to the sea.
Breakers result in a buildup of water along the beach that moves horizontally along the beach as a longshore current. Where it finds a shore bottom that allows it to return to the sea it surges out in a strong flow called a rip current.
Ocean Currents • Ocean currents are streams of water that stay in about the same path as they move through other seawater over large distances. • A density current is an ocean current that flows because of density differences due to differences in temperature, salinity, and suspended sediments. The current originates because of bodies of water with distinct properties moving towards each other.
Seawater Colder surface waters near Greenland more dense.
Seawater More dense water sinks to join even more dense water from Antarctica.
Seawater Surface currents – set up by winds.
The Ocean Floor • The seawater dropped about 460 ft during the most recent major ice age and exposed the margins of the continents to erosion. These margins are now flooded with seawater and form a relatively shallow area of water called the continental shelf. • It is considered to be part of the continent and not the ocean even if it is covered by about 425 ft of seawater. It retains some of the general features of the adjacent land such as hills, valleys, and mountains but they have been eroded and smoothed off. • Beyond the gently sloping continental shelf is a steeper feature called the continental slope. It is the transition between the continent and the deep ocean basin. • At some places around the world the continental slopes are cut by long, deep and steep-sided submarine canyons similar to the Grand Canyon.
The ocean floor Continental shelf – part of continent that retains some of the same features. Gradual slope. Use to be above sea level. Continental slope – steep feature connecting shelf to deep ocean basin. Ocean basin – bottom of ocean floor containing an abyssal plain, ridges, trenches and seamounts.
The ocean floor Deepest part of ocean floor found in trenches.
Review Exercises • P. 590-591 Applying the Concepts: # 1, 2, 3, 5, 7, 8, 9, 12, 20
Less than 1% of water on earth is freshwater. More water is evaporated from the ocean than is returned by precipitation. More water is precipitated over the land than evaporates. The difference is returned to the ocean by rivers and streams. Precipitation can evaporate again (70%), become runoff (streams) (30%) or groundwater (1%). The watershed of a large river includes all the water sheds of the smaller streams that feed into the large river. Two adjacent watersheds re separated by a line called a divide. A continental divide separates river systems that drain into opposite sides of the continent. Water moving downhill is sometimes stopped by a depression in a watershed, where water collects in a pond or a lake,a natural depression or in an artificial dam. If these accumulations of water are used for water storage, flood control, or to generate electricity they are called a reservoir. There is only one continuous ocean with parts that have been given names. A seais a smaller part of the ocean or an inland body of salty water. Pacific ocean is deepest and largest and has rim of fire. The Atlantic ocean is shallowest and second largest. It has the mid Atlantic ridge. The Indian ocean is smallest and as deep as the Pacific. The Atlantic has shallow seas: Gulf of Mexico, Caribbean, Mediterranean. Na+ and Cl- are most prevalent ions in the ocean. Salinity is the mass of ions per 1 kg of seawater. It increases by evaporation of the water and formation of sea ice. It decreases by heavy precipitation, the melting of ice, and the addition of freshwater by a large river. Two types of motion on the surface of the ocean: waves that have been produced by some disturbance and have crests and troughs, and currents, which move water from one place to another. An ocean wave can be described by the wave height, the wavelength, and the wave period. Waves formed by wind speed, length of time and fetch. The wind blows in different directions and wave patterns are complex especially during storm. Eventually become swells. Swells have circular patterns becoming smaller as you go down and then stopping. They touch the ocean bottom as they reach shore and breakers are created at the surf. Water that returns to the sea by moving beneath the breakers creates an undertow. Water that moves parallel to the shore in the surf zone is a longshore current. A rip current is a strong stream of water that bursts out against the waves and returns water to the sea. Adensity currentis an ocean current that flows because of density differences due to differences in temperature, salinity, and suspended sediments. The current originates because of bodies of water with distinct properties moving towards each other. Surface currents are set up by winds. The continental shelf is gently steeping, below the ocean water but shallower and used to be continent. It is followed by the steeper continental slope. There are also canyons along the continental slope and there are ridges and trenches at some places in the ocean floor. Summary