Lecture 8

OCEANS AND RESTLESS OCEANS

THE GLOBAL OCEANS

I. Introduction

A. Oceanography is the study of the oceans, its chemistry, biology, origin and processes.

B. Points of interest.

1. The Pacific Ocean contains the most water of all oceans in the world.
2. The volume of the oceans is many times greater than the volume of the land above sea level.
3. If all the water were evaporated from the oceans a layer of salt 60 meters thick would cover the entire ocean floor.

II. Composition of Seawater

A. About 3.5% by weight of the oceans are dissolved mineral substances.

B. Salinity is the proportion (in parts per hundred) of dissolved salts to pure water.

1. The average salinity of the ocean is 35 parts per thousand.
2. Sodium chloride is the principal salt making up most of the ocean water.
3. High salinity is found in areas of high evaporation.
4. The sources for salt to the ocean comes from chemical weathering of the rocks and volcanic outgassing.

III. Ocean's Layered Structure

A. The three layers of the ocean are based on temperature and salinity, the thickness of each varies with latitude, and season.

B. The surface mix zone makes up 2% of all three layers.

1. The surface mix zone is 0-450 meters thick.
2. This layer receives solar energy and the energy is transferred to depth by currents.
3. This layer has a uniform temperature (21-26 degrees Celsius) and thickness.

C. The transition zone makes up 18% of the three zones.

1. It may be 450-950 meters thick.
2. The temperature falls rapidly within this zone.
3. This layer of rapid change in temperature is called thermocline.
4. This layer marks the transition between the warm surface and cold deep ocean.

D. The deep zone makes about 80% of the three layers and is the largest layer of the three.

1. The water temperatures are typically 4 degrees Celsius.
2. Most ocean water is at 4 degree Celsius.

IV. Ocean Floor Topography

A. There are three major ocean topographic features.

1. Continental margins.
2. Ocean floors.
3. Mid-ocean ridges.

B. A crossection through the ocean floor.

V. Ocean Floor Sediments

A. Terrigenous sediments.

B. Biogenous sediments.

C. Hydrogenous sediments.

D. Sediment thicknesses.

THE RESTLESS OCEANS

I. Surface Currents

A. Ocean circulation patterns

1. Where the atmosphere and ocean are in contact, energy is passed from moving air to the water through friction.
2. The drag exerted by winds across the ocean surface causes the surface layer of water to move.
3. The winds are the primary driving force of surface currents.

B. Coriolis Effect is due to the Earth's rotation. The currents are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Therefore, the direction of surface currents do coincide with wind direction.

C. Upwelling: This is the process in which rising cold water from the deep layers replaces warmer surface water. This is a common wind induced vertical movement.

1.      1.      Upwelling occurs mostly along the coast of California, South America, and west Africa.

2.      2.      Upwelling occurs when winds blow toward the equator and parallel to the coast, the Coriolis effect deflects surface water from the shore. This water is then replaced by water that upwells from below the surface.

D. The importance of ocean currents are as follows:

1. they affect climate;
2. they help to maintain the Earth's heat balance.

II. Deep Ocean Circulation

A. Deep ocean circulation is affected by:

1. gravity;
2. density differences.

B. Temperature and salinity are two factors that create a dense mass of water.

1. Water that is cold and salty is denser than warmer less salty water.
2. Deep ocean circulation is call thermohaline circulation.
3. Surface ocean water that sinks will not reappear at the surface for an average of 500-2000 years.

III. Tides and Waves

A. Tides are daily changes in the elevation of the ocean surface.

B. Tides are caused by:

1.      1.      gravitational attraction between the Earth and moon;

2.      2.      the Earth's rotation.

C. Spring and Neap Tides.

1. Spring tides occur near the new and full moons. The sun and moon are aligned and the forces of these two bodies are added together. The combined gravity of sun and moon cause higher tidal bulges (high tides) and lower tidal troughs (low tide).
2. Neap tides occur when the moon is in its first and third quarters. During this time the gravitational forces of the moon and sun act on the Earth at right angles and partially offset each other's effect.

D. Tidal currents describe the horizontal flow of water accompanying the rise and fall of tides.

1.        1.      Flood currents are tidal currents that advance into the coastal zone as the tide rises.

2.        2.      Ebb currents are created as the tide falls.

3.        3.      Slack water occurs when there are periods of little or not current.

4.        4.      Tidal flats are areas affected by alternating tidal currents.

E. Waves

1.        1.      Waves are characterized by: waveheight (vertical distance between the top of waves, which are called crests), troughs, which separate crests, wave length, which is the horizontal distance separating successive crests, and wave period, which is the time interval between the passage of successive crests at a stationary point.

2.        2.      The height, length and period of a wave depends on the windspeed, length of time the wind has blow, and fetch, the distance the wind has traveled across open ocean.

3.        3.      Waves in the open sea are called wave of oscillation.

4.        4.      Wave of translation is when water advances up the shore and the wave front collapses or breaks.

5.        5.      The turbulent water created by the breaking wave is called the surf.

6.        6.      On the landward margin of the surf zone the turbulent sheet of water from the collapsing breakers are called swash, which move up the slope of the beach.

7.        7.      When the energy of the swash has been expended, the water flows back down the beach toward the surf zone as backwash.

F. Wave Erosion

1.        1.      Waves perform most of their erosional work during storms.

2.        2.      Each breaking wave may hurl thousands of tons of water against the land aiding in the erosion process.

3.        3.      Air in cracks or rock material is compressed due to water being forced into the cracks by waves. When the waves subside the air expands rapidly dislodging rock fragments and enlarging the extended preexisting fractures.

4.        4.      Abrasion is the sawing and grinding action of the water armed with rock fragments. Abrasion is more intense in the surf zone. Evidence for abrasion exists in the rounded and polished stones found along the beach.

G. Wave Refraction

1.        1.      the bending of waves is called wave refraction.

2.        2.      Wave refraction affects the distribution of energy along the shoreline and influences where and to what degree erosion, sediment transport, and deposition will take place.

3.        3.      Most waves move toward the shore at an angle. When the waves reach the shallow water of a smoothly sloping bottom they are bent and become parallel to the shore. Such bending occur because the part of the water nearest the shore touches bottom and slows first, while the end that is still in deep water continues forward at fullspeed. The result is a wave that approaches the shore nearly parallel.

4.        4.      Due to refraction, wave impact is concentrated against the sides and ends of headlands projecting into the water, while wave attack is weakened in bays.

H. Moving Sand Along the Beach.

3. Backwash is in the direction of the slope of the beach. The effect of this pattern of water movement is to transport particles of sediment in a zig zag pattern along the beach. This movement is called beach drift. Beach drift can transport sand and pebbles along the beach.
4. Oblique waves also produce currents within the surf zone that flow parallel to the shore. Since the water is turbulent, longshore currents can move fine suspended sand , and roll larger sand and gravel along the bottom.
5. Thus, beaches have been characterized as "rivers of sand".

I. Shoreline Features

6. Shoreline features vary depending on the rocks, current wave intensity, and whether the coast is stable, sinking or rising.
7. Wave-cut cliffs and platforms
• Wave-cut cliffs originate by cutting action of the surf against the base of coastal land. As erosion progresses, rocks overhanging the notch at the base of the cliff crumble into the surf and the cliff retreats.
• A relatively flat, benchlike surface called a wave-cut platform is left behind by the receding cliff.

8.      8.      Arches, stacks, spits and bars

§         §         When two caves on opposite sides of a headland unite a sea arch is produced.

§         §         Finally, the arch falls in leaving an isolated remnant, or sea stack on the wave-cut platform.

§         §         Spits are elongated ridges of sand that project from the land into the mouth of an adjacent bay. Often the end in the water hooks landward in response to wave-generated currents.

§         §         Baymouth bar is the term applied to a sand bar that completely crosses a bay, sealing it off from the open ocean. Such a feature tends to form across bays where currents are weak, allowing a spit to extend to the other side.

§         §         A tombolo is a ridge of sand that connects an island to the mainland or to another island, forms in such a manner as does a spit.

9.      9.      Barrier islands are low ridges of sand that parallel the coast at distances from 3 to 30 km offshore. Most barrier islands are 1-5 km wide and between 15-30 km long. The highest features are sand dunes, which may reach heights of 5-10 meters. Barrier islands may originate as spits that were severed from the mainland by wave erosion or by a general rise in sea level following the last episode of glaciation. Others are created when turbulent waters in the line of breakers heap up sand that has been scoured from the bottom.

J. Shoreline Erosion Problems

1. Shorline water movement is continuous and causes modest damage. The shoreline is a dynamic place that can change rapidly in response to natural forces.
2. Growing affluence and increasing demands for recreation have brought development to many coastal areas. Thus, efforts to protect beach property from storm wave action has resulted in the construction of engineered structures, such as groins and seawalls, and beach nourishment to protect the beach from erosion.

• Groins are constructed to maintain or widen beaches that are losing sand. A groin is a barrier built at right angle to the beach to trap sand that is moving parallel to the shore.
• Sea walls are massive barriers intended to prevent waves from reaching the areas behind the wall.
• Beach nourishment is another way of stabilizing shoreline sands. This practice involves adding sand to the beach system.

K. Emergent and Submergent Coasts

1. Emergent coasts develop because an area experiences uplift or as a result of a drop in sea level.
2. Conversely, submergent coasts are created when sea level rises or the land adjacent to the sea subsides.

Estuaries are produced when the sea inundates river mouth areas.