THE EARTH’S OCEANS

I. California K-6 Standards for The Earth’s Oceans

II. Goals of This Module

III. Introduction

IV. Layered Structure of the Ocean

V. Ocean Circulation Patterns

VI. More About Ocean Currents

VII. Continental Margin Features

VIII. Ocean Floor Features

IX. Mid-ocean Ridge Features

X. Sea-Floor Sediment

XI. Tides

XII. Possible Essay Questions

XIII. Practice Questions

I. California K-6 Standards for The Earth’s Oceans -

1. Kindergarten
   1. Earth Sciences Topics
      1. The Earth is composed of land, air, and water. As a basis for understanding this concept, students know:
         1. characteristics of mountains, rivers, oceans, valleys, deserts, and local landforms.

1. Grade 5
   1. Earth Sciences Topics
      1. Water on Earth moves between the oceans and land through the process of evaporation and condensation. As a basis for understanding this concept, students know:
         1. most of the Earth’s water is present as salt water in the oceans, which cover most of the Earth’s surface.

1. Grade 6
   1. Energy in the Earth System
      1. Many phenomena on the Earth’:s surface are affected by the transfer of energy through radiation and convection currents. As a basis for understanding this concept, students know:
         1. the sun is the major source of energy for phenomena on the Earth’s surface, powering winds, ocean currents, and the water cycle.

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II. Goals of this module -

• To learn about the composition of the ocean.

• To learn about the major currents in the ocean and their origin, including tidal currents.

• To learn about the characteristics of the bottom of the ocean.

• To learn the definitions of and proper spelling for scientific terms that are used commonly in newspapers, magazines, and books, and on television and radio programs to discuss the ocean.

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III. Introduction -

• Earth is known as the "water planet" or the "blue planet" because of its extensive oceans.

• Study of the oceans is called oceanography. Oceanography is a multidisciplinary science. Scientists who study the oceans are known as oceanographers.

• Distribution of oceans over the surface of the Earth -
  • World average = 71% ocean, 29% land.

  • Northern hemisphere = 61% ocean, 39% land.

  • Southern hemisphere = 81% ocean, 19% land.

• Volume of the ocean is 18 times greater than the volume of land above sea level.

• Origin of the oceans -
  • Comes from the water expelled from magma and lava during volcanic eruptions. This process is known as outgassing.

• Salinity of the oceans -
  • Comes from outgassing during volcanic eruptions and from weathering of the earth's surface.

  • Average salinity is 3.5% by weight.
    • Sodium chloride makes up 70% of the mineral salts dissolved in the ocean.

    • Magnesium chloride, sodium sulfate, and calcium chloride constitute most of the rest of the dissolved salts.

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IV. Layered structure of the oceans -

• Surface water between 50°N and 50°S latitude and to a depth of 1,500 feet is being constantly mixed and evaporated due to effects of the sun and wind. Temperature and salinity are rather constant: 21°C to 26°C (70°F to 80°F) and slightly higher salinity than normal.

• Below the surface mixed zone is a transition zone where the temperature lowers rapidly (thermocline) and the salinity decreases rapidly (halocline).

• The remainder of the ocean below and to the north and south of the transition zone is consistently below 4°C (39°F) and salinity is again constant.

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V. Ocean circulation patterns -

• Ocean currents are driven by the drag of the wind; hence are similar in pattern to atmospheric circulation.

• Ocean currents turn in large gyres, clockwise in the northern hemisphere and counterclockwise in the southern hemisphere due to the Coriolis effect.

• Currents moving away from shore on the eastern sides of oceans pull the surface water offshore and it is replaced by upwelling of cold, nutrient-rich bottom water.

Source for Diagram: http://www.geog.ouc.bc.ca/physgeog/contents/8q_1.html

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VI. More about ocean currents -

• Ocean currents affect ocean navigation and determine ocean trade routes.

• Ocean currents mix the ocean water, thereby moderating world climate.

• Deep-ocean currents are driven by density differences: the colder the water and the saltier the water, the heavier it is.
  • Ice formation in ocean water near the poles makes cold, salty water that sinks to the bottom of the ocean and moves along the bottom toward the equator. When the two currents meet near the equator they rise to the surface. Movement from the pole to the equator takes 500 to 2,000 years.

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VII. Continental margin features -

• Continental shelf - A flat, shallow-water platform around the edges of the continents; varies greatly in width.

• Continental slope - A relatively abrupt increase in slope off the edge of the continental shelf; actual gradient of the slope is just a few degrees.

• Continental rise - A gentle slope connecting the base of the continental slope to the ocean floor.

• Submarine canyon - A deep valley that is eroded into the continental shelf and down the continental slope.

• Somewhere under the continental slope or the rise is the boundary between continental crust and oceanic crust.

Source for Diagram: http://www.ngdc.noaa.gov/mgg/image/mggd.gif

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VIII. Ocean floor features -

• Ocean trench - A long, narrow, very deep valley located just oceanward of the continental slope.
  • Trenches are the deepest spots in the ocean (over 6 miles deep) and are the location of subduction zones.

• Abyssal plain - The flat, deep floor that covers most of the ocean basin.

• Seamount - An isolated shield volcano with gentle slopes that develops on the ocean floor above a hot spot in the mantle. Seamounts grow as underwater volcanoes that do not break through the surface to become islands.

• Guyot - A flat-topped seamount that forms when waves erode a wavecut platform across the top of an extinct volcano that later sinks below the surface of the ocean. A guyot will form generally in a temperate or cold climate, where coral growth is restricted or absent.

• Atoll - A ring of coral islands. Atolls will form generally in a subtropical or tropical climate, where coral growth is prolific.
  • Forms when an extinct volcano begins to sink below sea level and coral starts to grow just offshore in the form of a fringing reef.

  • As the island continues to sink, coral grows faster than the rate of subsidence, and a barrier reef forms.

  • The island becomes totally submerged, and the still rapidly growing coral forms an atoll.

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IX. Midocean ridge features -

• Midocean ridge - An elongate, ridge-like rise in elevation in the middle of the ocean
  • Midocean ridges are the location of spreading centers.

• Rift valley - a deep valley portion of a midocean ridge.
  • It has been discovered recently that rift valleys are the location of undersea hot springs where unique forms of life exist in the warm water and where mineral deposits are forming.

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X. Seafloor sediment -

• Terrigenous - Sand along the continental margins, mud on the rest of the ocean bottom.

• Biogenous - Skeletons of microscopic plants and animals. Calcite skeletons accumulate in areas shallower than 15,000 feet deep. Quartz and opal skeletons accumulate in water deeper than 15,000 feet because the cold water contains more carbonic acid, which dissolves any calcite shells.
  • The microscopic fossils in these layers can be dated and used to reveal past climatic changes.

• Hydrogenous - Direct chemical precipitation; mostly calcite in warm areas, some gypsum in nearshore lagoons.

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XI. Tides -

• The gravitational attraction of the moon and sun create a bulge in the ocean water on both sides of the earth.

• The rotation of the earth on its axis once each day creates two high tides and two low tides a day at any given spot on the earth.

• Highest of the two daily high tides is the tidal bulge that is under the moon.

• Spring tide period - Time of the month when the high tides are highest.
  • New moon - Produces the highest of the high tides and the lowest of the low tides.

  • Full moon - Produces moderate high tides and moderate low tides.

• Neap tide period - Time of the month when the high tides are lowest.
  • First quarter moon - Produces the lowest of the high tides and the highest of the low tides.

  • Last quarter moon - Produces the same tidal pattern as the first quarter moon.

• Because the moon revolves around the earth once a month, a complete tidal cycle lasts for one month. Spring tides for one week during new moon, neap tides for one week during first quarter moon, spring tides for one week during full moon, and neap tides for one week during last quarter moon.

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XII. Possible essay questions -

• Describe and illustrate the layered structure of the oceans.

• Describe and illustrate on a sketch map the general circulation pattern of the world's oceans.

• Describe and illustrate how upwelling occurs.

• Describe and illustrate, with a hypothetical cross section across an ocean, the major geographic features of the bottom of the ocean.

• Describe the three types of seafloor sediment.

• Describe and illustrate how the phases of the moon are related to the tidal cycle.

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XIII. Practice Questions

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