Earth's Oceans and Hemisphere Distribution - Chapter 9

The Vast World Ocean

• The Earth is often referred to as the water planet or the blue planet, with approximately 71% of its surface covered by the oceans and marginal seas.
• The distribution of water and land is not uniform between the Northern and Southern Hemispheres:
  • Northern Hemisphere: 61% water, 39% land (termed the land hemisphere)
  • Southern Hemisphere: 81% water, 19% land (termed the water hemisphere)
• Oceanography is the interdisciplinary science that studies all aspects of the world ocean.

Divisions of the World Ocean

• The world ocean can be divided into four main ocean basins:
  • Pacific Ocean: largest ocean, covering over 50% of the Earth's ocean surface, and the deepest ocean (average depth of 12,927 feet)
  • Atlantic Ocean: about half the size of the Pacific Ocean, with a slightly smaller depth
  • Indian Ocean: slightly smaller than the Atlantic Ocean, with a similar depth
  • Arctic Ocean: about 7% the size of the Pacific Ocean, with a depth of about 1/4 of the other oceans
• Oceanographers also recognize a fifth ocean basin: the Southern Ocean, located below 50° S latitude, surrounding the continent of Antarctica.

Composition of Seawater

• Seawater is not just saltwater; it contains a variety of other salts, metals, and gases dissolved in it.
• Every naturally occurring element is found in seawater, making it unsuitable for drinking or irrigation.
• The dissolved substances in seawater make up about 3.5% of its weight.
• Salinity, the total amount of solid, inorganic material dissolved in seawater, is typically expressed in parts per thousand (‰).
• The most abundant elements in seawater are Cl-, Na+, SO42-, Mg2+, Ca2+, and K+, which make up more than 99% of the dissolved substances.

Sources of Dissolved Components

• There are two sources of dissolved components in seawater:
  • Chemical weathering of rocks on the continents, which are transported to the oceans by streams
  • Earth's interior, through volcanic eruptions and other processes

Variations in Temperature and Density with Depth

• Surface water temperature varies depending on the amount of solar radiation received, with colder temperatures near the poles and warmer temperatures near the equator.
• Water temperature and density change with depth:
  • In low-latitude regions, water temperature decreases rapidly with depth, resulting in a thermocline and a pycnocline
  • In high-latitude regions, water temperature and density are relatively constant with depth
• The ocean is layered by density differences, with three main layers:
  • Surface mixed zone: nearly uniform temperatures, varying in thickness and temperature due to latitude and season
  • Transition zone: temperature falls with increasing depth, including a prominent thermocline and pycnocline
  • Deep zone: constant water temperature, just above freezing

An Emerging Picture of the Ocean Floor

• The ocean floor is not smooth, with topography more variable than the landscape of the continents.
• The science of measuring ocean depths and charting the shape of the ocean floor is called bathymetry.
• Three methodologies are used to map the ocean floor:
  • Sonar (Sound Navigation and Ranging)
  • Seismic Reflection Profiles
  • Satellite/Radar Altimeters

Continental Margins

• Continental margins are the submerged outer edges of a continent.
• There are two types of continental margins:
  • Passive continental margins: not associated with plate boundaries, with relatively little earthquake or volcanic activity
  • Active continental margins: near the edges of converging plates, with abundant earthquake and volcanic activity
• Features of passive continental margins include:
  • Continental shelf
  • Continental slope
  • Continental rise
  • Submarine canyons and turbidity currents

Deep-Ocean Basins

• Located between the continental margin and oceanic ridge system, deep-ocean basins cover about 30% of the Earth's surface.
• Features of deep-ocean basins include:
  • Deep-ocean trenches
  • Abyssal plains
  • Seamounts, guyots, and oceanic plateaus### Seamounts, Guyots, and Oceanic Plateaus
• Seamounts are conical volcanic projections that rise more than 0.6 miles above the seafloor, with over 1 million estimated to exist, mostly in the Pacific Ocean.
• Some seamounts grow to become islands, while others form linear chains or near oceanic ridges.
• Guyots, also known as tablemounts, are seamounts with flattened tops, formed when islands sink due to weathering and erosion.
• Oceanic plateaus are flood basalts formed on the ocean seafloor from mantle plumes, composed of basaltic and ultramafic rocks that can exceed 30 km in thickness.

The Oceanic Ridge

• The oceanic ridge is a divergent plate boundary where the ocean seafloor is elevated, characterized by extensive faulting, earthquakes, high heat flow, and volcanic structures.
• It is the longest continuous geographic feature on Earth, covering over 20% of the planet's solid surface and extending over 43,000 miles.
• The crests of the ridge can rise 2 to 3 km above the ocean seafloor, comparable to some mountain ranges on continents.

Seafloor Spreading

• Harry Hess proposed seafloor spreading in the early 1960s, where magma rises to the ridge axis, replacing oceanic material that has shifted horizontally.
• As rocks rise, they experience decompression melting, resulting in new oceanic crust being generated.
• New material is added equally to each side of the ridge, with the process taking approximately 80 million years for the newly formed material to cool and contract.

Seafloor Sediments

• Most of the ocean floor is covered with sediments of varying thicknesses, with marine sediments classified into four categories according to their origin: terrigenous, biogenous, hydrogenous, and cosmogenous.
• Terrigenous sediments consist of mineral grains weathered from continental rocks and transported to the ocean by streams or winds.
• Biogenous sediments consist of shells and skeletons of marine animals and algae, including calcareous ooze and siliceous ooze.
• Hydrogenous sediments consist of minerals that precipitate directly from seawater through chemical reactions, including manganese nodules, calcium carbonates, metal sulfides, and evaporites.
• Cosmogenous sediments, not discussed in the text, include tektites and meteorites.