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Unit 02

THE PLANET EARTH

Unit Overview

The three main topics of focus for this unit are:

  • Spheres of the earth system
  • Hemispheres
  • Continents and oceans

There are five spheres that constitute the Total Earth System: the atmosphere, the biosphere, the hydrosphere, the lithosphere, and the cryosphere. The atmosphere pertains to the gases that envelop the earth's surface. The biosphere pertains to all living organisms on the planet. The hydrosphere pertains to water in liquid and solid states that exists on and within the earth. The lithosphere¿or rock sphere¿is the outermost shell of solid earth. Finally, the cryosphere includes all forms of frozen water. These spheres are not independent; there is a continuous exchange of energy and material among the spheres. For example, the carbon cycle involves the exchange of carbon from the atmosphere (e.g., carbon dioxide), the biosphere (e.g., carbohydrates), the hydrosphere (e.g., dissolved carbon dioxide), and the lithosphere (e.g., calcium carbonate). Thus, these spheres can be viewed simultaneously as an Earth System.

The Earth can also be divided into geographically based zones known as hemispheres. The equator and the prime meridian demarcate these zones. Some salient characteristics of the hemispheres are as follows:

  • The Northern Hemisphere has much more land than does the Southern Hemisphere
  • The Western Hemisphere is centered on the Americas
  • The Eastern Hemisphere is centered on Asia.

From a physical geography perspective, the Northern and Southern Hemispheres are the most important due to their different proportions of land and ocean, as well as due to the seasonal impacts of solar heating. Further, the boundary is an actual terrestrial feature rather than an arbitrarily delineated line (such as the prime meridian).

The juxtaposition of land and oceans is a product of geologic processes, and this juxtaposition creates a climate control. Nearly all the major landmasses are very diverse with respect to climate, vegetation, and landforms. Many landmasses have long mountain chains¿either along their edges or extending across their interior portions. This information is important with respect to continental drift and plate tectonics, which will be presented in Units 32 and 33.

Unit Objectives

  • To define and highlight the spheres of the Earth System
  • To highlight the general characteristics of the Earth's continents
  • To introduce the world's ocean basins and the topographic characteristics of the seafloor


Glossary of Key Terms

Abyssal plains Large zones of relatively low-relief seafloor constituting one of the deepest areas of an ocean basin.
Atmosphere The blanket of air that adheres to the Earth's surface which contains the mixture of gases essential to the survival of all terrestrial life forms.
Biosphere The zone of terrestrial life, the habitat of all living things; include's the Earth's vegetation, animals, human beings, and the part of the soil layer below that hosts living organisms.
Continental rise Transitional zone of gently sloping seafloor that begins at the foot of the continental slope and leads downward to the lowest (abyssal) zone of an ocean basin.
Continental shelf The gently sloping, relatively shallow, submerged plain just off the coast of a continent, extending to a depth of ca. 180 m (600 ft/100 fathoms).
Continental slope The steeply plunging slope that begins at the outer edge of the continental shelf (ca. 180 m [600 ft.] below the sea surface) and ends in the depths of the ocean floor at the head of the continental rise.
Cryosphere The collective name for the ice system of the Earth.
Cultural landscapes The forms and artifacts sequentially placed on the natural landscape by the activities of various human occupants.
Earth System The shells or layers that make up the total Earth System range from those in the planet's deepest interior to those bordering outer space: this book focuses on the key Earth layers of the atmosphere, lithosphere, hydrosphere, biosphere, and cryosphere.
Ecumene The portion of the world's land surface that is permanently settled by human beings.
Hemisphere A half-sphere; used precisely, as in Northern Hemisphere (everything north of 00 latitude), or sometimes more generally, as in land hemisphere (the significant concentration of landmasses on roughly one side of the earth).
Hydrosphere The sphere of the Earth System that contains all the water that exists on and within the solid surface of our planet and in the atmosphere above.
Land hemisphere The roughly one-half of the Earth that contains most of the landmasses (see Fig 2.4); the opposite of the water (oceanic) hemisphere.
Lithosphere The outer most shell of the solid Earth, lying immediately below the land surface and ocean floor (lithas means rock).
Midoceanic ridge High submarine volcanic mountain ranges, part of a global system of such ranges, most often found in the central areas of the ocean basins; here new crust is formed by upwelling molten rock, which continuously moves away toward the margins of the ocean basins.
Natural landscapes The array of landforms that constitute the Earth's surface and the physical features that mark them.
Regional subsystem The particular interconnection, at any given place within the total Earth System, of the five spheres or subsystems (atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere).
Water (oceanic) hemisphere The roughly one-half of the Earth that contains most of the surface water; the opposite of the land hemisphere (see Fig. 2.4).


Unit Outline

  • Spheres of the Earth System (Figs. 2.1, 2.2)
    • The atmosphere
    • The lithosphere
    • The hydrosphere
    • The biospnere
    • The cryosphere
  • Hemispheres
    • Northern and Southern hemispheres
    • Eastern and Western hemispheres
    • Land and water hemispheres
  • Continents and oceans
    • The six continental landmasses
      • Africa
      • South America
      • North America
      • Eurasia
      • Australia
      • Antarctica
    • The five ocean basins
      • Pacific Ocean
      • Atlantic Ocean
      • Indian Ocean
      • Southern Ocean
      • Arctic Ocean
    • Ocean basin components
      • continental margins
        • continental shelf
        • continental slope
        • continental rise
      • abyssal zone
        • abyssal plains
        • seamounts
        • midoceanic ridges


Review Questions

  1. Draw and label the physical features that connect surface landmasses to the deepest abyssal zones of the ocean basins. Include the terms continental shelf, continental rise, and continental slope.
  2. Describe the Earth's cryosphere, and its components.
  3. State in very general terms the global distribution of population, using Fig. 2.5 as a reference.