Distribution of Oceans and Continents

The One Thing

The present distribution of continents and oceans results from the long-term movement and interaction of tectonic plates. Continental drift, seafloor spreading, and subduction together explain how ocean basins and continental landmasses are created, widened, closed, and rearranged over geological time.

Who and What

• Continental Drift: The idea that continents move slowly across Earth’s surface and have changed their relative positions over time.
• Alfred Wegener: The scientist who proposed the continental drift hypothesis in 1912. His theory was supported by several lines of evidence but initially lacked a convincing mechanism for continental movement.
• Pangaea: A supercontinent that joined almost all present-day continents. It began breaking apart approximately 200 million years ago during the Mesozoic Era.
• Panthalassa: The vast global ocean that surrounded Pangaea.
• Laurasia: The northern part of Pangaea, which later formed North America, Europe, and much of Asia.
• Gondwanaland: The southern part of Pangaea, including South America, Africa, India, Australia, Antarctica, and adjoining land areas.
• Tethys Sea: A long, shallow sea between Laurasia and Gondwanaland.
• Lithosphere: Earth’s rigid outer layer, consisting of the crust and uppermost mantle. It is divided into tectonic plates.
• Asthenosphere: The weaker, partly plastic layer beneath the lithosphere on which tectonic plates move.
• Plate Tectonics: The theory that Earth’s lithosphere consists of moving plates whose interactions shape continents, oceans, mountains, earthquakes, and volcanoes.
• Plate Boundary: A zone where two tectonic plates meet and interact.
• Convection Currents: Circular movements of heated and cooled material in the mantle that may help move tectonic plates.
• Seafloor Spreading: The formation of new oceanic crust at mid-oceanic ridges, followed by its movement away from the ridges.
• Mid-Oceanic Ridge: A long underwater mountain chain formed where oceanic plates move apart and magma rises to create new crust.
• Subduction: The sinking of one tectonic plate beneath another into the mantle, usually at a convergent boundary.
• Divergent Boundary: A boundary where plates move apart, allowing magma to rise and form new crust.
• Convergent Boundary: A boundary where plates move toward one another, causing subduction, mountain building, or continental collision.
• Transform Boundary: A boundary where plates slide horizontally past one another, commonly producing earthquakes.
• Magnetic Stripes: Symmetrical bands of alternating magnetic polarity on the ocean floor that record seafloor spreading.
• Fossil Evidence: Similar fossils on continents now separated by oceans, indicating that those continents were formerly connected.

Chronology

Causes and Consequences

• Pangaea and Panthalassa established an earlier arrangement of Earth’s surface. Pangaea consisted of Laurasia in the north and Gondwanaland in the south, separated by the Tethys Sea and surrounded by Panthalassa. Its breakup demonstrates that the present arrangement of continents and oceans is not permanent.

• Wegener’s continental drift hypothesis was supported by several types of evidence. The jigsaw-like fit of continental margins, matching rocks and mountain ranges, similar fossils, and ancient climatic indicators all suggested that separated continents had once been joined.

• Fossil evidence supported former continental connections. The fossil plant Glossopteris was found in India, Australia, South Africa, South America, and Antarctica. The freshwater reptile Mesosaurus was found in both South America and Africa, although it could not have crossed a wide ocean.

• Matching geological structures supported continental movement. The Appalachian Mountains in North America correspond with the Caledonian Mountains of Europe, suggesting that these regions were once part of a connected geological system.

• Ancient climatic indicators provided further evidence. Glacial deposits and striations in present-day tropical regions indicate that some continents were formerly located closer to the South Pole.

• Seafloor spreading supplied the mechanism missing from Wegener’s theory. Harry Hess’s proposal in the early 1960s explained that new oceanic crust forms at mid-oceanic ridges and moves away from them. The age of ocean-floor rocks increases with distance from a mid-oceanic ridge, while magnetic stripes record symmetrical changes in magnetic polarity.

• Oceanic crust is continually created and destroyed. New crust forms at mid-oceanic ridges, whereas older crust is destroyed at subduction zones. Consequently, oceanic crust is generally younger than the oldest continental rocks because it is continually recycled into the mantle.

• Plate movement changes the distribution of continents and oceans. The approximate rate of plate movement is a few centimetres per year, comparable to the growth rate of human fingernails. Earth’s lithosphere is divided into several major and minor plates, including the African, Antarctic, Eurasian, Indo-Australian, North American, Pacific, and South American plates.

• Different plate boundaries produce different geological effects. At divergent boundaries, plates move apart; at convergent boundaries, they move together; and at transform boundaries, they slide past one another.

• Continental collision produces major mountain ranges. When two continental plates collide, neither usually subducts easily, so large fold mountains may form, as in the Himalayas.

• Oceanic–continental convergence produces subduction-related features. When an oceanic plate meets a continental plate, the denser oceanic plate generally subducts beneath the continental plate, producing trenches, earthquakes, and volcanoes.

• Oceanic–oceanic convergence produces trenches and volcanic island arcs. When two oceanic plates converge, one may subduct beneath the other, forming an ocean trench and a volcanic island arc.

• Convection currents may help drive plate movement. Heated and cooled material circulating within the mantle can contribute to the movement of plates across the asthenosphere.

• The plate tectonic theory integrates several earlier ideas. It combines continental drift, seafloor spreading, and subduction into a comprehensive explanation of the changing distribution of continents and oceans.

• Ocean basins are temporary geological features. They can open through rifting and seafloor spreading and later close through subduction and continental collision. This occurs within a planet approximately 4.6 billion years old, whose surface is continually reshaped by plate interactions.

• Plate interactions create major physical features and hazards. Mountain ranges, ocean trenches, mid-oceanic ridges, earthquakes, and volcanoes result from the movement and interaction of lithospheric plates.

What Gets Asked

• How did the evidence from Glossopteris, Mesosaurus, matching continental margins, the Appalachian and Caledonian Mountains, and glacial deposits support Wegener’s continental drift hypothesis?
• Why was Wegener’s theory initially incomplete, and how did Harry Hess’s concept of seafloor spreading provide a mechanism for continental movement?
• How do divergent, convergent, and transform boundaries differ in movement and geological consequences?
• Why does oceanic crust tend to be younger than the oldest continental rocks, and how do mid-oceanic ridges, magnetic stripes, and subduction demonstrate this?
• How do continental–continental, oceanic–continental, and oceanic–oceanic convergence produce different features, including the Himalayas, trenches, earthquakes, volcanoes, and volcanic island arcs?
• Why are ocean basins not permanent features, and how do rifting, seafloor spreading, subduction, and continental collision alter the distribution of continents and oceans?