Plate tectonics and continental drift revolutionized our understanding of Earth's dynamic surface. These theories explain how Earth's lithosphere is divided into moving plates, driven by convection currents in the mantle below.

Evidence from coastline shapes, rock formations, and fossils supports the idea that continents were once joined. Seafloor spreading at mid-ocean ridges creates new crust, while subduction zones recycle old crust back into the mantle.

Plate Tectonic Theory

Lithosphere and Asthenosphere

• Earth's outermost layer is the lithosphere, which consists of the crust and uppermost mantle
• Lithosphere is rigid and brittle, averaging about 100 km in thickness
• Below the lithosphere lies the asthenosphere, a layer of the upper mantle that is partially molten and behaves plastically
• Asthenosphere extends from the base of the lithosphere to a depth of about 660 km
• Lithosphere is broken into several large plates that move relative to each other, riding on the underlying asthenosphere

Convection Currents and Mantle Plumes

• Convection currents in the mantle are the driving force behind plate motion
  • Hot, less dense material rises while cooler, denser material sinks
  • This circular motion creates convection cells that transfer heat from the Earth's interior to the surface
• Mantle plumes are localized upwellings of hot material that originate deep within the mantle
  • They are thought to be the cause of volcanic activity at hotspots like Hawaii and Iceland
  • Mantle plumes are not directly related to plate boundaries and can occur in the middle of plates

Continental Drift

Alfred Wegener's Theory

• In 1912, German meteorologist Alfred Wegener proposed the theory of continental drift
• Wegener noticed that the coastlines of continents, particularly South America and Africa, fit together like a jigsaw puzzle
• He also found evidence of similar rock formations, fossils, and glacial deposits on different continents
• Wegener hypothesized that all continents were once joined together in a single supercontinent called Pangaea, which broke apart and drifted to their current positions over millions of years

Evidence for Continental Drift

• Paleomagnetism provides strong evidence for continental drift
  • Earth's magnetic field is recorded in rocks as they form, preserving the orientation of the magnetic poles at that time
  • Paleomagnetic data from different continents show that they have moved relative to each other and the poles over time
• Fossil evidence, such as the Glossopteris flora found in South America, Africa, India, and Antarctica, suggests these continents were once connected
• Matching rock formations and mountain ranges, like the Appalachians in North America and the Caledonides in Europe, indicate past connections between continents

Seafloor Spreading

Convection Currents and Seafloor Spreading

• Seafloor spreading is the process by which new oceanic crust is formed at mid-ocean ridges and spreads outward from the ridge axis
• Convection currents in the mantle drive the divergence of lithospheric plates at mid-ocean ridges
  • As plates move apart, hot mantle material rises to fill the gap, cools, and solidifies to form new oceanic crust
• Seafloor spreading explains the young age of oceanic crust (less than 200 million years old) compared to continental crust (up to 4 billion years old)
• The rate of seafloor spreading varies but averages about 5 cm per year, roughly the speed at which fingernails grow

Mantle Plumes and Hotspots

• Mantle plumes can cause excessive volcanism and elevated topography at hotspots like Iceland, which sits atop the Mid-Atlantic Ridge
• As lithospheric plates move over stationary mantle plumes, a chain of volcanoes forms, with the youngest volcano at one end and progressively older volcanoes extending in the direction of plate motion
  • The Hawaiian Island chain is a classic example of a hotspot track, with the active Kilauea volcano at one end and older, extinct volcanoes and seamounts extending to the northwest
• Paleomagnetism can be used to track the movement of plates over hotspots, as the magnetic orientation of the volcanic rocks records the position of the plate at the time of their formation