Plate Tectonics Flashcards

The Quiet Revolution of Plate Tectonics

In the 1960s, a revolution occurred, shifting our understanding of Earth. This wasn't a single idea from one person but a collection of scientific insights that converged into the theory of plate tectonics.

• Plate Tectonics: Explains the structure and behavior of Earth, including the formation of continents, oceans, mountains, valleys, volcanoes, and earthquakes.
• This theory describes the Earth's broken outer shell rising from and falling back into the mantle, a continuous process of creative destruction and reconstruction.
• Plate tectonics is a grand unifying theory in Earth science, akin to Darwin's theory of evolution in biology and Einstein's theory of relativity in physics.

Early Speculations and Continental Drift

In the 16th century, as a complete picture of the globe emerged, speculations arose about the shape of Earth's landmasses and whether they once fit together.

• Lithosphere: The rocky outer part of the Earth, including the crust and the uppermost part of the mantle. It is broken into tectonic or lithospheric plates that move independently.
• Alfred Wegener (1912): Proposed the idea of a supercontinent and backed it up with evidence such as the spatial distribution of fossils, location of rock types, and trends of mountain ranges.
• Pangaea: The single supercontinent that existed approximately 225 to 300 million years ago, meaning "all Earth" in Greek.
• Continental Drift: The process by which Pangaea broke apart, leading to the current distribution of landmasses.
• Wegener's proposal faced resistance due to a lack of explanation for the energy needed to move continents.

Evidence from the Ocean Depths

Post-World War II, new evidence from the ocean depths revived the idea of drifting continents.

• Bruce Heesen and Marie Tharp (1957): Published the first physiographic map showing the physical features of the Atlantic Ocean floor.
  • Tharp converted raw data into maps, revealing the ocean's features as if drained of water. She was excluded from research expeditions because she was a woman.
• Mid-Atlantic Ridge: A vast mountain spine crisscrossed by huge fracture lines, extending for over 60,000 kilometers across different oceans.
• This discovery changed the understanding of the ocean floor from a flat, featureless plane to a landscape with mountains, valleys, and deep trenches.

Seafloor Spreading and Magnetic Barcodes

Further insights into seafloor dynamics provided critical evidence for continental movement.

• Harry Hess (1960): Proposed that magma spills out from the fracture lines of mid-oceanic ridges, creating new seafloor and pushing away the old seafloor.
• Seafloor as a Conveyor Belt: New seafloor forms at ridges, and old ocean crust is cooled, dragged down into the mantle at trenches, and recycled.
• Robert Dietz (1961): Published a similar idea he called the spreading seafloor theory
• Fred Vine and Drummond Mathews (1963): Published evidence recorded in the seafloor itself, related to Earth's paleomagnetism.
• Paleomagnetism: When magma cools and crystallizes, the alignment of the Earth's magnetic field is locked in place in the magnetic particles of rocks.
• Magnetic Barcode: Each time the Earth's magnetic field flipped, the magma erupting at mid-ocean ridges recorded the opposite polarity, creating a symmetrical pattern of magnetic stripes around the ridge.
• This evidence supported the seafloor spreading theory and provided the mechanism for how Earth's landmasses were moving.

Crustal Destruction and Subduction

Confirmation of crust destruction in ocean trenches completed the picture.

• During the Cold War, a global seismic surveillance system was created to monitor underground blasts, providing observations of deep earthquakes beneath ocean trenches.
• Subduction: The process where the denser oceanic crust dives beneath the lighter continental crust at subduction zones.
• Oceanic crust has a greater density than continental crust, causing it to subduct.
• Subduction zones are marked by giant trenches on the surface.

The Unifying Theory of Plate Tectonics

Precise mathematical calculations combined with new computing power allowed geophysicists to calculate how the coastlines fit and predict plate movements.

• A revolution in Earth science was led by young scientists in North America, Britain, and Europe.
• Years of research led to a map of the world divided into moving plates, creating the theory of plate tectonics.
• Plate Movement: The seven major plates and scattering of microplates glide across the weaker, hot, plasticky section of the mantle called the asthenosphere.
• Plate boundaries are dynamic places where much of the planet's geological action happens, such as earthquakes and volcanic activity.
• Pacific Ring of Fire: The area surrounding the basin of the Pacific Ocean, characterized by approximately 75% of all volcanoes and 90% of earthquakes.

Types of Plate Boundaries

Plates interact in various ways at their boundaries.

• Divergent Plate Boundary: Plates move away from each other, allowing magma to well up and create new seafloor.
  • Example: East Pacific Rise, where the Nazca Plate moves East and the Pacific Plate moves Northwest.
• Convergent Plate Boundary: Plates collide, and one plate is forced beneath the other (subduction).
  • Example: Peru-Chile Trench, where the denser oceanic Nazca plate is pulled beneath the lighter continental South American plate.
  • This process causes earthquakes and the formation of volcanoes, like the Andes.
• Transform Boundary: Plates slide horizontally past each other, causing stress to build up and release as earthquakes.
  • Example: San Andreas Fault, where the North American plate moves Southwest past the Pacific plate moving Northwest.
• Continental Collision: When two continental plates collide, neither is subducted, resulting in the compression and uplift of huge mountain ranges.
  • Example: Himalayas, where the Indian plate converges with the Eurasian plate.

Ongoing Refinements and Future Research

Plate tectonics theory continues to be fine-tuned.

• Continents grow from a nucleus of ancient and stable igneous and metamorphic rocks called a continental shield.
• Fragments of crust, such as island arcs and undersea volcanoes, are added to the main continent by collision.
• Plate motion is detected by satellites like the European Sentinel series, recording changes in Earth's surface down to the millimeter.
• Scientists are exploring what caused the outer shell to crack apart and how crustal recycling began, comparing Earth's plate tectonics with Venus.
• Connections between deep Earth processes and the evolution of complex life are emerging.
• Continental collisions and mountain building events may have supplied large pulses of nutrients to the biosphere during key moments of evolution
  • Example: the Cambrian explosion of 500 million years ago.