Sevier Orogeny and Continental Drift Overview

Sevier Orogeny and Plate Tectonics

• A layer of shallow ocean limestone was brought to the top of a mountain by the convergent forces of the Sevier Orogeny near Sun River Canyon, Montana.
• "Revolution" in science refers to significant paradigm shifts that force scientists to re-evaluate their understanding, like Darwin's evolution, Mendel's genetics, Watson-Crick-Franklin DNA model, and Einstein's relativity, impacting various fields.
• Plate tectonics is a revolutionary geological theory explaining Earth's dynamic surface, attributing earthquakes, mountains, and volcanoes to the movement of massive sections of the Earth's outer layers.

Wegener's Continental Drift Hypothesis

• Wegener synthesized evidence from similar rocks, mountains, fossils, and glacial formations across oceans, proposing continents once formed a single landmass.
• Continental coastlines had a jigsaw-like fit (e.g., South America and Africa).
• Evidence included fossils of Mesosaurus and Lystrosaurus found on separate continents, implying land bridges couldn't explain the trans-oceanic distributions of land-dwelling creatures.
• Glacial deposits in warm areas and tropical plant fossils in frozen regions were evidence against land bridges.
• Wegener couldn't explain the mechanism of continental drift, a critical gap in his theory that hindered acceptance.

Evidence supporting Plate Tectonics

• Ocean floor mapping revealed the Mid-Atlantic Ridge, a basaltic mountain range.
• Harry Hess proposed seafloor spreading, a process where new oceanic crust forms at mid-ocean ridges.
• Seafloor sediment thickness provided evidence against stationary ocean floors, and supported the concept of seafloor spreading and plate movement.
• Paleomagnetism (study of magnetic fields in rocks) provided crucial evidence for plate motion with symmetrical magnetic patterns on either side of mid-ocean ridges, revealing sea floor spreading.
• Earthquake epicenters followed the shapes of oceanic plates sinking into the mantle, forming Wadati-Benioff zones. Linking earthquakes to plate motion.

Layers of the Earth: Chemical and Physical

• The Earth has three major chemical layers: crust, mantle, and core. The crust has continental and oceanic forms.
• Seismic waves provide understanding of the Earth's interior and have been used to identify the Mohorovičić Discontinuity (Moho), the boundary between the crust and the mantle.
• Physical layers include lithosphere, asthenosphere, mesosphere, outer core, and inner core.
• Lithosphere is broken into plates, with plate boundaries actively shaping Earth's surface.
• Asthenosphere is the mechanically weak layer below the lithosphere that allows for plate movement.

Types of Plate Boundaries

• Plate boundaries are categorized as convergent, divergent, and transform.
• Convergent boundaries (destructive): Plates move towards each other, leading to subduction (one plate descending beneath another) or collision (both plates have similar densities, causing uplift).
• Divergent boundaries (constructive): Plates move apart, creating new crust, like at mid-ocean ridges or rift valleys. This can be narrow, like the East African Rift, or broad, like the Basin and Range region of the western United States.
• Transform boundaries (conservative): Plates slide past each other, creating earthquakes. A notable example is the San Andreas Fault.

The Wilson Cycle

• The Wilson Cycle describes the continuous opening, closing, and reopening of ocean basins, generating supercontinents like Pangea.
• Driving mechanisms of plate movement include ridge push, slab pull, and gravitational sliding.
• Hotspots are fixed points of volcanic activity unrelated to plate boundaries, creating island chains as the plate moves over them, like the Hawaiian Islands.

Additional Concepts

• Active margins are locations where oceanic and continental plates meet with significant movement, leading to mountain building and earthquakes. Passive margins form at the juncture of plates where one plate is made up of both continental and oceanic lithosphere.
• Subduction processes often lead to the formation of volcanic arcs, which are curved chains of volcanoes.
• Collision zones result in the formation of tall, non-volcanic mountains, high earthquake activity, and little volcanism.