Plate Tectonics: Review, Measurements, Driving Forces, and Hotspots

Archimedes principle and Isostasy

  • Archimedes principle (floating object in fluid): the weight of the object is balanced by the weight of the displaced fluid. In equations:
    W<em>extobject=W</em>extdisplacedwater = rho<em>extfluid  V</em>extdisp  g.W<em>{ ext{object}} = W</em>{ ext{displaced water}} \ = \ rho<em>{ ext{fluid}} \; V</em>{ ext{disp}} \; g.
  • Earth application: isostasy. “Archimedes’ principle applied to the Earth.” The ocean basins are lower because their oceanic lithosphere is denser, while continents sit higher because their lithosphere is less dense and thicker on average.
  • Classic example: oceanic plates sit lower than continental plates because oceanic lithosphere is denser; this difference explains elevations and topography differences across the globe.
  • Big-picture numbers mentioned in lecture:
    • Ocean depth below sea level is about 4 km4\,\text{km} (roughly the depth of the ocean basins).
    • About 77%77\% of the planet is below sea level (and about 30%30\% is above sea level in the crude, illustrative balance described in lecture).
    • Continental crust is roughly four times as thick as oceanic crust (thicknesses vary, continental crust substantially thicker than oceanic crust).
  • Isostasy in plate tectonics terms: continental lithosphere sits high because it is thicker and less dense, while oceanic lithosphere sits lower because it is thinner and denser; the balance acts like a floating cork in a denser mantle.
  • Lithosphere vs asthenosphere:
    • Plates consist of crust plus lithospheric mantle (collectively known as the lithosphere).
    • The boundary below the lithosphere is the asthenosphere, which is convecting but behaves as a solid on geological timescales.
  • Structural details from the lecture (reviewed as context):
    • Upper mantle (below crust) starts around the bottom of the upper mantle at about 410 km410\ \text{km} depth and goes up to the Moho (crust–mantle boundary).
    • The lithospheric mantle is typically around 100 km100\ \text{km} thick but can be thicker (e.g., thick lithosphere under Hudson Bay) or thinner near active rifts and plate boundaries.
    • The upper mantle thickness is often described as ~370 km370\ \text{km} between the Moho and the deeper 410 km marker, with variations by region.
  • Key takeaway about density and elevation: thicker continental crust is less dense overall than thinner oceanic crust, which helps explain why continents sit higher relative to ocean basins.

Plate structure and review of concepts

  • Plates have two main components: the crust and the lithospheric mantle (together forming the plate).
  • The boundary between lithosphere and asthenosphere is not fixed in depth; it deepens with plate age and cooling.
  • As plates move away from mid-ocean ridges (where they form), the lithosphere cools and thickens; older plates are generally thicker and colder.
  • The plates are rigid blocks that move coherently; beneath them, the asthenosphere convects in a partly molten state, enabling or resisting plate motion via coupling.
  • Continental vs oceanic lithosphere:
    • Continental lithosphere: thicker and generally less dense overall, sits higher (near sea level).
    • Oceanic lithosphere: thinner and denser, sits lower (below sea level).
  • The “hip-symmetric” topography mentioned in lectures reflects this isostatic balance: roughly 70-77% of Earth’s surface is below sea level, with the remainder higher.

Plate boundaries and measurements of plate motions

  • Plate boundaries types:
    • Divergent boundaries (e.g., Mid-Ocean Ridges): plates pull apart; new plate forms here.
    • Convergent boundaries (e.g., subduction zones): plates move toward each other; one plate sinks (subducts) beneath another.
    • Transform boundaries (e.g., San Andreas Fault): plates slide past one another laterally.
  • Plate anatomy on maps:
    • Plates (e.g., North American plate) can contain both continental and oceanic lithosphere; passive margins illustrate continental-ocean transitions within a single plate.
    • The same plate can include oceanic regions that subduct beneath neighboring plates (e.g., the Pacific plate’s subduction around the Ring of Fire).
  • How we know plate motion today (measurement methods):
    • Global Positioning System (GPS) and geodetic monuments provide precise, time-stamped position data.
    • Permanent GPS stations measure how fixed points on Earth move over time; typical measurements yield plate velocities.
    • The velocity data are converted into plate motion vectors; longer arrows indicate faster plate motion on motion maps.
  • Example of present-day motion and velocity:
    • Pacific plate: fastest-moving plate, approx vPacific≈11 cm yr−1v_{Pacific} \approx 11\ \text{cm yr}^{-1}.
    • North American plate (the plate containing much of the U.S. and Canada): slower, approx vNA≈1.5 cm yr−1v_{NA} \approx 1.5\ \text{cm yr}^{-1}.
    • Transform boundary at California (San Andreas): relative motion along the fault is around several cm/yr in typical segments (e.g., ~5 cm/yr in some parts).
  • How GPS data are obtained and used:
    • A network of geodetic monuments and satellites allows distance measurements using the time-of-flight of light (GPS signals).
    • With 4 or more satellites, precise 3D positions are computed; repeated measurements yield velocities.
    • The technology originated in part from military uses (navigation for missiles); selective availability was removed about ~15 years ago, enabling near-real-time, high-precision data.
  • Plate-motion models:
    • Arrows on maps show directions and speeds of plate movement; red arrows often indicate plate-boundary velocities, blue arrows indicate interior plate motion.
    • California example (San Andreas): movement between Pacific and North American plates is visible as ~5 cm/yr along the boundary, with distributed deformation across parts of the North American plate.
  • Data sources for past plate motions:
    • Magnetic stripes on the seafloor record reversals of geomagnetic polarity; dating those stripes yields ages of oceanic lithosphere and hence past plate motion histories.
    • The magnetic-polarity data robustly date oceanic lithosphere up to about 2×108 years2\times 10^8\ \text{years} (200 million years); beyond that, the signal becomes unreliable or absent.
    • Oceanic stripes combined with plate kinematic models give a well-constrained history for roughly the last 2×108 years2\times 10^8\ \text{years}.
  • Hotspots as a time-machine for plate motions:
    • Hotspots are surface expressions of deep mantle plumes that rise from the core–mantle boundary and pierce the lithosphere to form volcanoes.
    • The plume itself is relatively stationary, while the tectonic plate moves over it, creating a track of volcanoes (a hotspot track).

- Examples: Hawaii and Yellowstone.

Mantle convection, plumes, and how they drive plates

  • Convection in the mantle: hot material rises; cold material sinks. In the Earth, the source of heat is primarily the core (outer core is hot molten iron).
  • Visual analogy: convection as a pot of soup on a stove:
    • Hot material rises in the center; cooler material sinks toward the edges.
    • A mantle convection cell acts like a circulating soup, with slabs of colder, denser lithosphere sinking (subducting) and hot mantle plumes rising.
  • Mantle plumes (mantle upwellings):
    • Rising columns of hot mantle material from deep within the mantle (often linked to core–mantle boundary heat).
    • When plumes reach the base of the lithosphere, they can melt rock and create volcanism, forming hotspot volcanism (e.g., Hawaii, Yellowstone).
    • The bottom of the mantle (the core–mantle boundary region) acts as a “slab graveyard” where subducted slabs eventually accumulate and may contribute to plume formation or perturbation later on.
  • Laid-out consequences of plumes:
    • Mantle plumes can drive continental rifting and the breakup of continents (e.g., East Africa Rift, Somali plate formation).
    • The example of East Africa: a large plume rising from deep within the mantle causing the African plate to tear apart and form a new Somali plate in the rift zone.
    • Big plume beneath Africa (Somali plume) and another beneath the Pacific plate create hotspot tracks in the oceans (Hawaii, Emperor Seamount chain).
  • Hotspot tracks and plate motion storytelling:
    • A hotspot track length corresponds to the distance the plate has moved over the plume during the time since the volcanoes formed.
    • For example, modern Hawaii sits atop a plume beneath the Pacific plate; as the plate moves over time, older volcanic islands form a chain in a direction indicating plate motion history.
    • Emperor Hawaiian Seamount Track shows a bend around ~48–85 million years ago, indicating a major plate-motion reorganization (the lecturer cites a bend around 48 Myr, linked to a global reorganization around the India–Eurasia collision ~50 Myr ago).
  • A thought experiment used in lecture:
    • Imagine a sheet of paper and a lighter underneath; as you slide the sheet over the lighter, you burn a track in the paper in the direction the plate is moving. The track points toward older volcanoes and thus indicates plate motion direction.
  • Why hotspot tracks are more visible in the oceans:
    • Oceanic lithosphere is thinner and younger in many places and easier for a hotspot to burn through; continental lithosphere is thicker and more buoyant, making hotspot tracks less obvious there outside specific settings like Iceland (which sits near a boundary).
  • Important nuance about volcano formation:
    • Volcanoes at hotspots are formed when a plume melts rock at the base of the lithosphere as it ascends; if enough melt is produced, volcanism occurs at the surface (e.g., Kilauea at Hawaii).
    • The plume can be partially molten (roughly 1–2% melt initially) and melting increases as it ascends and pressure drops; later melting mechanisms (decompression melting) continue to be studied.
  • Plume morphology and plate motion interaction:
    • Plumes themselves are relatively stationary; plates move over them; as a result, hotspot tracks record historical plate motion.
    • The plume’s activity can be long-lived, but the conduit feeding it can become blocked or reoriented, causing changes in eruptive styles or volcanic chains.
  • Volcano examples and notes:
    • Hawaii: Big Island (Kilauea) is currently active; the chain of older (extinct) volcanoes extends to the west as the Pacific plate moves over the stationary plume.
    • Yellowstone: hotspot track is debated but widely discussed as a major mantle plume; the plume may extend down to the core–mantle boundary.
    • Iceland: hotspot located on a plate boundary; a notable exception to typical, off-boundary hotspot tracks.
  • Sliding forces driving plate motion (three components):
    • The three drivers of mantle convection that move plates are commonly summarized as: slab pull, ridge push, and basal drag.
    • Slab pull: the gravitational driving force from dense, sinking slabs at subduction zones; this is widely considered the dominant force.
    • Ridge push: a misnamed but commonly used term; the actual process is gravitational sliding of older, thicker, heavier lithosphere away from the bounding ridge as a result of plate divergence; the ridges themselves are passive cracks rather than driving flows.
    • Basal drag (basal shear): coupling between the flowing asthenosphere below the lithosphere and the lithosphere itself; the strength and importance of basal drag remain debated and are less well constrained.
  • Which force is most important?
    • The consensus since the Forsyth & Uyeda (1975) paper: slab pull is the most important driving force; ridge push is the least important; basal drag remains uncertain.
    • Evidence: fast-moving plates (e.g., Pacific, Nazca, Cocos) have many subduction zones; slow-moving plates (e.g., North American) have few subduction zones.
  • Quick note on the Ring of Fire and Mount St. Helens:
    • Mount St. Helens is associated with the Cascadia subduction zone (not a hotspot).
    • The Ring of Fire is a broad zone around the Pacific where many subduction zones occur, driving significant volcanic activity.

Quick wrap-up data and formulas mentioned

  • Plate velocities (example values):
    • Pacific plate: vPacific≈11 cm yr−1v_{Pacific} \approx 11 \ \text{cm yr}^{-1}
    • North American plate: vNA≈1.5 cm yr−1v_{NA} \approx 1.5 \ \text{cm yr}^{-1}
  • Velocity relation used to estimate plate speed from hotspot tracks:
    • v=ΔsΔtv = \dfrac{\Delta s}{\Delta t}
    • Where (\Delta s) is distance moved over time interval (\Delta t). In hotspot tracks, (\Delta t) is the age difference between volcanoes and (\Delta s) is the along-track distance.
  • Magnetic stripes as dating tool:
    • Oceanic lithosphere ages are inferred from magnetic stripe patterns corresponding to polarity reversals; reliable up to roughly 2×108 years2 \times 10^8\ \text{years} (200 Myr).
  • Melt generation mechanisms (brief):
    • One mechanism discussed is decompression melting due to pressure decrease as mantle material rises; the lecture alludes to a second mechanism yet to be discussed in detail (to be covered in future sessions).
  • Notable examples to connect concepts:
    • East Africa Rift and Somali plate formation via a mantle plume scenario.
    • Hawaii hotspot chain and Emperor Seamount Track illustrating plate motion history.
    • Yellowstone plume and discussion of plume depth beneath North American plate.
    • Iceland as a hotspot associated with a plate boundary.

Connections and implications

  • Foundational principles linked:
    • Archimedes’ principle and isostasy explain why continents protrude above sea level while oceans lie in depressions.
    • Mantle convection (and basal drag) is the broad engine that drives plate motions, with slab pull identified as the strongest individual component.
  • Practical implications:
    • Understanding plate motions helps explain the distribution of earthquakes and volcanic activity (e.g., subduction zones align with high seismicity).
    • Hotspot tracks provide a record of plate motion history and can be used to reconstruct past plate configurations.
  • Ethical and broader context:
    • The study of plate tectonics integrates observation, modeling, and interpretation of signals across deep time; errors or mislabeling (e.g., names like “ridge push”) can lead to conceptual confusion, hence careful terminology and clear teaching are important.

Quick glossary reminders

  • Lithosphere: rigid outer shell of the Earth including crust and the rigid upper mantle.
  • Asthenosphere: ductile, convecting layer beneath the lithosphere which enables plate motion.
  • Moho: the crust–mantle boundary.
  • Isostasy: vertical equilibrium between crustal columns and the mantle allowing for topographic variations.
  • Slab pull: gravitational pull from sinking dense slabs at subduction zones.
  • Ridge push: gravity-driven sliding of older, thicker lithosphere away from a mid-ocean ridge; often considered a misnomer due to the ridge itself being a passive fracture.
  • Basal drag: coupling between the convecting asthenosphere and the lithosphere at the base of the plate.
  • Hotspot: surface expression of a deep mantle plume that can create long-lived volcanic chains as plates move over a relatively stationary plume.
  • Plate boundary types: divergent (ridge), convergent (subduction), transform (sliding).
  • Magnetic stripes: seafloor magnetic anomalies that record past geomagnetic polarity reversals and can be used to date oceanic lithosphere.