CC

Notes on the Origin and Evolution of the Marine Environment (MSC I112)

Introduction

  • Plate tectonics represents a fundamental revolution in geology.
  • In the early 20th century, most geologists believed the positions of continents and ocean basins were fixed and did not change.
  • The foundations for the revolution in geology were laid with the Continental Drift Hypothesis introduced in the early 20th century.

Continental Drift Hypothesis

  • Proposed by Alfred Wegener in 1915.
  • Supportive evidence includes:
    • Continental fit (coastlines aligning when assembled).
    • Glacial/paleo-climate evidence suggesting past global connections.
    • Fossil evidence showing similar organisms across continents.
    • Matching rocks and geological features across continents.
  • Concept of Pangaea (late Paleozoic to Mesozoic) as a single supercontinent.
  • Ocean surrounding Pangaea: Panthalassa.
  • Pangaea began to break up about 200\,\mathrm{Ma} into smaller continents that drifted toward their present positions.
  • Since Wegener, geologists have proposed that supercontinents existed at least 4 times in Earth history (more than one Pangaea).

Pangaea and Supercontinents

  • Wegener proposed that the continents were once part of a single supercontinent named Pangaea ("all land").
  • The surrounding ocean was Panthalassa.
  • Breakup occurred circa 200\,\mathrm{Ma}, leading to the present-day continental arrangement.
  • Evidence supports the idea that supercontinents formed and broke apart multiple times (at least four major events).

Four Types of Evidence for Continental Drift

  • Past evidence: data from ancient geology and paleoclimatology.
  • Present evidence: observations of current plate movements.
  • Continental fit, glacial deposits, fossil records, and geological continuity across continents constitute the four major evidence types.
  • Each type contributed to the broader argument that continents drift across the oceans.

Continental "Fit" (Continental Fit)

  • Notable matches include:
    • West coast of Africa ↔ east coast of South America.
    • Northwest coast of Africa ↔ southeast coast of North America.
    • West coast of Greenland ↔ northeast coast of North America.
  • A global fit improves when considering continental shelves; supports past connection.

Glacial Evidence

  • Glacial deposits dated to about 300\,\mathrm{Ma} found in Antarctica, southern Africa, southeastern South America, India, and southern Australia.
  • Explaining these deposits requires fitting the continents together and placing them near the south pole in the past.
  • Evidence also includes glacial striations and terminal moraines.

Fossil Evidence

  • Identical fossils found on continents that are now widely separated, implying past connectivity.
  • Examples mentioned include Mesosaurus and Glossopteris.
  • Alternative explanations were considered but fossil distribution supports past continental connections.

Geological Evidence

  • Old rocks of Proterozoic mountain belts are continuous from one continent to another, e.g.,
    • South America
    • Africa
    • Greenland
    • Europe
    • North America
  • The continuity of ancient crust and mountain belts across current continental boundaries supports past supercontinent configurations.

The Great Debate

  • Wegener’s idea faced substantial skepticism and ridicule.
  • Major criticisms included the lack of a mechanism for continental drift.
  • The drift hypothesis faded for a period but was revived by subsequent discoveries:
    • Harry Hess used sonar in 1962 to map the ocean floor and proposed sea-floor spreading as a mechanism.
    • Later, paleomagnetism investigations in the late 1960s and 1970s provided further support.

Earth’s Magnetic Field

  • Earth has a dipolar magnetic field with polarity that can be observed as North/South magnetic poles.
  • Key concepts:
    • Declination and Inclination.
    • Magnetic field lines.
    • Intensity and polarity.
  • Visual analogies include aligned iron filings and a bar magnet with the magnetic poles aligned with the field.
  • The North Magnetic Pole moves relative to the North Geographic Pole.
  • When ocean-floor rocks form, iron minerals align with the ambient magnetic field; as rocks cool past the Curie point, this magnetic information is preserved.

Paleomagnetism

  • Paleomagnetism is the study of magnetic information preserved in rocks.
  • Rock magnetism can be measured in the laboratory.
  • Ancient rocks reveal latitudes/longitudes of magnetic poles that differ from today, enabling reconstructions of past plate positions.

Polar Wander

  • Paleomagnetism from ancient lavas indicates past magnetic polar wandering.
  • Polar wander paths can be shown on graphs; they indicate how the apparent position of the pole moved over time.
  • Visualization often includes numbered time markers (e.g., 0 D, 1000, etc.) to represent time and latitude/longitude changes.

Polar Wander and Apparent Polar Wander

  • Polar wander paths built from rocks of different continents do not line up, suggesting either multiple magnetic poles or plate motion.
  • The concept of apparent polar wander shows that the observed wander can be explained by plate movements rather than true pole motion.
  • Apparent Polar Wander implies that: magnetic polar wander is not the signature of a wandering pole on a fixed continent; rather, it is the signature of a fixed pole on a wandering continent.
  • Apparent Polar Wander (APW) can be reconciled by moving continents together, yielding consistent pole positions over time.

The Ocean Floor – Sonar

  • Oceanographers discovered several key features:
    • Deepest parts of the ocean occur near land.
    • A continuous or nearly continuous underwater mountain range runs through all ocean basins.
    • Submarine volcanoes form linear arrangements across the ocean floors.

The Ocean Floor – Key Features

  • Deep-ocean trenches.
  • Mid-ocean ridges (ridge axes).
  • Fracture zones.
  • These features are present in ocean basins and are central to plate tectonics.

Oceanic Crust

  • Oceanic crust is covered by sediment; thickness is greatest near continents and thins toward mid-ocean ridges.
  • Oceanic crust is mafic (basaltic) in composition.
  • High heat flow is characteristic of the mid-ocean ridge system.

Seismic and Structural Evidence

  • Earthquake belts were discovered along with oceanic fracture zones, mid-ocean ridge axes, and deep-ocean trenches.
  • These seismic patterns align with plate tectonic concepts and support the existence of plate boundaries.

Sea-Floor Spreading – Harry Hess (1962)

  • Core idea: mantle upwelling at mid-ocean ridges creates new crust that moves away from the ridge.
  • At ocean trenches, the sea floor subsides and sinks back into the mantle.
  • This process provides a plausible mechanism for continental drift and explains the creation and destruction of oceanic crust.

Magnetic Reversals

  • Layered lava flows record reversals in Earth’s magnetic polarity over geological time.
  • Frederick Vine and Drummond Mathews (1960s) used magnetic anomalies to test and support the sea-floor spreading hypothesis.

Sea-Floor Spreading – Magnetic Anomalies

  • Magnetic anomalies are symmetric about a mid-ocean ridge axis.
  • Positive anomalies indicate a field aligned with current polarity; negative anomalies indicate the opposite polarity.
  • These anomalies reflect the history of magnetic field reversals as new crust forms at ridges and moves outward.

Sea-Floor Spreading – Age Patterns

  • Age of sea floor increases with distance from the mid-ocean ridge.
  • The age distribution is a mirror image across the ridge axis, consistent with symmetric spreading from the ridge outward.

Hot Spots

  • John Tuzo-Wilson proposed the hot-spot idea.
  • Hot spots are volcanic plumes that are relatively stationary and independent of moving plates.
  • Plate motion can be tracked by the alignment of hotspot volcanoes (e.g., Hawaii-Emperor seamount chain) as plates move over stationary plumes.
  • This helps to reconstruct plate motion histories.

Transform Faults

  • Transform faults segment the middle of ocean ridges.
  • John Tuzo-Wilson proposed that transform faults connect offset ridge segments.
  • They accommodate the lateral movement of plates past each other.

Mantle Convection

  • Dan McKenzie proposed a two-layer mantle convection model (1966).
  • Convection in these mantle layers can drive the movement of tectonic plates on the surface.

Plate Tectonics – Synthesis

  • The modern theory of plate tectonics emerged in the 1960s as a synthesis of:
    • Alfred Wegener’s evidence for continental drift (1915).
    • Harry Hess’s sea-floor spreading (1960s).
    • Frederick Vine and Drummond Mathews’s magnetic anomaly evidence for sea-floor spreading (1960s).
    • John Tuzo-Wilson’s concepts of hot spots and transform faults (1960s).
    • Dan McKenzie’s two-layer mantle convection model explaining the driving forces behind plate movement (1960s).
  • The integrated theory explains the movement of lithospheric plates, the creation and destruction of oceanic crust, the distribution of earthquakes and volcanoes, and the past configurations of continents and oceans.

Notes on Implications

  • Philosophical and scientific implications include a paradigm shift from static-to-dynamic Earth models.
  • The discovery of plate tectonics unified geology with geophysics and geochemistry, illustrating a coherent, testable framework for understanding Earth’s history.
  • Practical implications encompass improved understanding of natural hazards, resource distribution, and crustal evolution.