Lecture 8a: Plate Tectonics, Geologic Principles & Geologic Time

Plate Tectonics

• Movement of tectonic plates with respect to each other, floating on the asthenosphere, caused by convection in the mantle.
• Early idea: Alfred Wegener (1912) proposed Continental Drift based on coastlines fit, fossils, mountain ranges, and glacial erosion patterns.
  • However, mechanism was considered physically not possible at the time, so his idea was largely rejected.
• 1920s–1940s: Discoveries about how continents separate began to emerge.
• Late 1940s–1950s: Discovery of mid-ocean ridges.
• 1960: Harry Hess proposed sea-floor spreading along mid-ocean ridges.
• Mid-1960s: Sea-floor spreading confirmed by evidence such as magnetic patterns and dating of rocks.
• 1960s–present: Technologies like GPS and InSAR enable precise measurements of plate motions.
• Plate tectonics hypothesis (today): A unifying explanation for the movement of crustal plates and global geologic activity.

Fossils and Plate Configuration (Evidence for Plate Tectonics)

• Fossils & Plate Configuration as key evidence for plate tectonics.
• Gondwanan distribution and the concept of Pangaea:
  • Fossils of Mesosaurus found in Argentina and Africa but nowhere else, suggesting these landmasses were once connected.
  • Glossopteris (ferns) fossils found across southern land masses (e.g., Africa, South America, Antarctica, India, Australia), implying a connected southern supercontinent.
  • Late Paleozoic configurations show the arrangement of continents and oceans like the Tethys Sea and the proposed supercontinent behavior behind today’s plate movements.
• Wegener’s observations supported a once-linked supercontinent (Pangaea) and subsequent separation of landmasses.
• Modern implication: Fit of coastlines, matching mountain belts, fossil correlations, and glacial patterns collectively support continental drift and plate tectonics.

Geomagnetics & Paleomagnetics

• Geomagnetics:

  • The Earth's magnetic field is generated in the fluid outer core.
  • The geomagnetic field exhibits periodic reversals in polarity (magnetic north/south flip).
  • The magnetic field’s direction at a location records the inclination of the field as rocks form, which helps reconstruct past plate motions.
  • Magnetic field lines bend around the Earth; the inclination of the field changes with latitude and over time.

• Paleomagnetics (magnetic stratigraphy):

  • Oceanic crust records magnetic polarity reversals as basaltic rocks cool and lock in the prevailing magnetic field direction at that time.
  • 1 reversal takes ≈ 7{,}000 ext{ years}.
  • There have been ≈ 184 reversal events in the past 83 ext{ Ma} (million years ago), with the latest reversal occurring about 0.78 ext{ Ma} ago.
  • These records provide a time scale that correlates magnetic anomalies with sea-floor spreading and plate motion histories.

• Paleomagnetics & plate motion:

  • Finding the “Past North Direction” helps delineate the historical movement of tectonic plates.
  • Magnetite (Fe-oxide mineral) and other iron-bearing minerals (e.g., hematite) in cooling basalt preserve the direction and polarity of the current magnetic field at deposition.
  • The preserved magnetic signatures along mid-ocean ridges serve as a record of plate motion over geological time.

• Video resource mentioned (example): a visualization contrasting pole wander paths vs. plate tectonics.

• Question (quiz): What is the oldest age of preserved sea-floor magnetic signature?

  • Options: A) Miocene, B) Late Jurassic, C) Early Cretaceous, D) Middle Cretaceous

Uniformitarianism

• Uniformitarianism:
  • The present is the key to the past; the same natural laws operate today as in the past, though rates can vary.
  • Proponents: James Hutton (1726–1797) and Charles Lyell (1797–1875).
  • This principle underpins the interpretation of geological records and the use of present processes to infer past conditions.

Relative Dating: Principles & Concepts

• Goal: Determine the relative order of past events (i.e., whether one rock unit is older or younger than another).
• Core concepts used for Relative Dating:
  • Principle of Original Horizontality: Sediments are deposited in horizontal or near-horizontal layers; folding or tilting later can deform them.
  • Principle of Lateral Continuity: Sedimentary layers extend laterally in all directions until they thin out at the edges of their deposition environment.
  • Principle of Superposition: In undisturbed sequences, younger layers are deposited on top of older layers; oldest at the bottom.
  • Principle of Inclusions: Any fragment included within another rock is older than the surrounding rock.
  • Principle of Cross-Cutting Relationships: A feature that cuts across another feature is younger than the feature it cuts.
  • Unconformities: Gaps in the geological record representing erosion or non-deposition.
  • Principle of Faunal Succession: Fossil content of successive layers allows correlation and dating; same fossils indicate similar ages across different locations.
• Recap: These principles provide a framework for establishing the relative ages of rocks and events before precise absolute dating methods.

Law of Superposition

• In an undisturbed sequence, the oldest strata are at the bottom and the youngest at the top.
• Example reference: Grand Canyon sequence is often used to illustrate young-to-old stacking, with younger deposits over older ones when not disturbed.
• Key takeaway: Relative age ordering is based on vertical position in sequences (assuming no tilting, folding, or overturning).

Law of Lateral Continuity

• Horizontal layers extend laterally until they thin out at the edge of their deposition environment (e.g., ocean or lake basins).
• Practical example: Rainy Cove, Nova Scotia demonstrates how layers can be traced laterally and thinned toward the margins of their original basin.
• Implication: If a layer is laterally truncated or absent in a section, post-depositional processes may have removed it.

Sediments & Stratigraphy: Key Concepts

• Many sediments form horizontal layers and are deposited over wide areas (e.g., mud, shale, carbonate limestone, flat/rippled sandstone).
• Not all sediments are deposited in broad horizontal sheets: rivers create elongated, narrow sediment belts; deserts form localized dune fields.
• Large-scale sea level changes (e.g., during ice ages) expose marine layers at the surface where erosion can occur, erasing portions of the geologic record and creating hiatuses.
• Erosion erases the geologic record, causing gaps (hiatus) in the stratigraphic sequence.
• Sedimentary rocks and their contacts:
  • Generally flat-lying, with occasional deformation (tilting, folding).
  • A wide variety of grain sizes; most sedimentary rocks are clastic, but carbonate sediments (limestones) are also common.
  • Intrusions of igneous rocks can intrude sedimentary sequences; important features include sills (parallel to sedimentary layering) and dikes (cuts across layering).
• Intrusive relationships:
  • Sills: Contacts are parallel to sedimentary layering and have limited thickness.
  • Dikes: Cuts across sedimentary layers.
• Metamorphic rocks: Can deform or reinterpret original layering; any rock type can be metamorphosed depending on conditions.
• Field relationships are essential for interpreting the geologic history of a region.

Hiatus & Unconformities

• Hiatus: A time gap in the rock record due to erosion or lack of deposition.

• Unconformities: Record of erosion/time gap with new deposition on top. Types:

  • Non-conformity: A non-layered igneous or metamorphic rock overlain by sedimentary rock.
  • Disconformity: Sedimentary rock overlain by another sedimentary rock with a time gap but no angular difference.
  • Angular Unconformity: Sedimentary rocks tilted or folded above an older sequence, creating an angular discordance.

• Visual cues (field context) help distinguish types of unconformities and reconstruct missing time intervals.

• Example reference: Angular unconformity observed in the Andes (Argentina) illustrates an older surface truncated by younger deposition or tilting.

Principle of Inclusions

• If a rock includes fragments of another rock, the included fragments are older than the enclosing rock.
• Example: Erosion and deposition on older igneous rocks can yield inclusions within newer sedimentary beds.
• Younger magma intrusions can also break through and incorporate fragments, indicating relative timing of magmatic activity vs. sediment deposition.

Faunal Succession & Index Fossils

• Principle of Faunal/Fossil Succession:
  • Each sedimentary layer is characterized by its fossil content.
  • Layers at different locations can be correlated if the same fossils occur in them, allowing cross-site age comparisons.
  • Used to divide geological time into major intervals (Paleozoic, Mesozoic, Cenozoic).
• Index fossils:
  • Short-lived, easily recognized, and geographically widespread fossils.
  • Useful for identifying and dating rock layers across broad regions.
  • Prompt identification of age ranges even when rock types differ.
• Example note: The presence of easily identifiable short-lived fossils in multiple rock types across regions enables correlation of layers and construction of relative geologic time scales.

Practice Questions & Quizzes (From the Lecture Slides)

• Q1: What is the oldest age of preserved sea-floor magnetic signature?
  • Options: A) Miocene, B) Late Jurassic, C) Early Cretaceous, D) Middle Cretaceous
• Q2–Q3: Refer to Lecture 12; determine the relative order of events or the applicable principle for a given sequence (not shown here).
• Q4 (Index fossils):
  • 4-A: When did layers H–E become folded? Options: a) Before dike C was intruded, b) After layer E was deposited but before layer B, c) After layer B was deposited, d) While layer A was being deposited, e) After layer H, but before G was deposited
  • 4-B: Which type of fold? Options: A) Syncline, B) Anticline, C) No fold visible
• Cross-cutting relationships exercise (Page 23): By the Principle of cross-cutting relationships, C is younger than which layers? Options: a) H, b) E, c) B, d) A
• Unconformity identification (Page 24): What type of unconformity is represented by the bold heavier line? Options: a) Disconformity, b) Nonconformity, c) Angular Unconformity, d) None of these is shown here

Connections to Foundational Principles, Real-World Relevance & Implications

• Scientific process and paradigm shifts:
  • Wegener’s Continental Drift hypothesis illustrates how a bold hypothesis can be initially resisted due to missing mechanism, followed by accumulation of evidence leading to a broader theory (plate tectonics).
• Uniformitarianism as a foundation for geology:
  • Similar natural laws operate today and in the past, enabling us to interpret ancient processes from present-day observations.
• Modern geophysics and technology:
  • GPS and InSAR provide quantitative measurements of plate motions, refining our understanding of plate interactions, earthquake risk, and volcanic activity.
• Practical implications:
  • Knowledge of plate tectonics informs resource exploration (minerals, hydrocarbons) and hazard assessment (earthquakes, tsunamis, volcanism).
• Ethical/philosophical dimension:
  • The shift from a static to dynamic Earth requires updating curricula and public understanding; science advances through evidence, dialogue, and willingness to revise models.
• Interdisciplinary connections:
  • Geochemistry (paleomagnetism, mineral records), geophysics (core dynamics), sedimentology (stratigraphy), and structural geology (folding, faulting, dikes/sills) all contribute to a cohesive understanding of Earth history.

Notes on Notation & Parameters

• Geologic time units used in the materials: Ma = million years ago. Example: past 83\ Ma contains approximately 184 magnetic reversals.
• Typical reversal cadence: \approx 7{,}000\ \text{years} per reversal.
• Key events in the plate tectonics timeline:
  • Continental Drift proposal: 1912
  • Mid-ocean ridges recognized: 1940s–1950s
  • Sea-floor spreading proposed: 1960
  • Magnetic anomalies and dating corroborate spreading: mid-1960s
  • Modern plate-motion measurements: 1960s–present (GPS, InSAR)

Note: The figures and specific layer identities (e.g., layers H–E, C, dike C, etc.) referenced in the quiz prompts are described as part of the in-text questions and exemplars. In practice, these would be interpreted with accompanying stratigraphic columns, maps, or cross-sections from the course materials. The notes above capture the conceptual content and instructional prompts presented in the transcript.