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

Vocabulary flashcards covering key terms from Lecture 8a.

Plate tectonics

Theory that Earth's lithosphere is divided into rigid plates that move on the asthenosphere due to mantle convection; explains earthquakes, volcanoes, mountain building, and sea-floor spreading.

Continental drift

Wegener’s hypothesis that continents move relative to one another; evidence includes coastlines fit, fossil matches, and mountain belts; mechanism was not identified at the time.

Mid-ocean ridges

Undersea mountain ranges where new oceanic crust forms as magma rises and cools, fueling sea-floor spreading.

Sea-floor spreading

Process by which new oceanic crust forms at mid-ocean ridges and pushes older crust outward, driving plate motion.

Magnetic polarity reversals

Periodic flips in Earth’s magnetic field direction recorded in rocks, used to track plate movements and sea-floor spreading.

Paleomagnetism

Study of the ancient magnetic properties of rocks to infer past pole positions and plate motions.

Magnetic inclination

Angle between a rock’s magnetization and the horizontal, reflecting the latitude of formation and preserving the field orientation.

Geomagnetics

Science dealing with Earth’s magnetic field and its history, including reversals and rock magnetization.

Magnetite

Iron-oxide mineral that records the Earth's magnetic field as rocks cool and lock in magnetization.

Index fossils

Widely distributed, short‑lived fossils used to identify and correlate the ages of rock layers.

Mesosaurus

Permian reptile whose fossils are found in Africa and South America, supporting continental connection.

Glossopteris

Fossil fern found across southern continents, evidence for a former supercontinent (Gondwana).

Pangaea

Hypothetical supercontinent that included most or all of Earth's landmasses before their breakup.

Tethys Sea

Ancient ocean between Gondwana and Laurasia, later closed by plate tectonics.

Uniformitarianism

Principle that present processes shape the past; natural laws operate today as they did in the past.

Relative dating

Ordering of events or rocks by age relative to each other (not in exact years).

Original horizontality

Sedimentary layers are deposited horizontally; tilting or folding indicates later deformation.

Lateral continuity

Sedimentary layers extend laterally until they thin or terminate; gaps reflect erosion or nondeposition.

Law of superposition

In a sequence of undisturbed sedimentary rocks, oldest layers are at the bottom, youngest at the top.

Cross-cutting relationships

A rock feature that cuts across another is younger than the feature it cuts.

Inclusions

Fragments of older rocks included within a newer rock; inclusions are older than the host rock.

Faunal succession

Successive rock layers are characterized by their fossil content, allowing age correlations.

Unconformities

Surfaces that represent gaps in the geological record due to erosion or non-deposition.

Non-conformity

Sedimentary rocks overlie eroded igneous or metamorphic rocks, indicating a major erosion/deposition gap.

Disconformity

Unconformity where the contact between sedimentary layers is parallel, indicating a time gap in deposition.