Plate Tectonics: Movement, Types and Evidence
Plate Tectonics: Movement and Types
Plate tectonics describes the large-scale motion of Earth’s lithosphere. The lithosphere is broken up into large slabs, which are spherical in shape. These plates move slowly and slide on the upper mantle.
Types of Plates
- Mixed plates: Composed of both continental and oceanic lithosphere.
- Oceanic plates: Consist only of oceanic lithosphere. An example is the Pacific Plate, known for its wide extension.
Plate Boundaries
- Constructive (divergent) boundaries: These are edges where plates separate, such as oceanic ridges and intracontinental rifts.
- Atlantic dorsal type: Located in the center of oceans like the Atlantic and Indian, these are symmetrical with respect to the coasts, with a center fracture called a rift.
- Pacific dorsal type: Asymmetrical, with no rift, and located near the coast.
- Intracontinental rift valley: Diverging edges separating two plates on the same continent (e.g., Africa).
- Neutral (transform) boundaries: Plates slide horizontally in opposite directions along transform faults. These boundaries are also known as passive or conservative.
- Convergent boundaries: Plates move closer together, causing earthquakes or volcanoes. These are also known as destructive boundaries.
- Convergence types:
- Ocean-ocean: One plate subducts under another along the Benioff plane.
- Continent-continent: One plate sinks under the other, forming a smaller angle due to the lower density of the rocks.
- Ocean-continent: Ridges are formed.
Theory of Plate Tectonics
Plate movement is driven by the internal heat of the Earth. Contributing factors include mantle convection, sinking plates, and magmas at ridges.
Evidence for Plate Tectonics
- Paleontological evidence: The presence of the same types of organisms on widely separated shores.
- Geographical evidence: The matching of coastal areas that would overlap.
- Seismic evidence: Distribution of seismic foci along the Benioff plane.
- Oceanic evidence: Magma at ridges shows bands of different polarity.
- Paleoclimatic evidence: Evidence of past climates.
Types of Geological Stress
- Compression: Shortening and thickening of rocks, originating in convergent margins.
- Tension: Stretching and thinning of rocks at divergent edges.
- Shear: Pressure acting in opposite directions on transform faults and convergent margins.
Folds
Folds are plastic, continuous deformations that do not break the rock structure. They form curvatures in the rock structure.
Fold Elements
- Hinge: Area of maximum curvature. The line connecting hinge points is the hinge line.
- Axial plane: A plane containing all hinge lines, dividing the fold into two flanks or limbs.
- Dip slope: The angle between the surfaces of the flanks.
- Core: The innermost part of a tight fold.
- Direction: Angle between the fold axis and the north-south line.
Types of Folds
Anticlines and synclines, which can be symmetrical or asymmetrical.
Faults
Faults are discontinuous deformations where movement has occurred between blocks separated by a fracture.
Types of Faults
- Normal: One block moves down relative to the other.
- Reverse: One block moves up relative to the other.
- Strike-slip: Lateral movement of blocks.
Combinations of these can create structures like horsts (uplifted blocks) and grabens (down-dropped blocks).
Dating Methods
Dating determines the time of an event or object.
- Relative dating: Orders stratigraphic events in a time series and can specify the time elapsed since an event.
- Absolute dating: Measures the time elapsed since a geological event occurred.
Methods of Absolute Dating
- Biological methods
- Sedimentological methods
- Radiometric methods
Geological Principles
- Principle of cross-cutting relationships: Any geological feature that cuts across another is more recent than the feature it cuts.
- Principle of inclusions: Inclusions are more modern than the horizontal cut.
- Principle of lateral continuity: A stratum is the same age along its entire length.
- Principle of superposition: The oldest stratum is at the base, and the most modern is at the top.
- Principle of uniformitarianism (actualism): Current geological processes are the same as those in the past.
- Principle of faunal succession: Fossils found in different geological times are distinct.