Earth Plate Tectonics #14

Earth’s Interior

  • Earth’s Mass: 5.9 sextillion tons (59 followed by 20 zeros).

  • Composition and characteristics inferred from:

    • Seismic waves and gravity.

    • Density differences refract (bend) seismic waves:

    • Denser layers: Waves speed increases.

    • P-Waves: Pass through all material.

    • S-Waves: Do not travel through fluids.

Geothermal Gradient

  • Temperature increases with increasing depth:

    • Approximately 25° C per km.

    • Approximately 1° F per 70 feet.

  • Examples:

    • South African gold mines: 2.2 miles (3.5 km).

    • Kola Superdeep Borehole: 7.5 miles (12.3 km).

  • Note: Direct measurement is only in the crust.

Density Variation with Depth

  • Density increases with increasing depth:

    • Composition is approximately 80% iron.

    • Iron is one of the ten most common elements in the galaxy and is frequently found in meteorites, but it is not as common on the surface.

Density of Earth's Layers

  • Question on Density: Which layer has the lowest density?

    • A. Inner core

    • B. Outer core

    • C. Lower mantle

    • D. Crust

    • Answers:

    • A: 94%

    • B: 6%

    • C: 0%

    • D: 0%

Structure of Earth's Layers

Earth's Layers and Their Characteristics

  • Solid Mantle:

    • Crust: 8-40 km (5-25 mi).

    • Solid Inner Core: 3500 km (2200 mi).

    • Liquid Outer Core: 2900 km (1800 mi).

Inner Core

  • Mass and Composition:

    • 1/3 of Earth’s mass.

    • Radius – 3600 km (2100 mi), composed of iron and nickel.

    • Inner Core:

    • Radius: 960 km (600 mi)

    • State: Solid, high density (13 g/cm³).

    • Estimated temperature: 6900° C (12,400° F).

    • Outer Core:

    • Thickness: 2400 km (1500 mi).

    • State: Molten, lower density (10 g/cm³).

    • Estimated temperature: 4800° C (8600° F).

Mantle

  • Thickness: 2885 km (1800 mi).

  • Mass Composition: 2/3 of Earth’s mass.

  • State: Solid – top is a plastic solid that deforms and flows.

  • Density: Ranges from 3.4 g/cm³ – 5.6 g/cm³.

  • Composition: Silicate rocks and iron (Fe) and magnesium (Mg).

Lithosphere and Asthenosphere

  • Lithosphere:

    • Average thickness: 100 km (60 mi).

    • Characteristics: Cool, hard, and strong.

  • Asthenosphere:

    • Thickness: 600 km (375 mi).

    • State: Plastic layer that flows both horizontally and vertically (convection).

  • Source of Tectonic Forces:

    • Alters the lithosphere and causes volcanic activity and mountain building.

Mohorovičić Discontinuity (Moho)

  • Definition: The boundary that separates the mantle from the crust, determined by density differences.

  • Characteristics:

    • Depth varies and mirrors surface topography.

  • Crust:

    • Thin, solid exterior constituting only 1% of Earth’s mass.

    • Lowest densities range from 2.2 to 3.0 g/cm³.

    • Two Types of Crust:

    • Continental Crust:

      • Thickness: Up to 70 km (43 mi).

      • Composition: Granitic (silica-rich with aluminum [Al], potassium [K], and calcium [Ca]).

      • Cover: 35% of the crust.

    • Oceanic Crust:

      • Thickness: 3 to 5 km (1.9 – 3 mi).

      • Composition: Basaltic (iron-rich with silicon [Si] and Mg).

      • Cover: 65% of the crust.

Types of Crust

  • Question on Density: Which type of crust has the highest density?

    • A. Oceanic

    • B. Continental

    • Results:

    • A: 67%

    • B: 33%

Isostasy

  • Definition: The condition of equilibrium in the Earth's crust, where the crustal plates "float" on the Earth's mantle.

  • Crustal level changes due to density differences and the addition and removal of material, known as Isostatic Rebound.

Continental Drift and Plate Tectonics

  • Alfred Wegener (1915): Developed the theory of Continental Drift.

    • Evidence for the Theory:

    • Jigsaw fit of continents.

    • Fossil matches across continents.

    • Stratigraphic evidence.

    • Paleoclimatic patterns.

  • Plate Tectonics: Gained acceptance in 1968 due to ocean basin research.

    • Explanatory power for:

    • Volcanic patterns and hot spots.

    • Earthquake occurrences.

    • Geological landforms.

Fossil Evidence Supporting Drift

  • Hypothetical Fossil Distribution:

    • Fossils of Mesosaurus found in southern Africa and South America, existing during Early Permian (299 to 251 Ma).

    • Fossils of the trilobite Lystrosaurus indicating the existence of land reptiles in multiple continents (South America, Africa, India, Australia, Antarctica).

    • Fossils of the fern Glossopteris found on all southern continents indicates they were once joined.

Geological Structures

  • Stratigraphic evidence of mountain ranges showing continuity across continents.

    • Example: Appalachian Mountains in North America connected with the Caledonian Mountains in Scandinavia.

Paleoclimatic Evidence

  • Paleoclimatic data supports historical climate evidence correlating with geological configurations:

    • Locations of ice masses and other geological features that indicate past environmental conditions.

Active vs Passive Margins

  • Passive Margins:

    • Characterized by gentle slopes leading from continental shelves to the abyssal plains.

  • Active Margins:

    • Associated with geological activity including subduction and earthquakes along the edges of tectonic plates.

Plate Boundaries Types

  • Types of Plate Boundaries:

    1. Divergent: Plates move apart, creating rift zones or mid-ocean ridges.

    2. Convergent: Plates collide, leading to subduction of oceanic crust or continental collisions.

    3. Transform: Plates slide past each other, causing faults like the San Andreas Fault.

Convergent Boundaries

  • Types of converging plates include:

    • Continental - Continental Collision (e.g., Himalayas).

    • Oceanic - Oceanic Collision (e.g., Aleutian Islands).

    • Continental - Oceanic Collision (e.g., Cascade Mountains).

Mid-Ocean Ridge and Geochronology

  • Geological Mapping of Oceanic Crust:

    • Age variations: Youngest crust near the mid-ocean ridges, growing older away from them.

    • Identification of Magnetic Patterns indicating sea floor spreading.

Hawaiian Hot Spot

  • Formation of Hawaiian Islands:

    • Result of hot spot activity forming volcanic islands from mantle plumes beneath oceanic lithosphere.

Tectonic Plate Motion

  • Tectonic plates are driven by convection currents in the mantle, leading to the movement of continents over geological time scales.

  • Earthquake and volcanic activity at plate boundaries highlight active geological processes in motion.