Geology: Earth's Origin and Structure

Geology

• Study of Earth's structure and composition

Origin of the Solar System

• The Nebular Hypothesis suggests that the sun formed from a swirling cloud of dust and gas around 4.6 billion years ago
• As the cloud shrank, rotation increased due to gravity, leading to hydrogen fusion and heating

Earth's Size and Shape

• The spinning motion of Earth formed a bulge, making it an oblate spheroid, not a perfect sphere
• Measuring the weight of objects in different places shows that Earth is not spherical
• Weight of objects tends to be heavier near the Earth's poles

Earth's Interior

• Collision of material left over from the solar system's formation led to the formation of Earth's cores and layers
• Inner core: solid iron and nickel
• Outer core: liquid iron and nickel
• Mantle: iron, silicon, and magnesium; Earth's thickest layer
• Crust: outermost layer, where life has been found
• Lithosphere: upper mantle and lower crust, floating above the asthenosphere

Earth's Heat

• Factors contributing to Earth's heat: decay of radioactive isotopes, weight of materials, and meteorite impacts
• Earth's heat has been decreasing since its formation due to rock thickness, heat loss rate, and percentage of radioactive materials

Earth's Magnetic Field

• Hypothesis: liquid iron in Earth produces an electric current, leading to the formation of the planet's magnetic field

Early Plate Tectonics

• Theory: the formation and movement of the Earth's crust
• Early explorers noted the movement of continents by comparing Africa's coast with east South America
• Alfred Wegener proposed that continents have been drifting apart, using the Mesosaurus as evidence

Modern Plate Tectonics

• Discoveries of magnetism and sea floor age provided evidence for plate movement
• Scientists revised Wegener's theory, adding that plates are part of a lithospheric plate covering the Earth's surface
• Volcanoes and earthquakes occur in concentrated belts at theoretical plate boundaries

The Ocean Floor

• Igneous rocks of the ocean floor are magnetic, providing information about plate movement
• Mid-ocean ridges show rocks changing polarity with each magnetic field reversal, indicating plate movement
• Rocks near the ridge are hotter than those further away

Divergent Boundaries

• Two lithospheric plates are pulling apart, forming rift valleys
• Fracture zones line the boundary where a fault lies perpendicular to the divergent boundary
• The crust is pulled apart, forming a new ocean floor
• Example: Mid-Atlantic Ridge

Convergent Boundaries

• A place where two plates are moving towards each other