Introduction to Earth Sciences

Posted on Oct 10, 2024 in Geology

What is a Fossil?

A fossil is any evidence of past life that has been preserved. Fossilization usually happens like this:

  1. An organism dies and is covered by sediments.
  2. The soft parts rot, but hard parts like bones or shells may remain and leave an imprint in the sediment.
  3. Sometimes these hard parts are mineralized and preserved.
  4. Hard parts may dissolve, and the hole is filled with material from groundwater.

Fossils and Earth’s History

Marine Fossils in Mountains

The existence of marine fossils in mountains tells us two things:

  • Areas with marine fossils were once underwater.
  • Mountains are not as old as the Earth itself.

How Did Marine Areas Become Continental?

A marine area can become continental due to:

  1. Changes in sea level.
  2. Uplift or sinking of the continent.
  3. A combination of both processes.

Changes in Sea Level

Eustatic changes are broad, long-lasting changes in sea level that affect the entire planet. Two processes cause eustatic changes:

  1. Variation in the volume of ocean water: During ice ages, sea level drops. When ice melts, sea level rises.
  2. Variation in the shape of ocean basins: Internal processes can raise or lower the ocean floor.

Isostatic Changes

Isostatic changes are local changes in sea level. For example, when glaciers melt, the land may rise, causing the sea to retreat. This creates raised beaches or coasts of emersion. Conversely, when the land sinks, the sea may advance, creating submerged valleys or coasts of submergence.

The theory of isostasy explains these changes. According to this theory, the Earth’s crust is in gravitational equilibrium with the denser mantle. When the crust is unloaded (e.g., by melting glaciers), it rises. When it is overloaded (e.g., by sediment deposition), it sinks.

Earth’s Relief

A Relief with Two Large Steps

The hypsometric curve shows the percentage of Earth’s surface at each altitude. It reveals two main steps in Earth’s relief, corresponding to the continental and oceanic crust.

Continental and Oceanic Crust

The theory of isostasy helps us understand these features:

  1. Areas with thick crust are continental.
  2. Areas with thin crust are oceanic.
  3. Any process that increases crustal thickness will lead to higher altitudes.

Continental Drift

Alfred Wegener and the Theory of Continental Drift

Alfred Wegener proposed the theory of continental drift, arguing that continents were once joined together in a supercontinent called Pangea and have since drifted apart.

Background of Mobility Ideas

  • Alexander von Humboldt noted similarities between the Atlantic coastlines.
  • Frank Taylor was a precursor to Wegener, suggesting the possibility of continental movement.

Wegener’s Arguments

  1. Geographic: Continents fit together like puzzle pieces.
  2. Geological: Similar geological formations are found on opposite sides of the Atlantic.
  3. Paleoclimatic: Sedimentary rocks indicate past climates, and maps of these climates suggest continental movement.
  4. Paleontological: Fossils of the same species are found in widely separated locations.

Theory of Continental Drift

Wegener’s theory proposed:

  1. Pangea existed in the past.
  2. Pangea broke apart, and the fragments drifted to their current positions.
  3. Mountain ranges formed at the leading edges of the continents.

Wegener could not explain the cause of continental drift, but he suggested two possibilities: centrifugal force and tidal forces. Neither was adequate.

Exploring the Deep Ocean

Why Study the Deep Ocean?

  1. Oceans cover 71% of Earth’s surface.
  2. We expect to find layers of sediment dating back billions of years.

The Ocean Reveals its Secrets

  1. The existence of the oceanic ridge system.
  2. The lack of sediment and its unusual distribution.
  3. The young age of the seafloor.

Facts About the Earth’s Interior

  1. The Earth’s interior is denser than the surface.
  2. The Earth’s interior is hot; temperature increases with depth.
  3. The Earth has a magnetic field, suggesting a metallic core.
  4. The Earth is layered, as revealed by seismic waves.

Composition of the Earth

  • Crust: Thin outer layer, composed of granite and gneiss on continents and basalt in oceans.
  • Mantle: Thick layer beneath the crust, composed of peridotite.
  • Core: Central area, composed of iron and nickel.