Marine Fossils, Continental Drift & Earth's Layers

What Are Fossils?

Places with rocks containing marine animal fossils were once under the sea. These organisms lived in the sea; upon dying, their bodies were deposited on the seabed and subsequently buried by sediments. Mountains are not as old as the Earth itself. When the organisms that became these fossils were alive, the rocks that now contain them (and form the mountains) might not have existed as mountains yet.

Explaining Marine Fossil Presence in Mountains

The presence of marine fossils in mountains can be explained by two main processes, both of which have occurred throughout Earth's history:

Changes in Sea Level

• Variation in the volume of water in the oceans: Climate has changed many times throughout Earth's history, affecting global sea levels.
• Variation in the form of ocean basins: Various internal geological processes can raise or sink the ocean floor, as well as change its shape, displacing water and altering sea levels relative to land.

Continental Uplift and Crust Thickness

Continents can rise or fall, and this is related to crustal properties:

• Areas with thick crust are typically continental.
• Areas with thin crust are typically oceanic.
• Any geological process that increases the thickness of the crust (e.g., tectonic collision) can lead to higher altitudes, potentially lifting ancient seabeds to form mountains.

Wegener's Arguments for Continental Drift

Alfred Wegener proposed the theory of continental drift based on several lines of evidence:

• Geographical Arguments: Wegener's initial observation was the apparent fit of continental coastlines, like pieces of a giant puzzle (e.g., the Atlantic coasts of South America and Africa).
• Geological Arguments: He found continuity in geological formations, such as mountain ranges and specific rock types, across continents now separated by vast oceans.
• Paleoclimatic Arguments: Wegener used certain sedimentary rocks as indicators of past climates. For example, evidence of ancient glaciers (tillites) was found in present-day tropical regions, and coal deposits (formed in warm, swampy environments) were found in cooler or polar regions. Other indicators included gypsum and halite (evaporites formed in arid climates).
• Paleontological Arguments: He analyzed the distribution of fossils and found that identical or very similar species of land-dwelling organisms and plants (e.g., Mesosaurus, Glossopteris) were found on continents now widely separated by oceans, suggesting these landmasses were once connected.

Understanding Earth's Interior

Key Characteristics of the Deep Earth

• The Earth's interior is denser: This is inferred from calculations involving the planet's total mass and volume. Density generally increases with depth towards the Earth's center.
• The Earth's interior is hot: Temperature increases significantly with depth, as observed in deep mines and boreholes. This internal heat originates from residual heat from Earth's formation and ongoing radioactive decay of elements within the Earth.
• The Earth acts like a magnet: Earth possesses a significant global magnetic field, primarily generated by convection currents in its liquid iron-nickel outer core. This field is what causes a compass needle to orient towards magnetic north.
• The Earth is structured in layers: The behavior of seismic waves generated by earthquakes (how they travel, reflect, and refract as they pass through the Earth) reveals that the Earth has a distinct layered internal structure.

Earth's Compositional Layers

Based on chemical composition, the Earth is primarily divided into three main layers:

• Crust: The outermost, relatively thin, and rocky layer. It varies in thickness and composition, distinguished into less dense continental crust and denser oceanic crust.
• Mantle: A thick layer beneath the crust, extending to a depth of approximately 2900 km. It is composed primarily of silicate rocks richer in magnesium and iron than the crust.
• Core: The central, innermost zone, composed mainly of an iron-nickel alloy. It is further divided into a liquid outer core and a solid inner core.

Earth's Geodynamic Units (Mechanical Layers)

Based on physical properties (especially rigidity, viscosity, and state of matter), the Earth is divided into these geodynamic units:

• Lithosphere: The rigid outermost layer, comprising the crust and the uppermost part of the mantle. It is broken into tectonic plates that move over the asthenosphere.
• Asthenosphere (Sublithospheric Mantle): A highly viscous, mechanically weak, and ductile (plastic-like) region of the upper mantle, located just below the lithosphere. It is capable of flowing slowly, which allows for the movement of tectonic plates.
• Mesosphere (Lower Mantle): Below the asthenosphere, the mantle becomes more rigid again due to increasing pressure, though it still convects very slowly over geological timescales. It extends down to the core-mantle boundary.
• Outer Core: A liquid layer below the mantle, extending from about 2900 km to 5150 km depth (the latter being the Lehmann discontinuity). Its fluid motion is responsible for generating Earth's magnetic field.
• Inner Core: The centermost part of the Earth, extending from about 5150 km depth to the Earth's center (around 6371 km). Despite its extremely high temperature, it is solid due to immense pressure.