Earth’s Dynamic Processes: Sea Level, Fossils, and Plate Tectonics
Changes in Sea Level
- Change in volume of ocean water: The climate has changed many times throughout Earth’s history. During glaciations, ice sheets covered much larger areas than today. Much of the seawater evaporated, precipitated as snow on the continents, and the ice accumulated. This water did not return to the ocean, causing a decline in sea level.
- Variation in the shape of ocean basins: Various internal processes can raise or sink the ocean bottom, changing its shape. This variation can cause sea level changes of about 350 m.
What are Fossils?
Fossils tell us about the environment where the rock was formed, whether it was marine or continental, and the climate (tropical or temperate). This leads to two conclusions: places with rocks containing fossils were once under the sea, and mountains are not as old as Earth.
Isostatic Theory
Iso = equal, stasis = stability. The Earth’s crust behaves as if floating on a denser material.
Wegener’s Arguments
- Geographic: Wegener noted that the continents seemed to fit together like puzzle pieces. While the fit wasn’t perfect, considering changes in sea levels and coastal erosion, the fit would be more accurate.
- Geology: Wegener found that some geological formations have continuity on both sides of the Atlantic.
- Paleoclimatic: Using sedimentary rocks as climate indicators, Wegener mapped ancient climates. Their distribution was inexplicable if the continents had remained in their current positions.
- Paleontological: Analysis of fossil distribution showed that organisms of the same species are found in widely separated locations today.
Continental Drift
In the past, all landmasses were united into a large continent called Pangea.
- Pangea split, and the resulting fragments displaced, leading to the current continents.
- Wrinkles formed at the advancing front of continents, creating mountain ranges like the Cordilleras.
Surprises from the Ocean
- Mid-ocean ridge: An undersea ridge 65,000 km long, rising 2 or 3 km above the abyssal plain.
- Shortage of sediment and its odd distribution: Instead of the expected 17 to 20 km of sediment, only 1.2 km was found, with almost none on the ridge.
- Youth of the ocean floor: All rocks found were less than 185 million years old.
Details Inside the Earth
- Density: The Earth’s average density is 5.5 g/cm³.
- Heat: Temperature increases with depth, as evidenced by volcanic eruptions.
- Magnetism: The Earth has a magnetic field, indicating a core.
- Layered Structure: Seismic waves reveal the Earth’s layered structure.
Composition of the Earth
- Crust: The thin, outermost layer. Continental crust averages 35 km thick, composed of granite and gneiss with a sediment cover. Oceanic crust is 8 km thick, dominated by basalt.
- Mantle: A thick layer reaching 2900 km, separated from the crust by the Moho discontinuity. It is composed of peridotite.
- Core: The central area, separated from the mantle by the Gutenberg discontinuity, composed of iron and nickel.
Geodynamic Units
- Lithosphere: The outermost, rigid layer, including the entire crust and part of the upper mantle. Continental lithosphere is 100-200 km thick, while oceanic lithosphere is 50-100 km thick.
- Sublithospheric Mantle: A plastic layer beneath the lithosphere, bordering the core. Rocks are solid but near their melting point. Includes the asthenosphere (670 km to 2900 km).
- Mesosphere (Outer Core): Below the mantle, reaching the Lehman discontinuity at 5150 km. It is liquid.
- Inner Core: The rest of the core, in a solid state.