Understanding Earth’s Surface: From Erosion to Plate Tectonics
Modeling Water’s Superficial Effects
Erosion
Water, especially in the form of streams and rivers originating from torrents, causes significant erosion. Different forms of erosion create inequalities in the ground, resulting in various landforms:
- Scarps, Waterfalls, and Chimneys: Differential erosion creates these features.
- Grooves and Gullies: Streams widen existing rows and form a network of gullies, eventually leading to valleys.
- V-Shaped Valleys: Rivers deepen their channels, carving out valleys with a V-shaped profile.
- Valleys with a Flat Trough: Over time, rivers tend to occupy a smaller portion of the valley, except during flood periods when they cover a wider area.
Deposition
Water also plays a crucial role in depositing materials:
- Alluvial Fans: Accumulated eroded materials transported by torrents are deposited in flatter areas, forming fan-shaped deposits.
- Floodplains: Rivers deposit sediments during flood periods, creating flat areas adjacent to the river channel.
Combined Forms
- Meanders: Curves that rivers describe. The outer edge of the curve experiences higher speed and greater erosion.
- Fluvial Terraces: Old floodplains that have been eroded by the river, leaving behind elevated flat areas.
Influence of Structure on Relief
Disposition of Stratification
- Structural Plains: Found in areas where the strata are horizontally arranged. Hard materials resist erosion, while softer rocks erode more rapidly, leading to the formation of plateaus.
- Sloping Relief: In areas with inclined stratification, asymmetric relief with gentle slopes and abrupt cuts develops.
- Ridges or Strings: Vertical stratification with hard layers can result in the formation of ridges due to differential erosion.
Folds and Fractures
Folds in rock layers can form anticlines (A-shaped) and synclines (U-shaped). Rocks often have fractures or joints, which are planes of weakness that encourage weathering and erosion by water.
Isostatic Changes
Coastal Changes
- Submergent Coasts: Rising sea levels can penetrate river valleys, forming rias or fjords in glacially carved valleys.
- Emergent Coasts: Land uplift leads to the emergence of coastal areas. Beaches become exposed, and according to the theory of isostasy, the Earth’s crust behaves as if floating on a denser material. Subsidence occurs when materials are deposited in basins, and these landmasses move until they reach isostatic equilibrium.
Continental and Oceanic Crust
The continental crust is formed by thinner, denser materials, while the oceanic crust is composed of thicker, less dense materials.
Continental Mobility
Early Theories
Early theories suggested that continents were fixed in place (fixism). However, Alfred Wegener proposed the theory of continental drift, which challenged this idea.
Wegener’s Arguments for Continental Drift
- Geographic: Continents seemed to fit together like pieces of a puzzle.
- Geological: Some geological formations showed continuity across the Atlantic Ocean.
- Paleoclimatic: Wegener used rocks as indicators of past climates and drew maps of these ancient climates.
- Paleontological: Studies of fossils revealed the existence of the same species in distant locations.
Theory of Continental Drift
Wegener proposed that in the past, all landmasses were joined together in a supercontinent called Pangaea. Pangaea then split apart, giving rise to the present-day continents. This process also formed wrinkles in the Earth’s surface, leading to the formation of mountains.
Challenges to Wegener’s Theory
Wegener’s theory faced criticism due to his inability to explain the mechanism behind continental drift. He incorrectly suggested centrifugal force and the gravitational pull of the sun and moon as driving forces.
Ocean Floor
Oceanic Ridges
Oceanic ridges are underwater mountain ranges that rise 2-3 km above the abyssal plains. They are characterized by a lack of sediments and a surprisingly young age.
Age of the Seabed
Rocks on the seabed are all less than 185 million years old, which is relatively young compared to the age of the Earth.
The Deep Earth
Data about Earth’s Interior
- The Earth’s interior is denser than the surface.
- The Earth’s interior is hotter than the surface.
- The Earth behaves like a magnet, suggesting the presence of a metallic core.
- The Earth is structured in layers, as revealed by the behavior of seismic waves during earthquakes.
Composition of the Earth
- Crust: A thin surface layer dominated by granite and gneiss.
- Mantle: A thick layer composed of peridotite.
- Core: The central sphere, composed mainly of iron and nickel. It is divided into the outer core (liquid) and the inner core (solid).
Geodynamic Units
- Lithosphere: The rigid outer layer, including the crust and the uppermost part of the mantle.
- Asthenosphere: The plastic layer beneath the lithosphere, where rocks are in a semi-molten state.
- Outer Core: Situated below the mantle and is in a liquid state.
- Inner Core: The innermost part of the core, which is in a solid state.
Fossils
Definition and Formation
A fossil is any preserved remains or traces of past life. The process of fossil formation is called fossilization.
Process of Fossilization
When an organism dies and is buried, its soft parts decompose, while hard parts may be preserved. Over time, these hard parts can be mineralized, replacing the original material with minerals. Sometimes, the hard parts dissolve, leaving a cavity that can be filled with other materials.
Evidence from Fossils
- The presence of marine fossils in rocks now found on land indicates that these areas were once underwater.
- Fossils demonstrate that mountains are not as old as the Earth itself.
Processes Causing Changes
- Eustasy: Variations in the volume of water in the oceans.
- Changes in the shape of ocean basins.