Earth’s Surface: Landforms, Fossils, and Tectonic Shifts
**Modeling Farm**
For wind activity to act effectively, there must be plenty of loose materials, little vegetation, and low humidity.
**Forms of Erosion**
- Wind picks up loose material and moves it (deflation), polishing the rock surface (abrasion).
- Alveoli: Small cavities produced in rocks.
- Mushroom Rocks: Mushroom shapes are provided to isolated rock masses.
- Desert Pavement: An area of desert covered by a thick carpet of rocks.
**Forms of Sedimentation**
- Dunes: Deposits of sand transported by the wind.
- Loess: Deposits of very fine materials conveyed by wind over long distances.
**Coastal Modeling**
Marine waters are subjected to three types of movements: waves, currents, and tides.
**Coastal Features Originated by Erosion**
- Cliff: Characterized by a leakage that separates the sea from the land surface. Erosion is intense. Fragments of rock are removed, creating undercuts. The rocks located on the undercuts form overhangs.
- Abrasion Platforms: Horizontal surfaces at the foot of the cliff, formed by the retreat of the cliff.
- Promontories and Inlets: Coastlines with alternating promontories and coves or bays.
- Islets: Remains of ancient coastal promontories that have lost their connection to the continent.
- Natural Arches: Cavities crossing promontories or located at the front of a cliff, resulting from differential erosion.
- Rasas: Terraces situated on the cliffs; they are old abrasion platforms.
**Coastal Features Originated by Sedimentation**
Ocean currents are responsible for transportation.
- Beaches: Coastal strips of sand or gravel deposits.
- Deltas: Accumulated deposits at the mouth of a river, similar to alluvial fans of torrents.
- Bars: Sand deposits that originate as elongated islands parallel to the coast, formed by seabed currents and littoral drift currents.
- Spits: Bars with one end connected to the coast.
- Tombolos: Deposits that connect an island with a continent.
- Lagoons: Coastal lagoons partially or totally separated from the sea by a bar.
**Influence of Structural Relief**
The arrangement of stratification, alternating hard and soft layers, favors the formation of various reliefs:
- Structural Plains: In places where the layers retain their original horizontal position. Hard materials protect the softer ones beneath them, forming plains, high plateaus, tables, and isolated mountain elevations.
- Delivery in Costs: In places with gently sloping stratification.
- Crest or Rope: In vertical stratification, projections are originated by hard coatings produced by differential erosion.
Folds and fractures provide an original relief that conditions the modeling in the form of antiforms and synforms.
**The Evolution of the Relief**
William Davis proposed a general model:
- Youth: Topographic relief; erosion is very intense, and rivers form deep valleys.
- Maturity: Relief is softened, erosion is less intense, mountains are rounded, and valleys are more open.
- Senility or Old Age: Almost flat relief (peneplain), erosion is almost nonexistent. Rejuvenation: phases of maturity and old age may occur.
**Fossils**
A fossil is any remains of an organism from the past or evidence of its activity. The process by which a fossil originates is called fossilization.
- An organism dies and is buried by sediment.
- Soft parts decompose; hard parts remain long enough for their outer surface to be printed in the sediment.
- This item may mineralize and become hard if preserved.
- The hard parts are dissolved, and the gap is filled by water supplied by subterranean material, forming an external mold of the body.
**What Fossils Reveal**
A fossil indicates the environment in which the rock was formed, whether marine or continental, warm or temperate climate. The existence of fossils in the mountains allows us to reach these conclusions:
- Places where there are rocks with fossils of marine animals were once under the sea.
- Mountains are not as old as the Earth.
**How to Explain the Presence of Fossils**
Either the sea level has changed, or the continent has risen or fallen; both processes have occurred.
**Sea Level Changes**
Tides cause the sea level to rise and fall every day. The changes in sea level discussed here are eustatic, wider, longer, and affecting the entire planet. These changes are caused by:
- Changes in the volume of water in the oceans due to climate change.
- Variation in the shape of ocean basins; various internal processes can raise or sink the ocean floor.
**Isostatic Changes**
In some coastal areas, the sea has penetrated river valleys, forming rias, or glacial valleys. While some coasts are sinking, others have raised beaches (coasts of emersion). This is explained by the theory of isostasy.
According to this theory, the crust behaves as if it were floating in a denser material. Subsidence occurs in a basin where materials are deposited. Land masses move slowly to reach a steady state, known as isostatic equilibrium. The Earth’s crust is in gravitational equilibrium with denser interior materials, rising when unloaded and sinking when overloaded.
**Relief in Two Major Steps**
The hypsometric curve shows the percentage of the Earth’s surface area at each elevation.
**Continental and Oceanic Crust**
There are two major steps: thick and thin oceanic crust, and thick and sparse continental crust. Major relief features:
- Areas with thick crust and low density are continental.
- Areas with dense, thin crust are oceanic.
- Any process that increases crustal thickness will cause it to reach higher altitudes.
**Mobility**
In 1915, Alfred Wegener published *The Origin of Continents and Oceans*, maintaining that the continents moved (mobility theory). Theories according to which continents have shifted over the history of the Earth are called mobilist theories. Those that deny the possibility of horizontal movement of continents are called fixist theories. Wegener’s ideas were not entirely new. He presented the following arguments:
- Geographic arguments
- Paleoclimatic arguments
- Geological arguments
- Paleontological arguments