Land Relief and Geological Processes
1. Introduction
Land relief refers to the roughness or deformation of the Earth’s crust. Several factors contribute to the modeling of land relief:
1.1. Lithological Factors
The rocks that form the ground and their characteristics influence landforms and relief.
1.2. Structural Factors
The initial geological structure of a zone influences its relief.
1.3. Dynamic Factors
These factors are related to the processes involved in building landscapes.
1.4. Climatic Factors
Climatic conditions influence relief, with each climate having different agents of erosion and weathering.
1.5. Anthropic Factors
Human activity can significantly alter land relief.
2. Internal and External Processes
Internal processes are driven by energy stored within the planet, while external processes result from the unequal distribution of solar energy. Both are subject to the influence of the gravitational field.
2.1. Internal Processes
- Volcanic processes: Involve the movement of magma (molten rock) and gases.
- Seismic processes: Tensions and friction in rigid rocky areas can release energy in the form of earthquakes.
- Tectonic processes: The movement and interaction of tectonic plates shape the Earth’s crust.
Internal processes are responsible for the major architectural features of the landscape.
2.2. External Processes
External processes modify and model land relief through constant activity. They require agents and sources of energy to act upon a material body capable of producing geological changes. Water is the most active material, with solar radiation as its main source of energy. The gravitational attraction of the moon and sun also plays a role.
3. Geological Processes
Geological processes are a set of phenomena, states, and forms that result from the geological action of different agents. These agents can be grouped as follows:
- Aeolian: Air and wind.
- Fluvial: Fresh water.
- Marine: Seawater and oceans.
- Glacial: Masses of ice.
- Biotic: Living beings.
- Anthropic: Human activity.
The climate, composition, and structure of rocks are considered conditioning factors that determine the effectiveness of these agents and processes.
3.1. Weathering
Weathering occurs when external agents act by fragmenting rocks. It facilitates the action of erosion.
- Physical weathering: Rocks are fragmented by mechanical processes such as pressure, temperature changes, and freeze-thaw cycles.
- Chemical weathering: Atmospheric agents, primarily water, act on rocks and transform their minerals. Gases and organic compounds secreted by living beings can also alter rocks. Reactions of chemical weathering include hydration, dissolution, hydrolysis, carbonation, and oxidation.
3.2. Soil Formation
Soil is a natural aggregate of variable thickness that covers the Earth’s surface and supports plant and animal life. It is a loose and porous material composed of mineral particles, organic matter, water, and gases. Five factors influence soil formation:
- Parent material: The original geological material from which the soil is formed.
- Climate: Water and temperature are important climatic factors.
- Living organisms: Animals and plant roots help mix materials and aerate the soil. Humus, decomposed organic matter, enriches the soil.
- Topography: The position in the landscape influences soil evolution.
- Time: Soil formation is a gradual process that takes time.
3.3. Fluvial Processes
Wild waters flow with no fixed course, while streams flow through small channels. The erosive action of these waters affects unconsolidated soils. Erosion is a serious environmental problem, although it can produce picturesque landscapes such as badlands (abrupt areas with deep ruts) and fairy chimneys (conical shapes with a spring at the top).
Rivers are natural watercourses that flow with a stable water supply. They are the principal means of carrying surface water from the continents to the sea. Fluvial courses erode their channels if the water speed increases enough to carry particles. Different forms of erosion include gorges, ravines, waterfalls, and meanders.
Particles transported by rivers move in different ways: dissolved, floating, suspended, or rolling and dragging along the riverbed. The mode of transport depends on the size, weight, and speed of the water. Transport begins when water dissolves or picks up particles and ends when sedimentation occurs.
Sedimentation of different-sized particles occurs when the water speed decreases. The most common depositional landforms are:
- Alluvial plains: Extensive deposits of materials that fill valley bottoms.
- Fluvial terraces: Layers of river deposits situated at a higher level than the current floodplain.
- Deltas: Accumulations of sediments that appear in river estuaries when deposition occurs in calm waters.
3.4. Marine Processes
Movements of the seas and oceans depend on differences in insolation and the gravitational attraction of the sun and moon. Marine erosion, transport, and sedimentation are all driven by water movement. There are three main types of water movement:
- Waves: Movements of the surface water caused by wind.
- Tides: Regular rising and falling of water levels that occur every six hours due to the gravitational attraction of the moon. The maximum rise is called high tide, and the minimum is called low tide.
- Currents: Displacement of water masses in the sea caused by differences in temperature and salinity.
Marine erosion occurs as a result of wave action. The erosion caused by waves against the coastline is known as marine abrasion. Common forms of marine erosion include:
- Cliffs: Abrupt slopes carved out of hard rocks due to the progressive erosion of their base and the collapsing of higher zones.
- Abrasion platforms: Completely flat surfaces produced by the gradual receding of the cliff wall. When the sea level drops permanently, these platforms become coastal plains.