Geological Agents: Erosion, Transport, and Sedimentation
Geological Agents
These are fluid systems, formed by water, ice, or air that, due to a potential energy difference between the relief and sedimentary basins, perform the tasks of erosion, transport, and sedimentation.
Solar Energy
Maintains the active water cycle, atmospheric motions, waves, and ocean currents. It enables geological agents to operate.
Erosion
Produces a characteristic molding of the relief that allows us to recognize which geological agents have acted in a zone.
Transport
Geological agents modify the texture and composition of the clasts.
Sedimentation
Originates sedimentary bodies that allow us to recognize the conditions that occurred in the process and which geological agent originated it.
Human Interaction with Geological Agents
Humans exploit the resources provided by geological agents. This operation can produce environmental impacts (positive and negative) that affect the dynamics of geological agents. Human intervention in the dynamics of geological agents can influence or mitigate geohazards, or aggravate them.
Weathering
Physical Weathering
Causes the disintegration of rocks due to tensions that originate breakage into fragments:
- Gelifration: When water increases its volume when frozen into the cracks of rocks
- Thermoclatia: Occurs due to changes in temperature, causing rocks to heat and expand more on the surface than at the bottom.
- Bioturbation: Due to the mechanical action exerted by living things.
- Haloclastia: Occurs due to the pressure exerted by salt crystals growing into the cracks
- Decompression: Rocks formed at high pressure in the crust fracture and expand when they reach areas near the surface.
Chemical Weathering
Causes the disintegration of rocks due to the decomposition of the minerals that compose them:
- Carbonation: Occurs when water charged with CO2 reacts with minerals like feldspar or calcite, making them soluble or causing them to break down.
- Hydrolysis: The breaking of the crystal lattice of minerals by reacting with H+ and OH- ions of water.
- Oxidation: Occurs when minerals come into contact with air or O2 dissolved in water.
Relief
There are two types of reliefs:
- Deformations produced by volcanic activity: Linked to:
- Volcanism associated with hot spots. Originates isolated reliefs like Hawaii.
- Volcanism associated with large faults like the Canaries.
- Volcanism produced by subduction. Results in orogenic thermal arcs that can form island arcs like the Philippines or Japan.
- Reliefs produced by folding and engrosamiento of the lithosphere: Originate from the collision of continental lithosphere, such as the Pyrenees or the Alps.
The formation of relief leads to a gravitational potential difference, which is greater the higher the height of the relief. This potential difference allows ice and water erosion to perform work and also enables slope phenomena.
Over a geological timescale, the dismantling of a relief produced by erosive work is quite fast, and the end result is peneplain formation: very extensive surfaces, substantially flat, normally low altitude, and with a very gentle slope towards the sea.
Isostasy
Erosion and transport agents produce a redistribution of the mass of the continents, removing reliefs and storing them as sediments in sedimentary basins. As a result, the continental lithosphere sinks where mass accumulates and rises where erosion has the opposite effect. This explains the vertical movements of rising and sinking.
- The relief has an initial tendency to rise by reducing its mass due to the effect of erosion.
- Sedimentary basins have subsidence. When sediment begins to accumulate in them, the collapse accelerates.
Slope Phenomena
Movement down the slope of a hillside of unstable materials drawn only by their weight. Gravitational processes are not external geological agents.
- Emergences: Produced in vertical or very steep slopes. The phenomena may be falling, rolling, etc.
- Slides: The landslide deposit slips on the slope without losing contact with it, either on the concave surface or scarring.
- Raptation: In periglacial climates, the clasts are raised by small columns of frost and ice during the fall. When the ice melts, they fall.
- Solifluction: The field is moving very slowly, behaving like a viscous liquid.
Slope Phenomena Hazards
A serious risk, mainly in areas of steep reliefs and densely populated areas, where they produce damage to infrastructure and cause many casualties.
Slow phenomena such as solifluction cause slope deformations that can damage highways and rail lines, tilt or fell poles, and affect the foundation and stability of buildings, but rarely harm people.
Rapid and violent events such as falls and landslides block roads and are usually associated with events like heavy rain or earthquake energy that can cause catastrophic damage.
Slope Phenomena Forecast
Based on the detailed mapping of the territory, taking into account factors such as the susceptibility to sliding of the material, easily erodible or highly fractured strata inclined in favor of the slope, or the presence of a level-off.
Hazard maps of displacements are prepared based on the overlap of the slope and a geological map, land occupation, and the landslide susceptibility.
Slope Phenomena Prevention
Structural measures usually associated with land use to avoid occupation of risk areas. They include installing metal mesh, building walls and gabions, terracing of sloping land, concrete slope surface, and anchor installations and porous pipes for water drainage.
Prediction
Based on the observation of precursors, although one cannot know in advance the moment, it can work with preventive measures to avert catastrophe. Some of these signs are cracks on the top of the slope, steps on the ground, or leaning poles.
Wild Water
Water viewer formed by water that runs on the ground surface after heavy precipitation, before being channeled into a river system or torrent.
In areas of steep slopes, the erosive capacity is large and an erosion area widens in the upper valley.
Torrential Systems
These are not permanent water courses that are fed with water from melting groundwater outcrops or rainfall. Its large transport capacity during torrential floods successfully mobilizes large clasts. Sedimentation occurs violently, and deposits are characterized by being a mixture consisting of clasts of various sizes.
Dynamic Risk for Torrential
- Erosion: This is the start and disposal of soil materials.
- Elimination of fertile soil and causes of desertification
- Slope instability phenomena can lead to slope collapse.
- Flow or dendritic torrential flood: These occur when a severe storm suddenly increases the erosion and transport capacity of a downpour. These processes are destructive and pose a significant risk to buildings and infrastructure built on these slopes.
Torrential Hazard
Consists of hazard mapping that considers the steepness of the slope, the cohesion of materials, ground permeability, frequency and intensity of the downpours, and territorial occupation.
Prevention of Torrential Dynamics
The following measures are applied:
- Restraint of soil and increased permeability of the land in the watershed by reforesting and planting herbaceous species.
- Correction of the slopes of the reception basins by terracing.
- Works rolling by building dams on the drainage channel
- Rectify the causes in the alluvial fan with a driver that directs the flow and avoids spilling over the dimension of the fan.
- Construction of building walls to protect
Prediction
Torrential flood prediction is based on early warning systems. When storms are anticipated, the alert is activated, it alerts the population at risk, and intervention devices (fire brigade, civil protection…) are deployed. The measurement stations send data in real time, assess the situation, and determine whether one can expect the formation of a dendritic flow, in which case we proceed to the evacuation of the area.