Sedimentary Rocks and External Geodynamics Hazards
Sedimentary Rock Characteristics
- Stratification is characteristic of sedimentary rocks, which are often found in overlapping horizontal layers called strata, limited by a roof and wall. The distance between the ceiling and the wall is the power.
- The structure is the geometric arrangement of the components of a sediment or sedimentary rock.
- Texture is the internal order of the rocks as to disposition, shape, and relative size of its components. It may be clastic, consisting of bits of minerals or rocks, and non-clastic, formed by precipitation of salts.
- The composition is related to the conditions under which it is formed.
Sedimentary Rock Classification
- Detrital sedimentary rock: an accumulation of rock fragments.
- Non-detrital organogenic sedimentary rock: accumulation of organic remains.
- Biochemical and Chemical Sedimentary Rocks (Non-detrital): Formed by chemical precipitation reactions or precipitation of substances favored by the activity of living organisms.
Uses of Sedimentary Rocks
- Industrial use: quartz sandstones (cuarzoarenitas) are used to make glass.
- Construction rock: gravel, along with cement and steel, is used to make reinforced concrete.
- Energy production: coal and oil.
Geohazards Related to External Geodynamics
These are events that can cause damage to persons, property, or the environment. Their origins may be natural, but they can also be caused or exacerbated by human action; named in this case, induced risks. The main risks associated with external geological geodynamics are caused by floods and gravitational movements.
Risks of Flooding
Flood risks occur when temporary flooding affects usually dry land due to heavy rainfall, river flooding, obstruction of watercourses, or tidal waves. The causes are often favored by natural processes or human action through the construction of dams, the modification of natural drainage of rivers, the modification of their banks, the elimination of existing soils in their catchments, and so on.
Actions to prevent damage from floods include:
- Structural: through the construction of containment dikes or dams, soil conservation of watersheds, reforestation, or cleaning the channels.
- Non-structural: involving risk mapping, management of land use, or flood insurance contracts.
- Administrative: such as civil protection plans or the establishment of warning systems and hydrologic data, which can predict the behavior of streams.
Gravitational Movement Risks
Natural causes can lead to the fall of blocks, landslides, or slippage of the surface layer on a detachment surface. Human intervention often influences these movements through the settlement of villages on unstable slopes, deforestation, steep slopes left by public works, and the accumulation of spills and debris on cliffs and slopes.
The prediction of these movements in restricted areas can be performed with a high degree of reliability, considering the following factors:
- Non-uniform gradients and lobes with concave areas.
- Determination of movements in former slopes.
- Scaling the volume of rock displaced and determining how far it could travel horizontally.
- Existence of subsurface lithology with low cohesion, slip planes, and alternating layers with different permeabilities.
Actions to minimize damage caused by these phenomena are grouped into 3 categories:
- Structural: standardizing hillside slopes, removing material from the head of the slopes and stabilizing their bases, preventing subsoil saturation by drainage to evacuate water, building walls, buttresses, and cliffs, using bolts to hold blocks and rock masses that could come loose, and so on.
- Non-structural: risk mapping, preventing the settlement of populations on unstable slopes.
- Administrative: advance planning through civil protection plans, landslide reports, and so on.
Other Minor Risks
- Expansive clays: when saturated with water, they increase in volume and can damage buildings and civil works.
- Subsidence: by dissolution of the subsoil.
- Advancing dunes: which can affect buildings, crops, and roads.