Coastal Plains, Horizontal Layers, Domes, and Volcanic Forms: A Comprehensive Guide
Coastal Plains
Coastal plains are recently emerged coastal strips formed by layers of sedimentary rocks that rest on older continental rocks. These sedimentary rocks dip towards sea level. For a plain to originate, the coast must be lifted by tectonic activity. As sea level rises, the plain is revealed. It presents a flat surface extending towards the sea.
Dendritic Drainage Pattern
This is the most common drainage pattern in nature, resembling the branching of a tree. It develops on rocks of equal strength.
Types of Rivers in Coastal Plains
- Subsequent Rivers: Develop parallel to the coastline, following the lowlands.
- Consequent Rivers: Flow perpendicular to the coastline.
- Resequent Rivers: Flow in the same direction as consequent rivers but only reach the back end of the slope.
- Obsequent Rivers: Flow in the opposite direction to the general dip of the zone and the direction of consequent rivers.
Horizontal Layers
Horizontal layers are sedimentary strata, primarily horizontal in elevation, that have emerged over a large area but have been dislocated.
Origin of Horizontal Strata
Initial Stage
The terrain is relatively flat, and consequent rivers follow the gentle slope of the surface. If the initial plateau is very high, rivers quickly carve canyons. The proximity of the base level prevents the water from deepening its channel, resulting in a less pronounced relief.
Youth Stage
The region, with high or low relief, develops a network of river valleys at the expense of the initial surface.
Maturity Stage
The original plain has been almost or completely consumed, and the region has reached its maximum roughness. As erosion continues, the relief softens, and the slopes become gentler.
Aging Stage
The region has been reduced to an undulating plain where major rivers have built large floodplains.
The difference between a plateau and a mesa is that a mesa is wider, while a table is surrounded by cliffs.
Importance to Man
Due to the low relief, population density is low, concentrated in the valleys. These areas are attractive for tourism, extreme sports, and climbing.
Limestone Caves
Limestone, composed mainly of calcium carbonate, is easily dissolved by weak solutions of carbonic acid in infiltrating or saturating water. Groundwater transports the dissolved carbonates, leading to the formation of underground cave systems.
Origin of Caves
Initial Phase
The limestone is strong and covered with a layer of sand, with water flowing on the surface.
- As the river deepens, water penetrates the joints of the limestone, reaching the same level and emerging on the riverbanks. The holes in the limestone enlarge and deepen over time.
Cave deposits, often called speleothems, are formations of calcium carbonate or gypsum that form after the cave has reached its peak and rapid solution is no longer occurring.
Slow dripping from the ceiling undergoes evaporation, leaving behind small amounts of calcium carbonate. The accumulation of these deposits results in beautiful limestone formations.
Formations hanging from the ceiling are called stalactites. Formations rising from the floor are called stalagmites. When stalactites and stalagmites join, they form columns.
Economic Importance
Deposits of bird or bat guano, rich in nitrates, are used in the manufacture of fertilizers and explosives. Caves also hold significant tourism potential.
Laccolith Domes
Initial Stage
A river system follows a consistent radial pattern, eroding the flanks of the dome and exposing the lower strata.
Maturity Stage
Characterized by a concentric arrangement of alternating hogbacks (ridges) and valleys. Tributary rivers flowing between hogbacks are resequent and obsequent.
Dendritic Drainage Patterns + Rectangular
Areas with Complex Structures
Some parts of the Earth’s crust, particularly continental shields, have been affected by multiple periods of folding, faulting, and volcanic activity.
Initial Phase
Mountains are formed by faulting, folding, igneous intrusion, and volcanic activity.
Middle Age
A completely irregular drainage system develops. The topography is steep, with faults, fault-line scarps, hogbacks, old volcanoes, and other structural forms.
Aging Stage
The region is reduced to a peneplain, with only the hardest rocks remaining as monadnocks (isolated hills).
Geo-economic Importance of Crystalline Masses
These formations are useful for extracting building materials such as marble and gneiss for slabs and countertops. They may also contain iron ore for exploitation.
Volcanic Forms
Volcanoes are formed by the emission of hot gases and molten rock under great pressure from a relatively small opening or crater that connects to a magma reservoir at depth. Eruptions can be explosive or quiet, influencing the morphology of the volcanic edifice.
Volcanoes Classified by Eruptive Type
Explosive Eruption Volcanoes
Cinder Cones
These are the smallest volcanoes, formed when the gas content of the lava is very high. The pressure of the lava propels material into the air.
- Larger pieces, weighing several tons, are called volcanic bombs. They solidify in the air, taking on a fusiform shape.
- Smaller fragments, a few centimeters in size, are called lapilli and are major constituents of the volcanic cone.
- The finest particles are volcanic ash and dust. Ash falls like snow over an area of several kilometers around the crater.
- The finest dust is transported by wind to regions far from the eruption zone and can remain suspended in the atmosphere for years.
- Cinder cones grow rapidly, with a slope angle between 26° and 30°. Erosion has little effect until weathering processes create soil that fills the spaces between volcanic material.
Composite Volcanoes (Stratovolcanoes)
These volcanoes consist of layers of pumice and ash alternating with lava flows. The slope of the volcano depends on the angle of repose of the lapilli and the proportion of lava layers.
Eruptions of large composite volcanoes are often accompanied by explosive emissions of gases, lapilli, bombs, ash, and lava flows. The crater can change shape rapidly due to both demolition and new accumulations of materials.
Water vapor (H2O) is the predominant gas in pyroclastic flows, which can remove a significant portion of the volcano, leaving a cylindrical depression called a caldera.
Volcanic basement blocks can collapse due to intense vibrations, ejecting tens of cubic kilometers of rock. A lake may form in the caldera, and if volcanic activity continues, a small cone may develop, forming a volcanic island.
Erosive Cycle of Volcanoes
Initial Phase
Volcanoes are in the process of growth. Lava flows spread around the river valleys, following the general slope.
Evolution
Youth Stage
The fault block is asymmetric and typically has uniform summits despite the presence of numerous small river faults that developed during uplift.
Maturity Stage
There are numerous major watersheds, spurs, and peaks separated by deep canyons. The main peaks of the massif retreat to a position near the center of the block, tilting away from the original appearance of the mountain block.
Aging Stage
The block has become a peneplain. The fault line is completely covered by alluvial deposits, and the surface no longer exhibits triangular facets or other physiographic features that would identify it. It becomes more difficult to locate the faults.
Geographic Importance – Geo-economic Importance of Faults
Advantages
- Faults can be used to locate metallic minerals such as gold.
- They are important for hydrocarbon exploration.
- They can be associated with the location of drinking water (oases).
- Hot springs associated with faults can be used for spas and tourist attractions.
Disadvantages
- Faults are associated with fractured rocks and landslides, which can cause earthquakes and other hazards.
- Active faults can generate earthquakes.