Thermal Plumes and Rock Deformations: Hotspots and Volcanism
Thermal Plumes and Hotspots
Thermal plumes are columns of hot rock material rising from the base of the mantle towards the surface. They often create zones of intense volcanism known as hotspots.
Hotspot Relief Types
There are three main types of relief associated with hotspots:
- Ocean ridges
- Plateaus
- High continental volcanic islands
Continental Lithosphere and Thermal Plumes
The continental lithosphere is thick, rigid, and dense. When a thermal plume rises beneath it, the rocks at its base expand and become less dense, causing the continent to bulge and potentially rupture. This process can lead to the formation of high plateaus, elevated by the pressure of the thermal plume.
A rift can evolve into an ocean ridge, separating continental fragments and creating an ocean basin.
Volcanic Archipelagoes
The oceanic lithosphere is thinner than its continental counterpart. When a thermal plume is situated beneath it, the lithosphere bulges, leading to volcanism and the formation of volcanic archipelagoes.
Hotspot volcanism can produce large volumes of basalt, forming volcanoes. Because tectonic plates are in constant motion, these volcanoes become extinct as they drift away from the thermal plume.
Thermal plumes are rising columns that, upon reaching the base of the lithosphere, spread radially. As the thermal plume cools, heat is conducted to the lithosphere, triggering magmatism and volcanism. The expansion of materials causes rocks to reach areas of lower pressure and cool. Eventually, the material sinks back into the mantle as downdrafts.
Rock Deformations
There are three primary types of rock deformation:
- Elastic: Reversible strain where the rock returns to its original form. This is often caused by seismic waves.
- Plastic: Irreversible folding of rocks due to compressive stress.
- Brittle: Fracturing of rocks.
Plastic Deformation: Folds
Folds are a result of plastic deformation in rocks. Key elements used to classify folds include:
- Hinge: The area of maximum curvature in the layers.
- Flank: The sides of the fold on either side of the hinge.
- Core: The central part of the fold.
- Axial Plane: An imaginary plane that passes through the fold’s hinge.
Folds are broadly classified into two types:
- Anticlines: Layers are folded with the newest layers on the outside and the oldest in the core.
- Synclines: Older layers wrap around more modern layers, with the youngest layers in the core.
Folds can be further classified based on the position of the axial plane and flanks, including upright, overturned, and recumbent folds.
Brittle Deformation: Joints and Faults
Joints are a form of brittle deformation characterized by rock breakage without significant displacement of fragments. Causes include:
- Shrinkage cracks: Occurring in clay and mud as they dry.
- Gelifraction: The action of ice wedging.
- Columnar disjunction: Resulting from the cooling of lava.
- Lajamiento (Decompression): Rocks deep within the crust expand and fracture due to reduced pressure.
Faults are fractures in rocks where there is a displacement of blocks or lips. The fracture plane is called the fault plane. There are three main types of faults:
- Normal (Gravity) Faults: Formed by extensional stress. The hanging wall moves down relative to the footwall.
- Reverse Faults: Formed by compressional stress. The hanging wall moves up relative to the footwall.
- Strike-Slip (Tearing) Faults: Result from shear stress. There is primarily horizontal movement along the fault plane.
A mantle thrust is a reverse fault with a nearly horizontal fault plane, where the raised lip (allochthonous) is moved over long distances. These are commonly formed in collision orogens.