Geological Processes: Magma, Faults, and Sedimentary Rocks

Magma Classification and Characteristics

Magmas are molten rock materials found beneath the Earth's surface, characterized by their temperature, silica content, and viscosity. These properties dictate their behavior and the types of igneous rocks they form.

Basic Magmas

• Temperature: High, typically 1000-1200°C.
• Silica Content: Low (around 50%).
• Viscosity: Low, allowing them to easily reach the surface.
• Resulting Rocks: Dark-colored, with densities around 3000 kg/m³. Examples include basalt (extrusive) and gabbro (intrusive).

Acid Magmas

• Temperature: Lower, typically around 700°C.
• Silica Content: High (around 75%).
• Viscosity: Very high, often preventing them from reaching the surface easily.
• Resulting Rocks: Lighter-colored, with densities around 2600 kg/m³. Examples include rhyolite (extrusive) and granite (intrusive).

Intermediate Magmas

• Temperature: Intermediate.
• Silica Content: Medium (around 65%).
• Viscosity: Medium.
• Resulting Rocks: Intermediate color and density (around 2700 kg/m³). Examples include andesite (extrusive) and diorite (intrusive).

Emplacement of Intrusive Igneous Rocks (Plutons)

Intrusive igneous rocks form large masses that crystallize within the Earth's crust. These bodies, known as plutons, exhibit various shapes and relationships with the surrounding host rocks.

• Laccoliths: Lens-shaped intrusions that are concordant (consistent) with the host rock layers, causing overlying strata to dome upwards.
• Batholiths: Very large, irregularly shaped intrusive bodies that are discordant (cut across) the host rock layers.
• Sills: Tubular or sheet-like intrusions that are parallel to the layering of the host rocks.
• Dikes: Tubular or sheet-like intrusions that cut perpendicularly or discordantly across the layering of the host rocks.

Faults: Rupture and Displacement of Rocks

A fault is a fracture or zone of fractures between two blocks of rock, where the blocks have moved relative to each other. This displacement occurs in the direction of least resistance.

Key Fault Terminology

• Fault Plane: The planar surface along which the fault rupture and displacement occur.
• Fault Blocks (or Lips): The two blocks of rock on either side of the fault plane. These surfaces often show slickensides (mirror-like streaks) or gaps due to movement.
• Fault Slip (or Fault Jump): The distance between two points that were originally together before the fault displacement.

Types of Faults

• Normal Faults:
  • The fault plane dips toward the downthrown block (the hanging wall moves down relative to the footwall).
  • Caused by tensile stress, leading to horizontal extension and separation.
• Reverse Faults:
  • The hanging wall moves up relative to the footwall.
  • Caused by compressional stress, leading to horizontal shortening.
• Thrust Faults:
  • A specific type of reverse fault where the fault plane dips at an angle less than 45°.
• Strike-Slip (Transcurrent) Faults:
  • Relative displacement is primarily horizontal, parallel to the strike of the fault plane.
• Rotational Faults:
  • One fault block twists or rotates relative to the other.
• Associated Faults:
  • Often form complex, staircase-like patterns, leading to areas of elevation (horsts) and depression (grabens).

Diagenesis: Post-Depositional Changes in Sediments

Diagenesis encompasses all the physical, chemical, and biological changes that occur to sediments after their initial deposition and before metamorphism. It transforms loose sediments into solid sedimentary rocks.

• Compaction: As new sediments accumulate, the increasing weight of the overlying column compacts the underlying sediments. This reduces pore volume and expels water.
• Cementation: Dissolved minerals precipitate in the pore spaces between sediment grains, acting as a cement that binds the grains together.
• Pressure Solution: Under deep burial pressure, contact areas between grains dissolve. The dissolved material then precipitates in adjacent pore spaces, often leading to tighter packing.
• Mineral Replacement: One mineral is replaced by another, often influenced by the circulation of mineral-rich water.
• Authigenesis: The formation of new minerals within the sediment's empty spaces. This process typically does not significantly affect the overall rock volume but allows for the growth of new crystals, such as pyrite, within the rock.

Sedimentary Rock Types

Sedimentary rocks are formed from the accumulation or deposition of mineral or organic particles at the Earth's surface, followed by cementation.

Detrital (Clastic) Sedimentary Rocks

Formed from the accumulation and cementation of fragments of pre-existing rocks or minerals.

• Conglomerates: Formed by the cementation of large, rounded rock fragments (clasts).
• Breccias: Formed by the cementation of large, angular rock fragments (clasts).
• Sandstones: Formed by the cementation of sand-sized grains. Common types include:
  • Orthoquartzites: Primarily composed of quartz grains with siliceous cement.
  • Greywackes: Contain a mixture of quartz, feldspar, and rock fragments, often with a muddy matrix.
  • Arkoses: Rich in feldspar grains, indicating a granitic source and rapid deposition.
• Shales: Composed of fine-grained clay minerals, often poorly cemented and friable. Examples include mudstones, claystones, and marls (if calcareous).

Chemical and Biochemical Sedimentary Rocks

Formed by the precipitation of minerals from water solutions or by the accumulation of organic remains.

• Limestones: Primarily composed of calcite (CaCO₃).
  • Chemical Precipitation: Formed directly from the precipitation of CaCO₃ from aqueous solutions. Examples include oolitic limestones, pisolitic limestones, travertine (stalactites and stalagmites), and lithographic limestones.
  • Biochemical Precipitation: Formed from the accumulation of skeletal remains of marine organisms (e.g., shells, corals) or by biological processes that induce calcite precipitation.