Geology: Sediment, Sedimentary Rocks, and Weathering Processes

Introduction to Sediment and Sedimentary Rocks

• Sedimentary rocks are defined as rocks that form from the weathering and erosion of older pre-existing rocks or sediment.

Fundamentals of Weathering

• Weathering consists of the natural processes that change or alter the physical and/or chemical state of rocks located at or near the surface of the Earth.

• Weathering processes interact dynamically across major Earth systems, including the atmosphere, hydrosphere, and biosphere.

Weathering and Earth Systems

• Atmospheric Interactions:

  • Oxygen ($O_2$) and carbon dioxide ($CO_2$) are key atmospheric gases that drive chemical weathering processes on Earth.

  • Chemical weathering reactions actively remove carbon dioxide ($CO_2$) from the atmosphere.

  • The net removal of $CO_2$ from the atmosphere via chemical weathering can cause planetary cooling.

• Hydrospheric Interactions:

  • Water ($H_2O$) in the hydrosphere combines with atmospheric or ambient carbon dioxide ($CO_2$) to produce carbonic acid ($H_2CO_3$), which functions as a weak acid.

  • Carbonic acid decomposes most rock-forming minerals during chemical weathering.

• Biospheric Interactions:

  • Physical (Mechanical) Weathering: Tree roots growing into fractures and burrowing animals breaking down rock physically fracture solid rock and aid in the production of soil.

  • Chemical Weathering: Decaying organic tissue from biospheric materials decomposes into water ($H_2O$) and carbon dioxide ($CO_2$), which produce additional carbonic acid to chemically break down rock.

Modes of Physical Weathering

• Pressure Release:

  • Occurs when overlying material is removed by surface processes, releasing the deep confining pressure on underlying rocks.

  • As the confining pressure is released, the rock body expands and cracks.

• Exfoliation:

  • A specific physical weathering process triggered when pressure is released from rock masses.

  • Cracks form parallel to the exposed surface, causing layers or slabs of rock to spall off in concentric sheets.

  • Exfoliation processes characteristically produce dynamic landforms known as exfoliation domes.

• Frost Action:

  • Mechanical weathering caused directly by the physical freezing of water within rock voids and fractures.

• Frost Wedging:

  • A specific mechanism of frost action resulting from the volume expansion of water as it freezes.

  • Water infiltrates cracks in rock, freezes, expands, and pries the rock apart, eventually loosening and shattering the rock mass.