Lecture 7 Flashcards
Where do sedimentary rocks fit in the broader rock cycle?
The rock cycle illustrates how igneous, sedimentary, and metamorphic rocks can transform into one another over time. Sedimentary rocks form when pre-existing rocks undergo weathering, become sediment, and then are transported, deposited, and lithified.
What is weathering, and how does it produce sediment?
Weathering is the process that breaks intact rock into loose debris (sediment). It can be physical (mechanical) or chemical. Sediment forms from the clasts or dissolved ions released during weathering.
How does physical (mechanical) weathering break rock apart?
Common mechanisms include: 1) Jointing: Cracks form due to changes in temperature, pressure, or tectonic forces, splitting rock into pieces. 2) Wedging: Water (frost), salt, or roots expand in cracks. 3) Thermal expansion: Repeated heating and cooling can crack a rock’s outer layers. 4) Animal/human activity: Burrowing or excavation can fragment rocks.
What are key chemical weathering processes, and which minerals are most affected?
Dissolution (water dissolves salts, carbonates, etc.), Hydrolysis (water reacts with silicate minerals to form clays), Oxidation (iron-bearing minerals rust in the presence of oxygen), and Hydration (minerals absorb water and expand). Feldspars, carbonates, and iron-rich minerals are particularly affected.
How do physical and chemical weathering processes reinforce each other?
Physical weathering increases rock surface area, accelerating chemical reactions; chemical weathering weakens rocks, making them easier to break apart physically.
What is differential weathering, and why does it occur?
Differential weathering means different rock types or minerals weather at different rates. Minerals like quartz are more resistant to chemical weathering, while others like calcite dissolve quickly in acidic solutions, leading to uneven surfaces or varied landscapes.
What are the major classes of sedimentary rocks?
1) Clastic (detrital) – cemented clasts of pre-existing rocks/minerals. 2) Biochemical – formed from organisms’ shells or skeletal debris (e.g., limestone). 3) Organic – carbon-rich remains of organisms (e.g., coal, oil shale). 4) Chemical – precipitated from mineral-rich water (e.g., rock salt, travertine).
How do clastic sedimentary rocks form?
1) Weathering creates detritus. 2) Erosion removes clasts from parent rock. 3) Transportation moves clasts via wind, water, ice, or gravity. 4) Deposition occurs when clasts settle out. 5) Lithification (compaction and cementation) transforms sediment into rock.
What factors help classify clastic sedimentary rocks (e.g., sandstone vs. shale)?
Grain size (clay, silt, sand, gravel), grain shape (angular to rounded), sorting (poorly to well-sorted), composition (quartz, feldspar, lithic fragments), and type of cement (calcite or quartz).
What is the significance of grain size, shape, and sorting in clastic rocks?
They indicate transport distance and energy. Well-rounded, well-sorted grains suggest long transport or extensive reworking, while angular, poorly sorted clasts indicate shorter transport or high-energy environments.
How do biochemical sedimentary rocks form, and what are some examples?
They form when organisms produce shells or skeletons that accumulate after death. Examples include limestone (calcite shells, such as fossiliferous limestone or chalk) and chert (formed from silica-secreting plankton shells).
What defines organic sedimentary rocks, and what are two main examples?
They consist of carbon-rich remains of once-living organisms. Coal (derived from plant remains in swamps) and oil shale (which contains kerogen mixed with fine-grained sediment) are key examples.
What are chemical sedimentary rocks, and how do they precipitate?
They form when minerals precipitate from water solutions due to evaporation or chemical changes. Evaporites (like rock salt), travertine (chemical limestone in caves or hot springs), and some types of chert (silica replacing calcite) are common examples.
