TOPIC QUESTION Define weathering
The various mechanical and chemical processes that cause exposed rock to decompose
The physical breakdown (disintegration is mechanical) and chemical alteration (decomposition) of rick at or near the earth’s surface
TOPIC QUESTION Define and provide examples of mechanical weathering
Rocks broken down into smaller pieces by physical forces but the mineral composition remains unchanged
- e.g. frost wedging, pressure release (granite), wind sculpting (Bolivia), clam boring
- mechanical weathering adds to the effectiveness of chemical weathering because it increases the surface area of the rock exposed to chemical attack
TOPIC QUESTION Define and provide examples of chemical weathering
Alternation of internal structures by removing or adding elements which leads to solid particles and ions in solution
- weak acids like carbonic acid (H2CO3) in rain and groundwater react with minerals to destroy their crystalline structure as can oxygen (oxidation) in the atmosphere
- dissolution - rock is dissolved usually by an weak acid like carbonic acid making limestone caves, rarely rocks are dissolvable in water (halite is water soluble)
- hydrolysis - chemical reaction with water, silicates decompose primarily through hydrolysis, hydrogen atom attacks and replace K+ in Feldspar structures
- clay minerals are end products of weathering and are very stable under surface conditions
- shale the most abundant sedimentary rock is made mostly of clay
- quartz is highly resistant to chemical weathering
TOPIC QUESTION What are the three types of chemical weathering
- Hydrolysis
- Dissolution
- Oxidation
TOPIC QUESTION Describe the chemical weathering process of oxidation
Oxidation can only occur after Fe (iron) is freed from the silicate structure by hydrolysis
- rust color that appears in some rocks from removal of an electron from iron 4 Fe + 3O2-> 2Fe2O3
- presence of water speeds up the oxidation reaction
TOPIC QUESTION What factors control weathering?
- Rock type
- mineralogy -sensitivity of ions in crystal to chemical weathering
- strength - interlocking crystals (igneous, metamorphic) or cemented (sedimentary)
- structure and loading history
- thermal expansion rates - Climate
- temperature
- moisture
- availability
- vegetation
- soil/ground water chemistry - Diagenesis
- Lithification
TOPIC QUESTION What is the definition of diagenesis?
Collective term that includes all chemical, physical, and biological changes that take place after sediment is deposited and during and after lithification
What are the two main process of lithification
- Compaction - packing sediment grains through burial, reduces volume by 40% and desiccation (loss of water from pore spaces resulted by from compaction/evaporation)
- Cementation - minerals precipitate from sediment pore fluids to bind grains together most common cements are calcite (CaCO3) and quartz (SiO2)
TOPIC QUESTION What are the classes of sedimentary rocks
- Detrital - solid particles from weathered rocks - sandstone, shale, claystone
- Chemical - soluble materials (chemical weathering) are re-precipitated either in an inorganic or biochemical process (limestone)
- Organic (coal) - carbon rich remains of organisms
What are the properties of clastic/detrital sedimentary rocks?
- solid particles from weathered rocks
- most common mineral in detrital sedimentary rocks is quartz
- other minerals like Feldspar and micas may be present
- particle size and roundness is used to classify
TOPIC QUESTION What does the shape of sedimentary rocks indicate?
Degree of weathering or time involved in transportation of sediments by current of air or water
- more rounded means greater degree of abrasion and therefore transportation
Substantial weathering and long transportation lead to gradual destruction of weaker or less stable minerals.
- quartz is durable but Feldspar and ferromagnesian silicates are not
TOPIC QUESTION: What are the different by particle sizes in sedimentary rocks?
- Boulder size (>256mm)
- Cobble size (64-245mm)
- Pebble size (4-64mm)
- Granule (2-4mm)
- Sand size (0.06-2mm)
e. g. sandstone - Fine grained or slit particles (0.06- 0.004mm)
e. g. shale/siltstone - clay size particles (<0.004)
e. g. clay stone
TOPIC QUESTION explain sorting and particle shape
Sorting refers to the range of particle sizes present in the rock - can be very poorly sorted (wide range of size of particles) to very well sorted (rocks of particles are all very nearly the same size)?
Shape described in terms of angularity which is the degree to which edges and corners can be are rounded and sphericity (how close the shape is to a sphere)
- shape can be angular, subrounded or well rounded and high sphericity or low sphericity
- e.g. conglomerate has rounded particles
while breccia has angular particles
TOPIC QUESTION Describe the characteristics of chemical sedimentary rocks
- soluble materials produced by chemical weathering
- precipitation results in chemical sediments which can then undergo a physical or biochemical process to form sedimentary rocks
e. g. salt left behind as water evaporates (DeadSea) or gypsum and sylvite
TOPIC QUESTION What is the definition of erosion
Process by which soil and rock particles are worn away
TOPIC QUESTION What is transport
Process of moving rock particles by gravity or a transporting agent - wind, water or ice
What is a sedimentary environment?
Geographic setting where sediment is accumulating
TOPIC QUESTION Explain how different sedimentary rocks form
- Detrital sedimentary rocks are from weathered rocks
- Chemical sedimentary rocks are soluble materials from chemical weathering that are re-precipitated; also biochemical process such as water dwelling animals extract dissolved minerals to form shells
- organic sedimentary rocks are carbon rock remains from organisms
TOPIC QUESTION define lithification
Is the process in which unconsolidated sediments are converted to sedimentary rock
2 processes
- compacting
- Cementation
TOPIC QUESTION identify different continental, marine and transitional sedimentary environments, their depositional energies and size of sediment deposited
Continental environments 1. Alluvial fan 2. Fluvial river 3. Lacustrine (freshwater lake) 4. Deserts 5, glacial 6. Swamp (Paludal) 7. Evaporitic lake (inland sea) Transitional environments 8. Deltas 9. Beaches and barrier islands 10. Lagoons 11. Tidal flats Marine environments 12. Coral reef 13. Continental shelf 14. Continental slope and rise 15. Abyssal plain
High energy - waves and currents have larger grain size, well to poor sorted dependent on transportation Nd rounded grains
examples - fluvial (stream), alluvial fan, beach, desert, glacial deposits, delta, reef
Low energy - small grain sizes, generally well sorted, quiet waters for five clays and silts to settle
Examples - lakes, abyssal plain, lagoon, swamp
TOPIC QUESTION identify and Describe several sedimentary structures and correlate them to possible depositions environments
- Stratas and beds are the most common and characteristic feature of sedimentary rocks
- variations in texture, composition and thickness reflect the different conditions under which each layer is formed
- ripple marks (high energy, beach)
- helminthopsis
- graded bedding (delta
- mud cracks
- trace fossils (lakes)
- cross bedding (sand dunes, river deltas and steam channels)
TOPIC QUESTION Describe transport and deposition of sediments
- soluble products of chemical weathering become dissolved in groundwater and streams
- chemical and mechanical weathering decompose and disintegrate rock
- gravity moves solid particles down slope
- rivers, glaciers and wind transport sediment
- deposition of solid particles produces glacial ridges, dunes, flood plains, deltas, and ultimately reaches the ocean floor
- as sediment is buried it is compacted and cemented into solid rock
- when material dissolves in water precipitates it is the source of such features as reefs and deposits rich in shells
TOPIC QUESTION name the major sedimentary rock forms
1 detrital
- conglomerate, breccia, sandstone, siltstone, shale
2. Chemical and organic - crystalline limestone, travertine, coquina, fossikerous limestone, chalk, chert, flint, rock gypsum, rock salt, bituminous coal
