Lecture 6: Weathering and Sedimentary Rocks Flashcards
What are the 3 main types of sediment?
Clastic, Chemical, and Biochemical
What is Clastic sediment?
Consisting of solid particles (rock fragments and mineral grains) derived from pre existing rocks (e.g. sand)
What is Chemical sediment?
Mineral matter precipitated from a solution. Example: Evaporated water leaves salt at the end.
What is Biochemical sediment?
Particles made out of biological remains. Example: shells, leaves, teeth, bones.
What is weathering and what are the 2 different types?
When exposed at earth’s surface, rocks are broken down due to weathering (due to physical breakdown and dissolving). There is mechanical/physical weathering and chemical weathering.
What is chemical weathering?
This weathering follows the sand trend as Bowen’s reactions series. Example: quartz crystallizes at lower temperatures, tends to be stable. We have quartz left at the end. When olivine is exposed to low temps, not used to it and it breaks down.
Why is sand light coloured? (explain with chemical weathering)
Sand is light coloured because of weathering, which has left low temperature minerals… The clear parts in sand are quartz and the sparkly bits are muscovite.
What are the products of chemical weathering?
Produces residual minerals and free ions. Solid components are left as residue (as sediment particles) and the remaining material is dissolved in water, in the form of ions.
What is the idealized soil profile?
From top to bottom:
O-Horizon: organic matter
A-Horizon: organic matter and stable mineral grains
E-Horizon: not always present, zone of leaching (removal of soluble organic and mineral matter by water percolating above)
B-Horizon: materials such as iron and aluminium, produces oxides that become trapped… and clays
C-Horizon: partially decomposed bedrock
Bedrock
What is erosion and transport (for clastic sediment)?
Erosion is removal. Residual sedimentary particles from weathered rock and soil can be dislodged (eroded) and transported away from their source area.
What can transport weathered rock away from its source area?
Gravity (rockslides, mudflows)
Running Water (streams)
Wind (sand/dust storms)
Ice (glaciers)
What are the 4 sedimentary structures?
Ripple marks: produced by wave ripples
Mudcracks: indicating wetting and drying of mudflat
Crossbedding: produced by migrating dunes in current
Hummocky crossbedding: produced by storm waves
What is deposition?
Sedimentary particles ultimately come to rest once transporting medium can no longer carry them. Smaller/lighter particles are deposited in less-agitated conditions then larger/heavier particles when transported by wind or water.
What are depositional environments?
Characteristics of classic sedimentary rocks can provide info on where their constituent sediments were originally deposited.
How does sediment turn into sedimentary rocks?
- Lithification
- Compaction
- Cementation
What is lithification?
once buried, sediment undergoes changes that transform it into rock
What is compaction?
as sediment layers are buried to deeper and deeper levels under successive sediment layers, sedimentary particles are squeezed together… thus the material becomes more rock like
What is cementation?
sediment grains can also become ____ together by minerals that precipitate from water remaining in the pore spaces between the grains… the grains therefore effectively become glued together
What are common sediment types?
Gravel (pebbles, boulders) , sand (sand sized particles, gritty) , silt (particles barely visible under low magnification) and clay (particles too small to see under low magnification)
What are the rock type(s) of Gravel?
Conglomerate (rounded clasts) and Breccia (angular clasts)
What are the rock type(s) of Sand?
Sandstone
What are the rock type(s) of Silt?
Siltstone
What are the common rocks of Clay?
Shale (fissile; splits easily) and Mudstone (massive; structureless)
What are evaporates?
when dissolved ions become too concentrated for the water to hold so positive and negative ions join together and precipitate as minerals
Can highly salt concentrated water escape back into the ocean?
No, it becomes further concentrated to the point that “evaporate minerals” are deposited
Contains Gypsum and Halite
What are chemical limestones?
Chemical sediment– Travertine forms in hot springs
What is fossiliferous limestone?
Biochemical sediment– Composed of calcite shells (skeletons) or organisms
What is chalk?
Biochemical sediment– a variety of fossiliferous limestone, made of microscopic skeletons or marine plankton
What is reef limestone?
Biochemical sediment– • marine organisms (such as corals) build rigid frameworks of limestone as new coral skeletons overgrow older ones
What is chert?
Biochemical sediment: • a few organisms, such as some microscopic planktonic organsms (as well as some ____) have a skeleton made of silica
• silica can dissolve and form a “gel” on the seafloor
• when this gel solidifies it forms a finely crystalline rock…. Called chert
• very hard, basically made of very fine-grained quartz crystals
What is coal?
Biochemical sedimentary– largely composed of ____ matter from plants. Seems represent large accumulations of organic matter that were deposited in swamps and were subsequently buried
What is the hybrid case? (partly biochemical, partly chemical)
Dolostone
