Lecture 6: Weathering and Sedimentary Rocks

Question 1

What are the 3 main types of sediment?

Answer 1

Clastic, Chemical, and Biochemical

Question 2

What is Clastic sediment?

Answer 2

Consisting of solid particles (rock fragments and mineral grains) derived from pre existing rocks (e.g. sand)

Question 3

What is Chemical sediment?

Answer 3

Mineral matter precipitated from a solution. Example: Evaporated water leaves salt at the end.

Question 4

What is Biochemical sediment?

Answer 4

Particles made out of biological remains. Example: shells, leaves, teeth, bones.

Question 5

What is weathering and what are the 2 different types?

Answer 5

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.

Question 6

What is chemical weathering?

Answer 6

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.

Question 7

Why is sand light coloured? (explain with chemical weathering)

Answer 7

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.

Question 8

What are the products of chemical weathering?

Answer 8

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.

Question 9

What is the idealized soil profile?

Answer 9

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

Question 10

What is erosion and transport (for clastic sediment)?

Answer 10

Erosion is removal. Residual sedimentary particles from weathered rock and soil can be dislodged (eroded) and transported away from their source area.

Question 11

What can transport weathered rock away from its source area?

Answer 11

Gravity (rockslides, mudflows)
Running Water (streams)
Wind (sand/dust storms)
Ice (glaciers)

Question 12

What are the 4 sedimentary structures?

Answer 12

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

Question 13

What is deposition?

Answer 13

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.

Question 14

What are depositional environments?

Answer 14

Characteristics of classic sedimentary rocks can provide info on where their constituent sediments were originally deposited.

Question 15

How does sediment turn into sedimentary rocks?

Answer 15

• Lithification
• Compaction
• Cementation

Question 16

What is lithification?

Answer 16

once buried, sediment undergoes changes that transform it into rock

Question 17

What is compaction?

Answer 17

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

Question 18

What is cementation?

Answer 18

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

Question 19

What are common sediment types?

Answer 19

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)

Question 20

What are the rock type(s) of Gravel?

Answer 20

Conglomerate (rounded clasts) and Breccia (angular clasts)

Question 21

What are the rock type(s) of Sand?

Answer 21

Sandstone

Question 22

What are the rock type(s) of Silt?

Answer 22

Siltstone

Question 23

What are the common rocks of Clay?

Answer 23

Shale (fissile; splits easily) and Mudstone (massive; structureless)

Question 24

What are evaporates?

Answer 24

when dissolved ions become too concentrated for the water to hold so positive and negative ions join together and precipitate as minerals

Question 25

Can highly salt concentrated water escape back into the ocean?

Answer 25

No, it becomes further concentrated to the point that “evaporate minerals” are deposited
Contains Gypsum and Halite

Question 26

What are chemical limestones?

Answer 26

Chemical sediment– Travertine forms in hot springs

Question 27

What is fossiliferous limestone?

Answer 27

Biochemical sediment– Composed of calcite shells (skeletons) or organisms

Question 28

What is chalk?

Answer 28

Biochemical sediment– a variety of fossiliferous limestone, made of microscopic skeletons or marine plankton

Question 29

What is reef limestone?

Answer 29

Biochemical sediment– • marine organisms (such as corals) build rigid frameworks of limestone as new coral skeletons overgrow older ones

Question 30

What is chert?

Answer 30

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

Question 31

What is coal?

Answer 31

Biochemical sedimentary– largely composed of ____ matter from plants. Seems represent large accumulations of organic matter that were deposited in swamps and were subsequently buried

Question 32

What is the hybrid case? (partly biochemical, partly chemical)

Answer 32

Dolostone