Download Notes: Sediments and Sedimentary Rocks

Notes: Sediments and Sedimentary Rocks
1. Weathering: the processes that break up and corrode solid rock transforming it into
sediment
a. Physical weathering: breaks rocks into intact smaller grains called detritus
1. boulders more than 256 mm
2. cobbles between 64mm and 256 mm
3. pebbles: between 2mm and 64 mm
4. sand: between 1/16mm and 2mm
5. silt: between 1/256mm and 1/16mm
6. mud: less than 1/256mm
b. Jointing: Happens when rocks that are buried deep in the earth under pressure
and heat are suddenly thrust to the surface and cool and expand at once and crack
because of this change
1. talus: a slope on a hill that is covered in loose broken rock
2. Frost Wedging: Happens when water gets into the joints in rocks and
freezes. When water freezes it expands and forces the joint to crack open
farther.
3. Salt Wedging: As water fills joints and evaporates away it leaves behind
salt grains. Over enough time the salt can build up to the point where it
fills the joint and stars to force it to open more.
4. Thermal Expansion: When the heat of an intense forest fire bakes a
rock, the outer layer of the rock expands. On cooling , the layer contracts.
This change causes this layer of rock to break into sheets.
5. Burrowing: Animals that burrow underground can dig through rock.
c. Chemical Weathering: Refers to the chemical reaction that alter or destroy
minerals when rock comes in contact with water solution or air.
1. Happens faster in the tropics than dry climates
2. Dissolution: when minerals dissolve into solution
a. this is how the ocean became salty
3. Hydrolysis: water has a chemical reaction with the minerals in a rock
and breaks the rock down.
a. this makes clays
4. Oxidation: a chemical reaction during which an element looses
electrons. Oxidation reactions in rocks transform iron bearing ores into
rusty ion-oxide rocks.
5. Hydration: the absorption of water into the crystal structure of minerals
causing some minerals to expand.
6. Roots of plants, fungi, and lichens: secret organic acids that help
dissolve minerals in rocks. These organisms extract nutrients from the
minerals and thus undermine the crystal structure of the rock.
d. Physical and Chemical Weathering often work together
e. Differential weathering: when different rocks in an outcrop undergo weathering
at different rates.
1. This is how you get the waterfalls that you can walk behind.
2. Clastic Sedimentary Rocks: consist of cemented together detritus (sediments of all
sizes)
a. Weathering: creates the sediment
b. Transportation: wind, water or ice transports the sediment some distance.
1. Sometimes sorting the sediment by its size
c. Deposition: how the sediment settles out when it reaches the end of its journey
1. Sorted by size (homogenous)
2. Heterogenous (all sorts of random sized sediments in one place)
d. Lithification (compaction and cementation): Geologists refer to the
transformation of loose sediment into solid rock.
1. Sediment must be buried under layers of other sediment
2. Pressure squeezes out water and air that was trapped between clasts and
the clasts compact together
3. Cementing: consisting of minerals (commonly quartz or calcite) that
precipitate from groundwater, partially or completely fills the spaces
between clasts and attaches each grain to its neighbor
e. Breccia: a sedimentary rock made of sharp detritus
f. conglomerate: a sedimentary rock made of water rounded detritus
g. Sandstone: quartz grains
h. siltstone: silt
i. shale: mud
3. Organic Sedimentary Rocks: Made from the remains of living organisms
a. Limestone: made from the shells of dead seafish
1. easily eroded by slightly acidic water (carbonate based rock)
2. you see lost of ocean fossils in limestone
b. Chert: formed from the silicate shells of plankton
c. Coal: Made from the remains of plants
4. Sedimentary Structures: features on sedimentary rocks that provide important clues as
to how the sedimentary rock formed.
a. Stratification: When a rock has obvious layers of rock
b. Ripple Marks:
c. Mud Cracks: Roughly hexagonal plates
d. Fossils