Lecture 5: Rock and Soil Formation Flashcards
What are the five controls on soil formation?
Parent material, climate, organisms, time, topography
Parent material is a heterogeneous layer of weathered material that overlies the bedrock - commonly regolith - but what does this include?
Weathered and unweathered rock, clay, iron and other metal oxides, other weathering products.
As soils are easily eroded, they form on…
Shallow slopes, plains and lowlands.
Position on slope controls…
soil moisture, nutrient supply, type of vegetation and mineral/organic matter accumulation.
The C horizon of soils contains the regolith. What is the difference between the A and B horizons?
The A horizon is the most organically rich and contains clay/insoluble compounds. Organics are sparse in the B horizon, where soluble minerals and Fe-oxides are found in lenses/coatings.
Laterites form in warm, humid climates by fast, intense weathering, what characteristics does this provide them with?
Residue of fast weathering = laterite. Red in colour, soluble minerals leached, Fe and Al oxides/hydroxides plus thin layer of humus present.
Pedocals form in dry climate with little vegetation and thin soil. Despite this, what is unusual in dominating their A horizon
A horizon dominated by parent material and soluble minerals.
What is lithification and when does it occur?
The process by which rocks solidify and sediments become rocks - commonly associated with burial. Occurs post-deposition.
After lithification, what do the following become: mud; sand; gravel; lime mud/sand/ooze?
mud = mudstone and shale
sand = sandstone
gravel = conglomerate
lime mud/sand/ooze = limestone and dolomite
Diagenesis imposes physical and chemical post-depositional changes to material. What is it replaced by and under what conditions?
Metamorphism at 300/350 degrees.
Describe the process by which chemical cementation lithifies sediments?
Minerals precipitate in rock (calcite, iron oxide, quartz). An insitu transformation occurs as pore spaces decreases (changing the porosity and permeability of rock) until bound together.
Physical cementation occurs via what process, and what is usually associated with this?
compaction - dissolution and overgrowths often associated.
Metamorphic rocks are those which have undergone changes to any of their
Mineralogy, composition or texture.
What are controls on metamorphic rock?
Temperature, pressure, parent rock, fluids.
What processes exert varying pressure-temperature controls on metamorphic facies?
Ocean extension, crustal extension and subduction zones.
