ESSC 110 Lecture Nine -Weathering, Sediments, Soil

Define Sediment

Physical and chemical breakdown of rock at earth's surface, produced by weathering, covering bedrock in various thickness; and can be transformed into soil through interaction with water and organisms

Define weathering

a combination of processes that corrode solid rock

define regolith

layer of debris resulting from wealthering

Define Physical weathering

also known as mechanical weathering, breaks rocks into unconnected clasts that vary in size

What are the processes included in physical weathering

Jointing, frost wedging, root wedging, thermal expansion

Define Jointing

natural cracks that from in rocks due to the removal of overburden or due to cooling, erosion, and plate tectonic movement causes exhumation: exhumation is when both temp and pressure drop, the rock stretches vertically and can crack

exhumation

exhumation is when both temp and pressure drop, the rock stretches vertically and can crack. creating joints

Frost wedging

Once water has entered a wedge in the rock, and the water freezes, it expands and opens the wedge of the rock

Root wedging

as a root grows, they exert a force on the surrounding rock

What are the two types of weathering

physical and chemical

Chemical weathering

chemical reaction that alters or destroys minerals when rock comes in contact with water solutions or air.

Types of chemical weathering

dissolution, hydrolysis, oxidation, hydration

Dissolution

minerals that dissolve into water, mostly affects rocks and salts

Oxidation

reactions during which an element loses electrons. Usually occurs when elements combine with oxygen, common in iron bearing minerals

Hydrolysis

water chemically reacts with minerals to form other minerals, common in feldspars and other silicate minerals. weathers into clay

Hydration

absorption of water into crystal structure of minerals causing some to expand

How does physical weathering speed up chemical weathering

physical weathering increases the surface area available to chemical weathering, speeding up weathering rate

how does chemical weathering speed up physical weathering

dissolving away grains and rocks that hold rock together, weathering happens fastest at corners and edges because exposed on multiple directions

Rate of chemical weathering

not all minerals chemically weather at the same rate, least stable minerals weathering the fastest

Differential weathering

when an outcrop of rocks undergoes different rate of weather, weathers fasters at edges and at layers with least stable minerals

Soil

rock or sediment modified by physical and chemical interactions with organic material and water that is capable of supporting plant life

Soil forms from

production of mineral debris, interaction with water and interaction with organisms

Soil forming processes

happens at different rates and at different depths, causing soil horizons, in specific orders called soil profiles

Soil Horizons lables

O horizon (organic layer)
A horizon (surface soil)
E horizon (washed out layer)
B horizon (Subsoil layer)
C horizon

Parent material

mineral material on which the soil is built, can be bedrock

O horizon

organic layer, partially decomposed organic matter

A horizon

surface soil, mineral particles mixed with organic material

E Horizon

washed out, depleted of soluble nutrients

B horizon

subsoil, often dense texture due to clays

C horizon

weathered rock fragments with little organic material

What are soil forming factors

rainfall, temp, vegetation substrate composition, and slope steepness

How does climate effect soil formation

total rainfall and average temperatures determine rate of chemical weathering and leaching

How does the substrate composition effect soil formation

some soils forms on basalt, granite, volcanic ash, so the substrates have different compositions have different resistance to weathering, impacting the composition and rate of soil formation

how does the slopes steepness, wetness, time and vegetation type effect soil formation

regolith easily washes away from steeper slopes, wetter soils hold more organic materials, flat soils hold more moisture and develop thicker soils. thicker soils take a long time to make, new soil is thin

Soil classification is determined by

the composition of sand, silt, and clay which also determines its texture

what causes soil destruction

nutrient removal and erosion

rate of soil formation and destruction

soil forms at 1mm/yr and erodes greater than 25/yr

soil conservation methods

contour plowing, strip farming, and terracing, also plating perennial species