GEOS 1004 - Weathering and Soils

Landscapes are shaped by turning rocks into…

sediments

What are sediments?

fragments of pre-existing rocks

What is physical weathering?

breaking rocks into smaller pieces

What is chemical weathering?

changing the chemistry of the rocks

True or False: physical and chemical weathering act in concert (simultaneously)

true

Smaller pieces of weathered rock —> ?

more reactions (some surface area is exposed to chemical weathering, but it increases as the reactions increase)

The weaker the rock is, the…

easier it is to break

Examples of Wedging (physical weathering)

frost forcing small cracks wider, mineral grains forcing small cracks wider, roots forcing small cracks wider

Unloading (physical weathering)

releasing stress and causing expansion; occurs spontaneously

Exfoliation

occurs everywhere (ex. fractures parallel to the surface)

Examples of Dissolution (chemical weathering)

water dissolving salt, water getting into fractures of rock and dissolving the rock, rainfall dissolving the atmospheric CO₂ creates natural acidity

Weak Acid + Limestone = ?

weathered rock

Oxidation (chemical weathering)

turns soluble Fe²⁺ into insoluble Fe^{3+ _/} removes iron from the solution

Hydrolysis (chemical weathering)

ions replaced by hydrogen: KAlSi₃O₈ + H₂O → HALSi₃O₈ + K⁺ + OH^- ; feldspar weathered along cleavage planes

What is spheroidal weathering?

combining physical and chemical weathering

Process of Spheroidal Weathering

1 - water penetrates extensively jointed rock; 2 - chemical weathering decomposes minerals and enlarges joints; 3 - rocks are attacked more on corners and edges and take on a spherical shape

What is weathering susceptibility?

a function of the: properties of the original rock, climate, presence or absence of soil and vegetation, and length of exposure

The minerals that form first will be…

the ones that weather first

True or False: minerals and rocks weather at the same rate

false

Rocks that resist weathering also resist…

erosion

What is a talus?

a slope of rock fragments

What is the middle ground made up of?

lithosphere (inorganic) and biosphere (organic); 50% pores and 50% solids

What is mineral matter known as?

regolith

What is organic matter known as?

humus

What is soil made of?

components of the physical and chemical weathering of rocks (quartz, clays, iron oxides)

What are soil profiles?

slices into the ground that show the differing soil horizons (horizons vary in size and not all may be present)

Process of Rain Going Into the Ground

rain enters ground —> plant debris accumulates —> worms churn —> microbes and fungi metabolize —> roots weather minerals —> downward-percolating water transports ions and clay —> Iions and clay accumulate

Order of Soil Horizons

O, A, E, B, C, U, unweathered parent material

O Horizon

loose and partly decayed organic matter

A Horizon

mineral matter mixed with some humus

E Horizon

zone of eluviation and leaching

B Horizon

accumulation of clay transported from above

C Horizon

partially altered parent material

Thicker soil develops on…

flat terrain

Thinner soil is on…

steep slope because of erosion

Transported soil can be developed on…

unconsolidated stream deposits

High Temperature + High Precipitation = ?

high vegetation

The tropics have the ________ soils, desert regions have the ________ soils.

thickest; thinnest

As more trees/plants grow, the _ _______ gets thicker, which in turn makes the other horizons much deeper.

O Horizon