Chapter 3 Weathering and Soils

Define and differentiate saprolite, soil, and regolith.

Saprolite - Soft, thoroughly decomposed and porous rock, often rich in clay, formed by the in-place chemical weathering of igneous, metamorphic, or sedimentary rocks. Saprolite is especially common in humid and tropical climates.

Soil is the mixture of minerals, organic matter, gases, liquids, and myriad organisms that together support plant life.

Regolith - the layer of unconsolidated rocky material covering bedrock.

Describe how physical, chemical, and biological weathering differ.

Physical weathering is broken down physically, chemical weathering is broken down chemically and biological weathering is related to physical breakdown of rocks from plant or animal life.

How does chemical weathering influence physical weathering and vice versa?

Physical weathering exposes more rock to be exposed to the elements through breakdown and will cause more of the rock to be broken down through chemical processes. Chemical weathering can weaken a rock so that more physical weathering can affect the rock.

List three mechanisms of physical weathering and describe how each works.

Freeze- thaw cycle - Ice expands and breaks apart the rock and then melts. This continues for thousands of years.

Heat/Cold Cycle - Extreme heat and cold through passage of time causes the rock to expand and contract.

Unloading - Plutonic igneous rocks form at depth in the crust with an immense volume of rock (overburden) above them. When these rocks are exposed, and the immense weight of the overburden rock above them is removed, they expand, and zones of weakness open up as joints.

What are borndhardts, tors, and inselbergs?

Bornhardt is a dome-shaped, steep-sided, bald rock outcropping at least 30 metres in height and several hundred metres in width.

Tor, which is also known by geomorphologists as either a castle koppie or kopje, is a large, free-standing rock outcrop that rises abruptly from the surrounding smooth and gentle slopes of a rounded hill summit or ridge crest.

Inselberg - isolated hill that stands above well-developed plains and appears not unlike an island rising from the sea.

What processes likely created felsenmeer?

exposed rock surfaces that have been quickly broken up by frost action so that much rock is buried under a cover of angular shattered boulders.

Predict what will happen to boulders or outcrops of rocks exposed to a forest or range fire or to extended heating in the desert sun?

They expand and can form fractures from the expansion of rock. This could let water enter into the cracks and begin chemical weathering.

Explain how salts can physically weather rock.

When salt crystals form they act like ice and can begin to create wedges in the rock and break it apart.

Why does chemical weathering occur?

Everything is chemical. Water reacts with gases in the atmosphere which in turn interacts with minerals and will breakdown the minerals through time.

Explain how silica and most rock forming minerals differ from calcium carbonate in terms of chemical weathering?

Calcium allows the movement of cations easily and the movement of silica is very hard. This makes limestone easier to erode and move cations.

Why are iron and aluminum oxides characteristic of materials left as residue after extensive chemical weathering?

Because they are the cations that are least mobile and are relatively insoluble, which means that they will be left behind in the residual soils over time as weathering strips away more mobile elements.

List the most important factors influencing the amount of leaching by solution of material from soil or rock

dissolved water will may move matter in or out of a layer by groundwater movement.

Discuss how various types of biological activity affect physical and chemical weathering rates of rocks.

Bacteria can create an acidic environment and increase chemical weathering. Turbidation is another example of physical weathering.

What controls the swelling potential of clay minerals?

The bonds between layers. If the bonds are weak then they have a propensity to swell. If they are strong they do not allow water to enter in-between the layers.

Define cation exchange, indentify which ions are most likely to be exchanged, and explain why cation exchange is an important process.

Cation exchange is the solubility and exchange of elements. Certain elements are very easily exchanged such as Ca, Na, and K. It is important because a lot of the building blocks of life for plants come from the Easily soluble elements.

What is oxidation of minerals and where can you see it occur?

The loss of an electron to another molecule. Typically in the soil which has a high redox potential or areas with available oxygen.

Sketch Goldich's Weathering Series.

Starting with feldspar, explain how it weathers over time to overlay become gibbsite.

Orthoclase reacts with acid to become illite. Illite reacts with carbonic acid and becomes Kaolinite. Kaolinite reacts with water and carbonic acid to become gibbsite which is no longer a clay but an oxide.

What is chelation and why is it important in soil formation?

is the process through wchich relatively immobile metal ions such as iron and aluminum in soils are mobilized by a soluble organic compounds that form metal structure around metal ions. IT facilitated by organic acids particularly fluvial acid produced by the breakdown of leaf litter, soil organic matter, and lichens.

Whys is the density of saprolite lests than that of the rock from which it was derived?

Because the heavy minerals have been eroded and clay is the remaining mineral left after everything has been eroded.

Describe the patterns of and balance between chemical and physical weathering on a global scale.

Explain how vegetation type and density affect soil formation rates and type of soils that result.

Vegetation also helps keep soil in place and shades the soil helping it retain moisture and increasing rates of chemical transformation. Soils created are heavy with organic material. Density of vegetation growing in the soil influences erosion resistance and the ratio of surface water runoff to infiltration after precipitation events.

explain how topography (slope steepness) affects soil thickness.

Rates of soil erosion are greater at steeper slopes. Gentle slopes will host more vegetation and accelerated soil development because they retain more moisture.

How is soil development related to the age of a soil?

Young surfaces have weakly developed soils which are thin and show little evidence for pedogenesis. Old surfaces typically have more strongly developed thicker soils with distance evidence for pedogenesis including color and soil structure

List three ways by which material can be added to and removed rom soils.

by ground water for both adding and taking away, biotubartion, chemically(?), Through organic processes.

Define translocation and explain how it can occur in soils.

the definitive pedogenic process.

o Translocation is the downward movement of solid soil particles or dissolved ions

List the most common soil horizons and explain how each forms.

A horizon - leached horizon.

o addition organic material, mostly vegetation, is a fundamental process.

o A horizons can look dark and mostly like organic material because the mineral grains get coated with organic compounds. However, the "mineral fraction" is dominant.

B horizon this is the horizon which is most useful to geologists and geomorphologists.

o clay, from dust or chemical weathering (Bt horizon).

o calcium carbonate (CaCO3) , from dust or chemical weathering . (Bk horizon).

o iron and aluminum oxides these are weathering products. In tropical climates concentrations of Al oxides can be economically rewarding ("bauxite deposits"). (Bs horizon).o w organic compounds: in humid, cold climates, especially where there are forests, organic compounds formed in the A or O horizons are translocated to the B horizon (Bh).

C horizon unweathered parent material if the soil is formed in unconsolidated sediment. If the soil is formed in bedrock, then the C horizon is transitional between the R horizon and the B horizon(s).

O horizon also known as the "duff layer". A surface accumulation of organic matter - more than 50% of the horizon is organic matter.

E horizon a bleached lower A horizon. It forms below the A horizon, and it is typically white, and often consists mostly of quartz. Forms as strong organic acids that develop in the A or O horizons percolate down through the soil. . REMEMBER: E IS FOR EMPTY!

Explain how bioturbation occurs and why it is important.

Critters live in the ground and eff it up. It is important because it can provide evidence of animal life in the area. It can also be used as a geopedal structure.

Describe the two general models relating soil production rates to soil depth.

First model - soil production declines with increasing soil depth because soil forming processes have less access to fresh bedrock beneath deeper soils. For example burrowing activity in thick soils breaks down less rock to produce fresh soil than does burrowing in thin soils.

Second model - Soils promote rock weathering because they hole more water and support more vegetation than bare rock outcrop.

Compare and contrast pedogenesis and soil production.

pedogenesis is the process of soil formation. Soil production is the rate at which the soil is produced.

Make a table listing the 12 soil orders, describing the salient characteristics, and suggesting in what environment each might be found.

List five factors thought to control of pedogenesis (soil development).

water content,. temperature, topography, rock penetration, and parent material.

Define a soil catena and explain differences observed across a landscape in terms of soil forming processes.

a suite of soils from differ position in a landscape. Catenas reflect local differences in soil formalin processes that result from topographic position and the hydrological and geomorphological processes that govern infiltration , runoff and soil erosion.

Suggest in what geomorphic environment you might find a chronosequence and explain why defining a chronosequence could be useful to you as a geomorphologist.

You might be able to find one in a desert area where the strata and cross cutting relationships are evident. It would be valuable because you would be able to tell the story of the area and interpret which processes led to the current area.

Compare a paleosol and cumulic soil.

Paleosol it is the typical and accepted practice to use the term "paleosol" to designate such "fossil" soils found buried within either sedimentary or volcanic deposits exposed in all continents

Cumulic soil - This is a layer or layers of mineral material in Organic soils.

List and describe several examples of weathering induced landforms at both small and large scales.

Arches National Park, Big Butte

List several types of duricrusts and explain how they help shape landscapes.

Silcrete, Laterite, Ferricite, Calcrete. They are resistant to erosion, they are hardened and turned back into rock within the pedogenic zone. They can cause a topographic inversion which occurs when low lying areas become more resistant and eventually become elevated as the surround terrain erodes away.

Compare rates of soil production to contemporary rates of soil loss and explain why this comparison is important.

`Soil production is a very slow process but soil loss is occurring quickly. Loss of nutrients in soils is also occurring because of over agriculturalization. We are losing more fertile soil than is being produced.