Chapter 15 - External Processes

CHAPTER 15 (external processes)

What is meant by denudation?

The overall effect of the disintegration, wearing away, and removal of rock material. Implies a lowering of continental surfaces.

The three stages in denudation are?

1. Weathering is the breaking down of rock into smaller components by atmospheric and biotic agencies.

2. Mass wasting involves the downslope movement of broken rock material due to gravity.

3. Erosion consists of more extensive and generally more distant removal of fragmented rock material.

Weathering involves the mechanical disintegration and/or chemical decomposition of rock, whether the rock is above or below ground…..depends on ‘openings’ which fall into 5 types…what are they? (microscopic, joints, faults, lava vesicles, solution cavities)

Microscopic: tiny and numerous, responsible for extensive weathering. May consist of spaces between crystals of igneous or metamorphic rocks, pores between grains of sedimentary rocks, minute fractures within mineral grains of any rock.

Joints: cracks that develop as a result of stress, innumerable in most rock masses dividing the blocks into various sizes. Most important of all rock openings.

Faults: cracks in bedrock along which there is relative movement of the walls making up the crack. Individual or occur in small numbers.

Lava vesicles: holes of various sizes, develop in cooling lava when gas is unable to escape as the lava solidifies.

Solution cavities: holes formed in calcareous rocks as the soluble minerals are dissolved and carried away by percolating water.

What is the basic difference between joints and faults?

Faults are individual and occur in small numbers, joints are multitudinous. Also, they sometimes appear as major landscape features. Faults allow easy penetration of weathering agents into subsurface areas.

What are the weathering agents? (p. 427) they are mostly atmospheric-explain

Atmosphere, Temperature changes, water, biotic agents.

Describe mechanical weathering. How does it differ from chemical weathering?

Mechanical weathering: the physical disintegration of rock material without any change in its chemical composition. Big rocks are mechanically weathered into small ones. Occurs at or very near to the surface. (frost wedging, salt wedging, temperature changes, exfoliation).Chemical weathering requires moisture. Chemical can lead to the disintegration of the entire rock mass.

What is exfoliation?

A weather process in which curved layers peel off bedrock, stripping away concentric rock slabs. Occurs mainly in granite.

What is chemical weathering? How does it benefit from mechanical weathering (see fig 15.13)? What are the principal reacting agents?(432)

Chemical weathering: the decomposition of rock by the alteration of its minerals. Mechanical weathering exposes bedrock to the forces of chemical weathering. The greater surface area exposed, the more effective the chemical weathering. Finer-grained materials decompose more rapidly than than coarser grained materials of identical composition.

Reacting agents: oxygen, water and carbon dioxide.

Processes: 1. Oxidation- oxygen atoms combine with the atoms of various metallic elements making up the minerals in the rock and form new products. 2. Hydrolysis: chemical union of water with another substance to produce a new compound that is nearly always softer and weaker than the original. 3. Carbonation: reaction between the carbon dioxide in water and carbonate rocks to produce a very soluble product that can readily be removed by runoff or percolation and can also be deposited in crystalline form if the water is evaporated.

What is the relationship between climate and weathering (fig. 15.17) ? And mass wasting (Fig. 15.15)?