Lecture 6 - Weathering and Erosion in the Parks of the Colorado Plateau

Outline

I. Weathering and Erosional Processes

II. Landforms Produced by Weathering and Erosion-Canyonlands, Arches and Bryce National Parks

I. Weathering and Erosional Processes

Weathering- mechanical breakdown and chemical alteration of rock

Erosion- removal of weathered material

Weathering and erosion followed by deposition and cementation are processes by which sedimentary rocks are formed and responsible for much of the spectacular scenery in the parks of the Colorado Plateau.

Mechanical Weathering- physical forces break down rock

  1. Granular disintegration- thermal fluctuations cause outer part of rock to heat up and expand and cool down and contract eventually fracturing the rock into smaller pieces. Rocks exposed to forest fires experience this effect most dramatically.
  2. Frost Action-
    frost wedging- water in rock fractures repeatedly freezes and thaws, expansion of ice fractures rock. Most effective at high altitudes where temperatures fluctuate greatly.
    frost heaving- water freezes and thaws beneath rock layer forcing it to move up and down and fracture
  3. Exfoliation- when massive rock formations are uplifted, the pressure on them decreases and this pressure change produces fractures that are parallel to the rock surface. These fractures cause large concentric sheets of rock to peel off (exfoliate) like the layers of an onion. (saw this in the Navajo Sandstone in Zion)
  4. Organic weathering- roots and burrowing animals can pry open fractures in rock

Chemical Weathering- breakdown and reforming of rock minerals in the presence of water

  1. Carbonation-rainwater dissolves CO2 in atmosphere producing carbonic acid which can very effectively dissolve limestone (calcium carbonate). This is responsible for some of the springs in the limestone units in the Grand Canyon. H20 + CO2 = H2CO3 + CaCO3 = Ca + 2(HCO3)
  2. Oxidation- reaction with oxygen (usually in water) to produce oxides. For example, iron combines with oxygen to form iron oxide (rust) this process produces much of the red in the sedimentary rocks of the Colorado Plateau
  3. Hydration- rock forming minerals combine with water to form new minerals usually hard minerals are converted to clay minerals which are softer than original minerals.

Factors that Influence Weathering

Weathering occurs at different rates depending on rock composition and structure, this is known as differential weathering which isresponsible for the terraced nature of the walls of the Grand Canyon and the "Grand Staircase" of the Colorado Plateau.

Composition- certain minerals like quartz are less susceptible to chemical weathering than others making rocks rich in these mineralsresistant

Structure- fractured rock weathers more quickly than solid rock also fine grained rock has more surface area and will weather fasterthan coarse grained rocks.

Topography- weathered material is more easily transported on steep slopes, therefore continually exposing fresh rock to weathering andaccelerating rock weathering

Climate- moisture and heat accelerate chemical weathering especially that of limestones, temperature extremes speed up frostwedging and other physical weathering processes

Erosional Processes

  1. Stream erosion-physical removal of weathered material by water in streams, responsible for carving the Grand and Zion Canyons
  2. Mass wasting - downslope movement of material under the influence of gravity, responsible for widening canyons produced by stream erosion rockfalls and rockslides are common on slopes that are undercut and accumulate rock piles called talus- very common in all Parks of the Colorado Plateau landslides, mudflows, and ground creep are other examples of mass wasting that are not very important on the arid Colorado Plateau
  3. Wind erosion or sandblasting where sand grains carried in the wind etch, pit or smooth rock surfaces

II. Landforms Produced by Weathering and Erosion-Canyonlands, Arches, and Bryce National Park

History of Important Events at Canyonlands and Arches

  1. Mesozoic-early Cenozoic forces building the Rocky Mtns. to the east mildly folded and fractured flat-lying sedimentary rocks of the Colorado Plateau.
  2. Pennsylvanian (Paleozoic) salt deposits of the Paradox Formation present in both Canyonlands and Arches began to flow and produced bowing and fracturing of the younger rock layers above.
  3. Weathering focused along the parallel fractures produced fins in the Permian (Paleozoic) Cedar Mesa sandstone in Canyonlands and in the Jurassic (Mesozoic) Entrada sandstone in Arches.
  4. Weakness of the underlying layers of the Entrada formation allowed fins to be more easily eroded on both sides to produce windows and arches at Arches National Park.

Slides showing these features in the Parks

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This page was last reviewed on 1/14/04.