CHAPTER 10 Weathering, Karst Landscapes, and Mass Movement

This chapter examines the forces/processes that “wears away or rearranges” landforms Text calls this denudation (I used to lecture these forces as degradation) – forces that collectively are “levelers” of the Earth’s surface --- weathering; mass movement; erosion; transportation; deposition Paradoxically, in conjunction with forces of transport, reducing agents are also aggradational forces building (deposition) the Earth surface

Dynamic Equilibrium Approach to Landforms In conjunction with tectonic forces, degradation and aggradation forces are part of a 4.5 billion year old Earth system … surface building and reduction [We have previously discussed the end to this system and the ultimate “leveling” of the Earth surface] This is the text’s dynamic equilibrium model

Equilibrium Stability DestabilizingNew Condition Event of Stability Period of Adjustment

Reducing forces are atmospheric and gravitational in origin and tied to geology and weather and climate Taken together these elements define a dynamic equilibrium state for the Earth surface A change in one or more of these elements may destabilize and trigger a geomorphic threshold … a point at which there is enough energy to overcome resistance against movement

Weathering Processes Processes by which rocks at or near Earth surface are physically or chemically disintegrated or decomposed in preparation for removal and transportation Weathering is the breakdown of bedrock (parent rock material) without transportation regolith is the term for surface/near surface material acted upon by weathering

With enough time, even the most massive of Earth materials will be decomposed and transported *Weather does not by itself level the landscape. It breaks rock material so that denudation (degradation) can take place

Weathering is traditionally divided by process agent into: (1) Physical (or Mechanical) Weathering [generally dominate in drier/cooler climate regions] (2) Chemical Weathering [generally dominate in warmer/wetter climates] Both are primarily surface phenomena (but may occur as deep as 600’ below ground)

Variations in weathering depth will depend on: (1) structure and composition of rock (2) climate (3) configuration of the land surface (4) vegetative cover

Physical Weathering - Physical / mechanical weathering is disintegration without chemical alteration --- Important: (1) Smaller rock material easier to transport (2) “ “ “ “ “ encourages greater chemical weathering by exposing more rock material

Examples of physical weathering (1) ice wedge (2) frost wedge / frost heave (3) salt-crystal growth (4) uneven expansion of rock material in a conglomerate or in the ultimate expansion of high-pressure-formed igneous plutons (5) root growth / burrowing by animals

Chemical Weathering Chemical weathering is decomposition and decay of the chemical structure of rock Prepares rock material for transport in several ways: (1) may form “new” softer rock material (2) by combining with water / through chemical change, No. 1, may create new structures more easily stressed / fractured (3) may create acidic compounds

Principal chemical weathering processes (1) oxidation (2) hydration and hydrolysis (3) carbonation and solution --- “chemical erosion”

Different rock types are affected by different weathering processes and at different rates Under differing geologies and geographies, this can lead to a highly diversified visual landscape

Karst Topography and Landscapes - Limestone is abundant on the Earth [look at Fig at global distribution of limestone / look especially at eastern U.S. distribution] - It is a very soluble rock - Where limestone is present, distinctive and spectacular landscapes may be produced

Karst - term given to any landscape having landforms evolved from solution of limestone … approx 15% of Earth surface - term derived from the Krs (Karst) plateau of the Dalmatian Coast of Croatia

Karst, cont Synonymous with limestone strata, it takes more than limestone to create karst landscape … many regions have karst features – true karst is rare (1) the limestone formation must contain 80% or more calcium carbonate (2) joints “cracks and conduits” must be present in the limestone (3) there must be an aerated zone between ground and water table (4) vegetative cover must be available (for organic acids)

Limestone Landscapes - Groundwater is a weak carbonic acid containing CO 2 - Limestone is primarily calcium carbonate (CaCO 3 ) - Together they form calcium bicarbonate Ca(HCO 3 ) 2 - CaCO 3 +H 2 O+CO Ca(HCO 3 ) 2

Limestone Landscapes, cont Created is a landscape of bumpy, pitted surface topography, poor surface drainage, and well developed solution channels We would expect to find (Fig 10.17): (1) sinkholes (solution sinkholes) (2) dolines (3) caves / caverns --- collapse sinkholes --- valley sinks (4) haystack hills (hums)

Limestone Landscapes, cont Limestone landscapes are as well known for underground features --- travertine (dripstone) --- stalactites --- stalagmites --- “soda straws” --- etc

Mass-Movement Processes Natural systems are noted for their attempts to seek steady state or equilibrium Gravity is an equalizing force that attempts to create this equilibrium - massive amounts of material are moved downslope by gravity - moved downslope as far as the defined angle of repose

Mass Movement In addition to gravity, any action that: (1) increases shear stress or (2) decreases friction between slope material particles will increase the potential effects of gravity and slope

Mass Movement [Fig 10.19] - collective term for any of the various forms of gravity-induced downhill material transportation - can either be natural or man-induced in origin - massive and rapid vs slow and constant - actual volume of global mass movement is unknown - many actions taken by man can either delay or accelerate these processes

Mass movement processes are classified: (1) movement fast or slow? - a continuum of “subjective generalities” slow (long-term observation – to – fast (occurrence over extremely short period)

(2) type of material moved? - range of loose Earth surface material in unstable (or potentially unstable) location --- ice, snow, debris,soil, rock, mud (3) kind of motion? - ways that slope material moves in response to gravity