Part 5Part 5

CHEMICAL AND CHEMICAL AND MECHANICAL MECHANICAL WEATHERING WEATHERING

PROMOTED BY PROMOTED BY SEEPAGE AND SEEPAGE AND

CATION EXCHANGECATION EXCHANGE

Atmospheric Atmospheric carbon cycle: carbon cycle: COCO22

concentrations, concentrations, weathering, and weathering, and climate climate

Susceptibility to weathering is proportionate to Susceptibility to weathering is proportionate to chemical exchange with oxygenated groundwaterchemical exchange with oxygenated groundwater

As As oxygenated groundwateroxygenated groundwater percolates through rock it leaches percolates through rock it leaches the ground mass of the ground mass of soluble ionssoluble ions and absorbs and absorbs free cationsfree cations. These . These chemical exchanges promote oxidation and loss of cementation, chemical exchanges promote oxidation and loss of cementation, which causes degrades tensile strength, thereby promoting which causes degrades tensile strength, thereby promoting mechanical weathering as an “after product.”mechanical weathering as an “after product.”

Honeycomb solution cavities, or Honeycomb solution cavities, or alveolar weatheringalveolar weathering (“Tafoni”), in (“Tafoni”), in cliff of Entrada Sandstone testify to differential weathering cliff of Entrada Sandstone testify to differential weathering caused by percolating groundwater dissolving carbonate cement caused by percolating groundwater dissolving carbonate cement beneath a more resistant crust. Between Kanab and Mt. Carmel beneath a more resistant crust. Between Kanab and Mt. Carmel Junction, Utah.Junction, Utah.

Demarcation between sandstone strata leached of iron Demarcation between sandstone strata leached of iron (above) and that below. Note large solution cavities in (above) and that below. Note large solution cavities in the lower (red) strata. Back country of Capitol Reef the lower (red) strata. Back country of Capitol Reef National Park, Utah.National Park, Utah.

Channel potholes are formed by the grinding action of Channel potholes are formed by the grinding action of stones or coarse sediment whirled around by eddies stones or coarse sediment whirled around by eddies or the force of the current at a bend.or the force of the current at a bend. Fiery Furnace Fiery Furnace area of Arches National Park, Utah area of Arches National Park, Utah

Series of channel Series of channel potholes carved into potholes carved into the Navajo Sandstone the Navajo Sandstone in backcountry of in backcountry of Capitol Reef. Capitol Reef.

Note master joint Note master joint providing structural providing structural control of the control of the cascading channel, cascading channel, which descends in a which descends in a series of falls and series of falls and pools.pools.

Standing/percolating Standing/percolating water accelerates water accelerates chemical weatheringchemical weathering

Double Arch at Arches Park was likely initiated by pothole seepage through the Slickrock Double Arch at Arches Park was likely initiated by pothole seepage through the Slickrock Member of the Entrada Formation. As carbonate cement was leached from the sandstone, Member of the Entrada Formation. As carbonate cement was leached from the sandstone, its tensile strength was diminished, leading to blocks in tension falling away, leaving the its tensile strength was diminished, leading to blocks in tension falling away, leaving the arch ribs that were in compression. The tensile strength of this material is between 1/30arch ribs that were in compression. The tensile strength of this material is between 1/30 thth and 1/70and 1/70thth of the compression strength. Note people for scale (arrows) of the compression strength. Note people for scale (arrows)

Bulk densityBulk density, , diagenesisdiagenesis and and susceptibility to susceptibility to weatheringweathering tend to be influenced by tend to be influenced by hydraulic hydraulic conductivityconductivity (permeability with respect to water). (permeability with respect to water).

The erosional escarpment formed by limestone and The erosional escarpment formed by limestone and marl spires of Brice Canyon NP. These were formed marl spires of Brice Canyon NP. These were formed by preferential solutioning along systematic regional by preferential solutioning along systematic regional joints.joints.

These spires at Brice Canyon are erosional remnants caused by These spires at Brice Canyon are erosional remnants caused by preferential flow paths of groundwater seepage. This view is preferential flow paths of groundwater seepage. This view is taken from near the base of the escarpment. For pioneer taken from near the base of the escarpment. For pioneer shepherd Ebenezer Brice it was a heck of a place to loose one of shepherd Ebenezer Brice it was a heck of a place to loose one of his sheep.his sheep.

Looking back at crest of the erosional escarpment at Brice Canyon. Looking back at crest of the erosional escarpment at Brice Canyon. Note the caverns and arching roofs developed by solutioning Note the caverns and arching roofs developed by solutioning along preferential flow paths. The roofs eventually collapse, along preferential flow paths. The roofs eventually collapse, leaving linear rows of spires. leaving linear rows of spires.

Sandstone fins formed by tension along the northern Sandstone fins formed by tension along the northern flank of the Salt Creek anticline in Arches National flank of the Salt Creek anticline in Arches National Park. The anticline was caused by a rising salt diapir Park. The anticline was caused by a rising salt diapir which was subsequently dissolved by Salt Creek.which was subsequently dissolved by Salt Creek.

Ground view of a Ground view of a sandstone fin at Arches sandstone fin at Arches Park.Park.

These are formed in the These are formed in the Slickrock Member of the Slickrock Member of the Jurassic age Entrada Jurassic age Entrada FormationFormation

The sandstone exhibits The sandstone exhibits extremely low splitting extremely low splitting tensile strength, due to tensile strength, due to the leaching of interstitial the leaching of interstitial carbonate cement.carbonate cement.

This low tensile strength This low tensile strength tends to favor the tends to favor the development of natural development of natural rock arches rock arches

The arches form in The arches form in thin sandstone fins thin sandstone fins along brittle horizons along brittle horizons with extremely low with extremely low splitting tensile splitting tensile strengthstrength

These are typified by These are typified by profuse exfoliation, as profuse exfoliation, as shown at middle leftshown at middle left

The stronger roof unit The stronger roof unit spans the opening in spans the opening in compression, without compression, without exhibiting any marked exhibiting any marked secondary fracturing.secondary fracturing.