ROCK JOINTS, ROCK JOINTS, SPECIFIC SPECIFIC

SURFACE AREA, SURFACE AREA,

and WEATHERINGand WEATHERING

Part 2Part 2

All rock is jointed All rock is jointed mediamedia

This shows orthogonal This shows orthogonal jointing in layered jointing in layered sedimentary strata.sedimentary strata.

GroundwaterGroundwater moves moves through the jointsthrough the joints

Joints often occur in Joints often occur in semi-parallel clusters, semi-parallel clusters, which are more which are more subject to weathering subject to weathering and disintegration; and disintegration; enlarging flow enlarging flow conduitsconduits

Regional systematic joints perturb layered sedimentary Regional systematic joints perturb layered sedimentary rock strata, generally orthogonal to the bedding. rock strata, generally orthogonal to the bedding. Navajo sandstone above Colorado City, ArizonaNavajo sandstone above Colorado City, Arizona

Stepped cut in volcanic strata illustrating problems Stepped cut in volcanic strata illustrating problems with raveling in upper portion of the weathered horizon. with raveling in upper portion of the weathered horizon. Note presence of trees, along mid-slope bench and Note presence of trees, along mid-slope bench and shear zones. shear zones.

Joints break the rock mass up into discrete Joints break the rock mass up into discrete pieces, creating greater specific surface area pieces, creating greater specific surface area for chemical reaction and cation/anion for chemical reaction and cation/anion exchange with oxygenated groundwater, or exchange with oxygenated groundwater, or enzymes secreted by tree and plant rootsenzymes secreted by tree and plant roots

Plot showing Plot showing asymptotic relationship asymptotic relationship between number of between number of particles and specific particles and specific surface area for one surface area for one cubic meter of rock cubic meter of rock divided into divided into increasingly smaller increasingly smaller blocksblocks

Intensely jointed or Intensely jointed or fractured rock can be fractured rock can be expected to undergo expected to undergo weathering much more weathering much more quickly than less quickly than less fractured, or lower fractured, or lower permeability materialspermeability materials

Blocks within Blocks within joint clustersjoint clusters can often weather out can often weather out completely, becoming filled with porous and low completely, becoming filled with porous and low cohesion grus, which is easily eroded, as shown here cohesion grus, which is easily eroded, as shown here near Olmsted Point in Yosemite Nat’l Park.near Olmsted Point in Yosemite Nat’l Park.

At greater depths, At greater depths, minerals in minerals in percolating percolating groundwatergroundwater can can foment foment cementationcementation along joint systems, as along joint systems, as shown here.shown here.

Most common Most common cementaceous agents cementaceous agents are are carbonatecarbonate, , silicasilica, , and and ironiron derivatives derivatives

Note maroon colored Note maroon colored zones of iron oxide zones of iron oxide extending outward from extending outward from orthogonal joint planes orthogonal joint planes

Precipitation of Precipitation of Iron OxideIron Oxide

Chemical reactionsChemical reactions involving involving iron-rich iron-rich mineralsminerals, such as , such as pyroxenepyroxene, in the , in the presence of presence of oxygenoxygen and and waterwater. .

Iron oxideIron oxide can be can be one of the one of the strongest binders strongest binders

Iron cementation Iron cementation along orthogonal along orthogonal joints in precambrian joints in precambrian granite in the western granite in the western Grand CanyonGrand Canyon

Note Note mineralization mineralization haloshalos radiating radiating outward from the outward from the original joints, at right original joints, at right angles to one another.angles to one another.

Note granite core Note granite core stones weathering stones weathering into into grusgrus between the between the ironstoneironstone halos halos