ROCK JOINTS, SPECIFIC SURFACE AREA, Part 2 ROCK JOINTS, SPECIFIC SURFACE AREA, and WEATHERING

All rock is jointed media This shows orthogonal jointing in layered sedimentary strata. Groundwater moves through the joints Joints often occur in semi-parallel clusters, which are more subject to weathering and disintegration; enlarging flow conduits

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

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

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

Plot showing asymptotic relationship between number of particles and specific surface area for one cubic meter of rock divided into increasingly smaller blocks Intensely jointed or fractured rock can be expected to undergo weathering much more quickly than less fractured, or lower permeability materials

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

At greater depths, minerals in percolating groundwater can foment cementation along joint systems, as shown here. Most common cementaceous agents are carbonate, silica, and iron derivatives Note maroon colored zones of iron oxide extending outward from orthogonal joint planes

Precipitation of Iron Oxide Chemical reactions involving iron-rich minerals, such as pyroxene, in the presence of oxygen and water. Iron oxide can be one of the strongest binders

Iron cementation along orthogonal joints in precambrian granite in the western Grand Canyon Note mineralization halos radiating outward from the original joints, at right angles to one another. Note granite core stones weathering into grus between the ironstone halos