Tuesday, December 30, 2014

What a Geologist Sees - Part 25 (Originally published 11/16/08)

Oh, the stuff we can see at construction sites and quarries!

[Disclaimer: I only enter construction sites on Sunday, when there is no activity, I stay away from the equipment and any obviously dangerous places and if there are any "No Trespassing" signs, then I don't go in.]

One "treat" at a construction site is to be able to see the effects of erosion and deposition in the exposed materials. In the uppermost photo, you can see the gulley erosion in the soft, graded soil. Just downslope from the gulley is a small "alluvial fan", where the eroded material was deposited. Larger examples of alluvial fans are seen at the mouths of mountain canyons.

In the second photo, in a sand pile at a quarry, as sand is removed from below, it triggers miniature slumps and landslides in an attempt to bring the slope back into equilibrium. In larger settings, slumps and landslides generally happen on slopes that have become destablized due to construction and heavy rainfall.

As one would expect, in a construction site, rocks are exposed that we usually wouldn't see at the surface. In the third photo, road construction has exposed a portion of a diabase (basalt) igneous dike that was most likely intruded during the Triassic or Jurassic Period. The iron-rich silicate minerals in the diabase are susceptible to weathering (by oxidation) in this humid climate, thus these blocks from the shallow sub-surface show a "rind" of oxidized material, with fresher rock material within the block.

In the fourth photo, we see "saprolite" that has been exposed during the construction of a drugstore. Saprolite is called "rotten rock" by some, it is rock that has been chemically weathered to the point that its structural integrity has been lost and the material can be easily crushed by hand. The "parent rock" - exposed nearby - is a biotite gneiss, similar to a granite, and in the case of the saprolite, the feldspars, micas, and other minerals (except for quartz) have been altered to clays. If not covered over quickly, this sort of material would wash into a nearby creek, resulting in "silting up" of the stream (and a probable EPA/Ga EPD fine).

The fifth photo, of another sand pile, shows how gravity, with the help of the wind, attempts to stablize the slope of this sand pile. Unconsolidated (loose) materials have a defined "angle of repose", which is the maximum angle-of-slope that particular sized material can sustain. If a slope is "oversteepened", miniature landslides and slumps carry material downslope in an "attempt" to establish equilibrium at the angle of repose, which generally varies between 25 and 35 degrees, depending on the size and angularity of the particles.

In the final photo, in this pile of mixed sand and gravel, rainfall has induced "rill erosion" (small erosion channels) on the slopes and small alluvial fans at the base of the slope.

[All of these photos were taken in the greater Atlanta area.]

In Geology, we term the mass downslope movement of material to be "mass wasting". [Yeah, I know Geologists can get mass-wasted after too many adult beverages, but that is another story.] Mass wasting occurs when gravity overcomes cohesion and internal friction. Water can be a facilitator of this process, as well as earthquake, traffic, and construction vibrations (as suggested above).

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