Physical Geology Slides-Soils and Weathering


Weathering Phenomena

weathering of tombstone Studies of rock weathering are so popular that people are just dying to participate. In a century and a half, this slate headstone has scarcely weathered. Even faint guide lines scribed by the stonecutter are still visible. Near Syracuse, New York.
weathering of tombstone In roughly the same length of time, this marble headstone has weathered appreciably. Lettering is rounded, the surface is rough, and the stone is stained by organic acids. (One of the James clan's distant relatives, named Jesse, went west and became rather famous. Really.)
weathering of marble, Greece 2500 years of exposure to salt air will do this to marble. A wall at Cape Sounion, Greece.
differential weathering, CA Rocks weather at different rates. The dark gray rock, rich in ferromagnesian minerals, is so crumbly it can be dug by hand. The dikes and sills, mostly quartz and feldspar, are much less weathered. San Bernardino Mountains, California.
differential wear on stone floor 400 years of pedestrian traffic has differentially eroded this marble floor in St. Peter's, in Rome.
differential wear on stone floor A century of foot traffic in the State Capitol, in Madison, has resulted in noticeable differential wear.
spheroidal weathering A rock from Saudi Arabia, showing exfoliation and spheroidal weathering.
These granitic rocks at the base of Mount Whitney, California, are spheroidally weathered.
spheroidal weathering This road cut in Colorado shows the relation between spheroidal weathering and joints. Below ground level the rocks are fresh and broken by joints into angular blocks. Above ground level the blocks are rounded by spheroidal weathering.
cavernous weathering In desert environments, moisture lingers under rocks, causing them to weather from below. The dark patina, called desert varnish, is actually a very complex product of chemical weathering and microbial action. Calico, California.
Liesgang rings These look spectacular but actually have little known geological significance. The color bands, called Liesgang Rings, are due to iron oxide transported through the rock by water.

Soil Types

spodosol, Wisconsin A typical soil profile, a spodosol from Menominee County. Thin organic debris (O horizon) caps the soil. Organic acids help dissolve materials out of the A horizon, creating a bleached zone of pure white quartz sand. Beneath is the brilliant orange B horizon where iron accumulates. Spodosols are typical soils of conifer forests.
soil horizons Scenes like this are common during the spring plowing. The variations in soil color are partly due to more drainage and oxidation in the high spots, but chiefly due to scalping. Soil is plowed and eroded off the high spots and deposited in the low spots, so that the B horizon shows through in the high spots of the fields. The very dark area is freshly plowed, still-moist soil.
gypsum from aridosol In arid climates, calcium dissolves out of the A horizon only to reprecipitate as calcite or gypsum in the B horizon. This gypsum came out of an Iraqi foxhole in Kuwait. It must have been tough to dig. Serves them right.
ultisol Very old temperate soils like this outside Birmingham, Alabama are called ultisols (ultimate soils). The pebbles are chert, all that remains of the limestone that once existed here. Unweathered bedrock is exposed at the bottom of the 2-meter deep gully.
ultisol This ultisol, in central Tennessee, has had most of its soluble components leached away but still preserves the layering of the original bedrock.

Fossil Soils (Paleosols)

paleosol This is a typical oxisol or laterite, a very old tropical soil, except this soil is in California. It is also a paleosol, a fossil soil about 50 million years old. Paleosols are rarely preserved because soils form in areas of weathering and erosion.
paleosol Another paleosol, this one preserved between two lava flows on the Isle of Skye, Scotland. By coincidence, this one is also about 50 million years old.

Mass Wasting


soil creep When soil creeps downhill, small trees tend to be bent over, but also have a biological drive to grow vertically (called geotropism). Thus, trees on creeping slopes tend to curve at the base.
soil creep Fenceposts and telephone poles, on the other hand, don't grow vertically and merely tilt on creeping slopes.
"mystery spot" Almost every State has a "mystery spot" where some strange force causes trees to be bent and twisted, structures to slide downhill, and so on. Most of the phenomena at these "mystery spots" can be explained in terms of soil creep.
colluvium The loose rubble on the hillside is a product of weathering and soil creep called colluvium. It creeps downhill slowly but is not very securely in place and can also give way as a debris flow if it becomes wet enough. Near San Bernardino, California.
mass wasting, CA A virtual textbook of hillslope processes near Livermore, California. A fresh mudflow can be seen as well as several older landslide scars in varying states of erosion. The fine horizontal terracing on the hillsides was made by the hooves of grazing cattle. This landscape was probably open grassland with scattered trees before European occupation.

Landslides and Avalanches

avalanche chutes, CO This gully in central Colorado has been scoured clean by repeated snow avalanches.
avalanche, Mount St. Helens The far end of the Mount Saint Helens avalanche. The mounds are blocks of the mountain, but were literally shaken to pieces as they moved and can be dug with a shovel.
Lituya Bay, Alaska, 1958 In 1958, a large earthquake shook a massive rock and ice avalanche loose at the head of Lituya Bay, Alaska, generating the highest wave ever documented. These photos show the bay before and after the avalanche. There were three boats in the bay and amazingly, two survived. One of the survivors recalled looking down on 100-foot trees as the boat was carried over the bar at the mouth of the bay. Although the wave was immense in the small enclosed bay, it rapidly dissipated to insignificant in the open sea.
Lituya Bay, Alaska, 1958 A view looking down on the area washed by the wave. That's a mature forest of 200-foot Douglas firs and Sitka spruces.
Lituya Bay, Alaska, 1958 The water swept away a mature forest right down to bedrock. The high water mark is 1,740 feet above sea level. Water sloshed an additional 200 feet or so higher, carrying debris into the forest but not uprooting it.
construction practices, California Your tax dollars at work. The depth of weathering is visible in the distant cut. These homes (costing $200,000 and up) were built on landslide deposits. The landslide deposits contain the ruins of the previous housing development built here. Guess who pays for the disaster relief when it happens again?

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Created 13 July 1998, Last Update 28 May 2003

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