Tuesday, June 13, 2017

Graphic granite

Graphic granite is relatively common rock consisting of alkali feldspar (i.e., rich in potassium, in some cases in combination with sodium) and quartz, but the rock has a very interesting texture, consisting of a distinctive repetitive pattern that resembles cuneiform writing.

The above picture and the following two pictures are of the same piece of graphic granite, which is about 7 inches long (= 18 cm; the scale is in centimeters).

The origin of graphic granite was debated for over a century. It is now known to be the result of simultaneous growth of quartz (gray color in the rock above) and feldspar (white color) under conditions that favor the planar growth of the feldspar host. 

The next two pictures are different views of the same piece of rock, but you can notice how the texture differs, depending on the view.

Graphic granite occurs in pegmatites, which form during the final stage of a magma's crystallization. The graphic granite illustrated in these three pictures came from the pegmatite at the Stewart Mine in San Diego County, Southern California (see my archived post for  September 30, 2016 which focuses on the mineral rubellite from the Stewart Mine).

Monday, May 29, 2017

Polka-dot granite

"Polka-dot granite" is a distinctive rock, which has been found at several localities in Southern California. It has been used to indicate offset along the San Andreas Fault system, but some geologists have reported that this distinctive granite might not all have been derived from the same magmatic source. More geochemical research is needed.

“Polka-dot granite” is a granite with clots of cordierite (a mineral containing magnesium), biotite, garnet, and other minerals surrounded by an irregular halo (absorption sphere) consisting of white granite with little or no mafic (dark) minerals. The inner and outer margins of the halo are irregular but distinct. The halo is surrounded by the same granite that occurs in the center of the halo. The result looks like large polka-dots, which range in diameter from less than a centimeter to 8 cm.

"Polka-dot granite" collected from Southern California by Dave Liggett.

Same specimen shown above but photographed in the shade.
"Polka dot" is 4.4 cm in diameter.

Close-up of previous photograph.

Another "polka dot" (1.5 cm diameter) from the same locality.

By the way, cordierite is known for its ability to withstand extreme temperatures. It is used for making "pizza stones" because you can take the stone from a low temperature to a very high temperature without the risk of breaking it.

Tuesday, May 16, 2017

Crystals That Show Twinning

In my March 31, 2017 post concerning a distinctive granite, I showed an orthoclase crystal with twinning, which occurs when two separate crystals of the same substance share some of the same crystal lattice. Instead of a normal single crystal, the crystalline structure appears doubled. 

In this new post, I show some other common examples from my personal collection of twinned crystals. They are aragonite, pyrite, gypsum, quartz, and staurolite. 

aragonite (40 mm length)
gypsum (37 mm length)

pyrite cubes (47 mm length)
quartz (47 mm length)
staurolite (18 mm length)
pseudomorph of staurolite (40 mm)

pseudomorph of staurolite (40 mm length)

pseudomorphs are formed
when a mineral is replaced
by a foreign substance

Monday, May 1, 2017

Saint Francis Dam Part 2: The Geology

See my previous post for an overview of the topic. A very simplified geologic map is shown below. Basic information is superimposed on a Google Earth (2016) image of the dam area. The information is a combination of my own observations and those taken from a published geologic map (Dibblee, 1997) of the Warm Springs, California topographic quadrangle. This map is available through the Dibblee Geologic Foundation, which is easily accessed via the internet.

The site of the St. Francis Dam was not a good place to build a dam, what with the San Francisquito Fault underneath the site. If you go to my previous post, you can clearly see a demarcation between the "white" lowermost part of the canyon and "gray" rest of the canyon. The "white" area is the Pelona Schist, and the line of demarcation is the San Francisquito Fault.

Although considered to be an inactive fault, it uplifted the Mesozoic (or possibly older) Pelona Schist, which makes up the east side of the fault. The actual fault zone consists of sheared and fractured rock with plastic clay gouge about one foot thick and abundant gypsum crystals (reportedly, up to 11 inches in length).

This fault had been mapped by professional geologists prior to the building of the dam, but that information was not utilized by the builders of the dam.

The San Francisquito Fault has been commonly regarded as a strike-slip fault (horizontal movement), but, in recent years, the thinking has been more inclined to call it a strike-slip fault that has been renewed with a dip-slip component (up and down movement). 

Simplified geologic map of the dam-site area on a Google Earth (2016) image.

This view is of the west side of the area immediately downstream from the dam site. The road is the abandoned highway, built many years after the dam collapse. Note the white rubble in the upper right-hand corner. This is what is left of the dynamited west wing. The abandoned highway is parallel and near the trace of the San Francisquito Fault. Most, but not all, of the rocks above this highway are the red rocks of the fluvial and poorly cemented Vasquez Formation (of Oligocene age = about 26 million years old), which were down dropped by the fault. Some of the rocks adjacent to the highway and all of the rocks below it are the uplifted Pelona Schist (of probable Mesozoic age). Many of the Pelona Schist outcrops have a superficial red color, which is derived from the overlying red rocks of the Vasquez Formation.

This picture is the view along the far left side of the previous photograph and shows the trace of the San Francisquito Fault along the west side of the dam site. The gray to yellow unstained Pelona Schist is left of the vertical to nearly vertical fault (i.e., the trace undulates). The beds of the sedimentary rocks of the red Vasquez Formation are to the right of the fault in this picture.

The surface of this piece of Pelona Schist from the San Francisquito Fault zone shows slickensides, which are smooth/polished surfaces created by frictional movement along fault zones. 

This photo, which is up the road a small distance from the previous photo, is a typical outcrop of the red-colored Vasquez Formation after a rainstorm. As evidenced by the rock fall on the abandoned highway, when this formation gets wet, it is prone to falling apart. It is a poorly cemented ancient river-bed sandstone, which also contains veins of gypsum. Early geologists knew this, but the engineers who built the dam did not deem this information to be significant. They were hugely mistaken. The next three pictures prove my point.

The three pictures above show the same orientation of a chunk of the Vasquez Formation sandstone. The first picture shows the chunk laying loose on the abandoned highway. Notice the white seam of gypsum crystals. I put this piece of rock in a bucket of water for two days. The next pictures show the results of my experiment. The middle picture is after only two days, and the last picture is after two weeks. As you can see, the rock mostly disintegrated via absorption of the water. This geological process is called "slaking." Just imagine how much hydrostatic pressure a rock like this would experience at the bottom of a dam after a short time. 

The next three pictures show the VERY close proximity of landslides along the east side of the dam.

The view is to the east and shows the landslide scarps (their "heads" are just below a road) on the east side directly opposite the dam. These scarps are very steep and are underlaid by the Pelona Schist, which is this area has a very high angle of foliation (i.e., its component layers are steep enough to easily fail and cause landslides). The white rubble in the foreground is what is left of the concrete "wing" along the west side of the dam. The "wing" was demolished by workers after the dam failed. 

This is a side view of the landslide scarps, which are immediately east of the dam (now just a pile of rubble). Notice the series of stair-step-like surfaces of concrete just above the road on middle left side of the photograph. I showed a closeup of them in my previous post. The main reason I used this photograph is to show the steep slippage planes in the landslides areas (mentioned and shown above) of the Pelona Schist. These landslides are mostly ancient landslides (paleo-landslides), which are obvious to the trained eye but were not recognized as such by the builders of the dam.

This last view shows the frontal view of the landslide scarps, with the remains of the dam in the foreground.

At the Govenor's request, a commission investigated the failure of the dam. They found that it did not fail because of weak concrete. It failed because of many problems. Some of these are recognizable geologic problems and include: 1) The inevitable landslides associated with the poor quality rock and steep inclination of the foliated Pelona Schist along the east abutment of the dam; 2) the even worst quality rock of the crumbly, porous Vasquez Formation along the west abutment; and 3) the crumbly and weak nature of the actual fault zone itself.

For those who want to see some really great vintage photographs of the dam immediately after its collapse, please Google the following phrase: slaking St. Francis dam disaster. You ought to check it out; it is worth the effort. By the way, you will also discover that there have been several books written about the failure of the dam. A few are still in print.

Saturday, April 15, 2017

St. Francis Dam Disaster, Part 1: Before and After

The March 12, 1928 failure of the "Saint Francis Dam" in northern Los Angeles County in Southern California was one of the worst "natural" disasters in U.S. history and resulted in the second greatest loss of life (about 450 lives). Only the 1906 San Francisco Earthquake and its associated fire claimed more lives.

My coverage of the St. Francis Dam is "broken" into two parts.  This first part concerns the physical aspects of the dam, and the second part (the next post) concerns the geology of the site.

This photograph (from Wikipedia.com) shows the completed dam in 1928, which was built in 1924 through 1926. The view is to the northeast. The dam was 185 ft. high (56 m). The length of the main dam was 700 ft. (210 m), and the length of the wing, on the west side of the dam, was 588 ft. (179 m). The reservoir was 3 mi. long, and the maximum water depth was 182 ft (55 m). The reservoir was at full capacity (38,000 acre-feet), just before the dam collapsed.

This photograph, upstream view, (also from Wikipedia.com) shows the remnants of the dam after its collapse on March 12, 1928. Only the central part and most of the wing dike (on the west side of the dam) remained intact. This dam, which was part of the Los Angeles Aqueduct system, collapsed just before midnight. The wall of water was initially 125 feet high, and its initial velocity was 18 mph. The 12 billion gallons of water roared down the Santa Clara River Valley to the city of Ventura, which is at the coast. The water traveled a total of 55 miles, and it took 5.5 hours to reach the ocean. The debris was a deadly mixture of mud, barbed wire, wood, etc. Nearly all the victims never knew what hit them.

All the subsequent photographs (in both of the posts) were taken by me in January, 2011 and in April, 2017.

This is all that is remains today of the main part of the dam. Although the central part did not collapse during the failure of the dam (see first picture), it was eventually dynamited in order to prevent climbers from attempting to climb it and possibly fall (i.e., liability issues).

Remnants of the "steps"which were on the downstream part of the face of the dam.

Rebar sticking out of large chunks of the concrete rubble. The staff is 1.5 m (in 10 cm increments). The flat surfaces were once part of the "steps," mentioned above.

Other sides of the same blocks shown in previous slide.

Piece of cable which was added to the concrete mixture. The concrete consisted of large angular chunks of cobbles and boulders of local rock types, including the Pelona Schist (e.g., gray angular piece in lower left side of picture, next to the cable).

This is the floodplain about 0.25 mi. downstream from the dam site. As you can see, there are building-size chunks of the dam that were broken up and transported by the wall of water suddenly being released from the reservoir behind the failed dam. Most of these chunks, some weighing as much as 10,000 tons came from the western side of the dam. Most of the eastern side collapsed in place and remained near the dam. Some of the transported chunks moved as far as 3,000 ft. downstream. For scale: Notice the cars on new highway along the left side of picture.

San Francisquito Creek once again flows freely, right through the middle of the collapsed dam.

Friday, March 31, 2017

San Andreas Fault Displacement of a Distinctive Granite

The San Andreas Fault is one of the most famous geologic features of the world. It is common knowledge that the sides of the fault, relative to each other, are moving. This post concerns a distinctive granite that has been displaced approximately 160 km (100 mi.) by this fault.
Map showing locations (in red) of the distinctive granite that has been offset by the San Andreas Fault in Southern California. The arrows show the relative sense of offset along the sides of the fault. Artwork is by R. Squires.
The rock that has been displaced is, indeed, a type of the igneous rock granite. To be more precise, however, it is a megaporphyritic monzogranite. "Megaporphyritic" means that the crystals are of vastly different sizes. A monzogranite is the commonest type of granite with roughly equivalent amounts of orthoclase and plagioclase, along with abundant biotite.

Megaporphyritic monzogranite hand specimen (20 cm length) from Mill Creek Canyon. The large pink crystals are orthoclase, the white ones are plagioclase, and the black ones are biotite. 

The orthoclase crystal just right of the center of the above picture show twinning. Crystal twinning occurs when two separate crystals of the same substance share some of the same crystal lattice. Instead of a normal single crystal, the crystalline structure appears doubled. There are several minerals that commonly have twinned crystals, and orthoclase is one of them.
An exceptionally large crystal (4.5 cm length) of orthoclase from the Mill Creek Canyon locale.
The rocks, which are found in the Liebre Mountain area in northern Los Angeles County and also in the Mill Creek Canyon area of San Bernardino County, are believed to have been part of the same intrusive magma body that crystallized during the Triassic, about 215 million years ago. The two rock masses, which were both derived from melted continental crust of Precambrian age, share identical chemical compositions and geologic age. They both also suffered a Late Cretaceous thermal event about 70–75 million years ago.

Geologists refer to localities that show precise offsets along faults as "piercing points." Liebre Mountain is 100 km northwest of the Mill Creek area. This distance of offset is less than the generally accepted distance of about 240 km of offset along the San Andreas Fault.

Several years ago, I noticed a boulder-size piece of granite in the office of one of my colleagues, Dave Liggett. Over the years, I kept admiring the rock, and, one day, he kindly gave me some nice representative specimens. These are the same specimens illustrated here, and they are from the Mill Creek Canyon locale. He also provided the background story of this rock. 

Monday, March 20, 2017

Epidote (the coating form)

Ever since I started collecting minerals, I noticed green splashes of a material coating various pieces of granite and other various rocks. I learned later that the coating consists of the common mineral epidote (pronounced “ep-i-dote”).

Epidote is a calcium, aluminum, iron, hydroxyl-silicate mineral typically found in metamorphic-rock areas where alteration or replacement took place in association with hydrothermal fluids. Epidote is especially common in fractures or joints. Fibrous crystals of epidote can be dark-green, black, or even yellow.

The epidote I find, however, has a very distinctive pistachio or pea-green color. It occurs primarily as surface coatings on cobbles and boulders of biotite-rich granite, which weather out from sedimentary rock conglomerates, as shown below. The name “epidote” is derived from a Greek word meaning “increase,” in reference to its crystalline shape.

Epidote coating a clast (maximum dimension 5 cm) of granodiorite
found on a hiking trail in the Santa Clarita area, Los Angeles
County, Southern California.