Visiting California's San Andreas Fault

Where can I actually see/touch the San Andreas Fault? I'm visiting California in a month, and have long wanted to visit this famous earthquake fault.
Marsha Beatty
Salt Lake City

Map of the San Andreas system. USGS

Marsha,
The San Andreas fault is visible in many places. You might want to get a copy of this book to serve as your guide: Field Guide to the San Andreas Fault  (Caveat: I haven't read it, and don't know the author.)
The San Andreas is the boundary between the North America and Pacific plates. The fault extends from the Gulf of California (Sea of Cortez) northwest for 810 miles across California, from the Salton Sea trough to north of San Francisco, entering the sea south of Fort Bragg. The fault is far longer than that, as it continues into the Pacific Ocean, where it separates the Pacific Plate from the Juan de Fuca Plate.
The fault runs right through Carrizo Plain National Monument, established in 2001 to highlight the geology. The USGS has an online field guide to San Andreas features there. The National Monument is located about 160 miles north of Los Angeles, 260 miles south of San Francisco, about 55 miles west of Bakersfield, and and 50 miles north of Santa Barbara. The San Andreas is a 'right lateral fault'. That means that if you are standing on one side looking

across the fault's trace, objects on the other side have been offset to your right.

Famous sign just south of Parkfield, California.

 

The Pacific Plate is the rock on the right.
North America is the tan rock above the car.
Photo by David Lynch.

The field guide mentioned above will tell you how to find this fantastic exposure:








The patch on this road repairs damage from motion along the
fault, and has to be periodically replaced.

Try this website to visit some features in the Hollister- Pinnacles National Monument area; here and here.

This blog has photos of features along the San Andreas in the LA area.

This website is a field guide to the famous Wallace Creek site on the Carrizo Plain, probably the best-known feature on the San Andreas.




Wallace Creek--- This view of the Wallace Creek on the Carrizo Plain is perhaps the most famous feature on the San Andreas Fault. As the stream continues to progressively erode into its channel, right-lateral motion along the San Andreas Fault has moved the downstream portion of the channel northward (towards the viewer). Sediments cut by Wallace Creek were radiocarbon dated at about 3,700 years old, and the stream channel has been offset about 430 feet since then. USGS.

What causes the Basin and Range?

Dave,
I drove across US 50 in Nevada this summer. Beside being hotter than blazes, I was struck by the repeated miles-long straightaways and then steep climbs over mountain ranges. This sequence was repeated over and over. This has got to be geology, right?
Joe DeMarsh,
Dallas

Dear Joe,

You have successfully learned Doctor Rock's First Law of Everything: GEOLOGY MADE THAT! Nevada is in the Basin and Range physiographic province. This huge area is characterized by alternating tilted mountain ranges and flat valleys. These are oriented more-or-less north-south. US 50 runs right across the middle of the state, from east to west, against the grain of the Basin and Range.

Characteristic Basin and Range topography- mountains rising
several thousand feet separated by 10-20-mile-wide sediment-
filled basins. Click to enlarge any figure.

 The Basin and Range is formed by stretching of the crust in the arid US west. The crust is thin here, only 18-20 miles thick. It is spreading laterally on top of the weak, ductile mantle. This motion causes the crust to fracture along faults that are angled downward at around 60 degrees. The rock on one side of these fractures slides down relative to the other, so these fractures become faults that occasionally slip as earthquakes.The movement along these angled faults allows the crust to 'extend' laterally, east to west. There has been so much extension in the Basin and Range that the crust has stretched until it is twice as wide as it was before extension began.

Development of the Basin and Range Province
http://www.iris.edu/hq/inclass/lesson/brittle_vs_ductile_rocks

The blocks of rock that drop down along the faults form the long valleys that separate the parallel rotated mountain ranges. This topography is the signature of the Basin and Range province.



Two ways to get alternating ranges and valleys.
The Basin and Range developed via the
model on the right.



The Hanaupah fault scarp in Death Valley National Park.

The fault cuts across an alluvial fan, uplifting the mountain

range relative to the valley floor. Many thousands of small

uplifts and downdrops along faults like this are needed to

create the basins and ranges.Photo by Marli Miller
http://geomaps.wr.usgs.gov/parks/province/basinrange.html

 

Extension in the Basin and Range began around 17 million years ago and continues today. You can read about a 1983 magnitude 7 Basin and Range earthquake in Idaho here.

Basin an Range earthquakes in the first week of August, 2013
http://earthquake.usgs.gov/earthquakes/map/.

A must-read laypersons book about the geology here is "Basin and Range" by John McPhee. The book is also included in his later anthology, "Annals of the Former World". Wonderful mind expanding reading by a nongeologist.

Geology tools to keep in your vehicle

Doctor Rock,
   I really enjoy stopping along roadsides to look at rocks. I don't know much about geology but love to collect cool-looking rocks. I've been learning some things reading your blog and want to get a bit more serious about geology. What tools and books do you recommend?
Fred Armens,
Chicago

Fred,
   Good idea to have the basic tools in your vehicle- you never know when you'll want them. These would include a rock hammer (not a carpenter's hammer!) and a hand lens. You'll want eye protection when you use the hammer. A geology guide book to the areas you visit is also a good idea. A geologic map is always a good idea to have handy. If you are in an area where there are surface deposits like ash layers or lake beds, a gardening hoe is useful to scrape at exposures so you can see the layers better.

Chisel style

pick style

Rock hammers- These are made for bashing rocks. The idea is to break a hand sample off of an outcrop, or to break open a stone so you can better examine the fresh rock inside. Rock hammers come in two basic styles. Some have a chisel-shaped end, and others have a pointed pick end. The pointed end is nice for sticking into a small crack to help pry a rock loose. The chisel end is better for splitting fractures in rocks. Both work fine; it is the hammer head that you will use 90% of the time anyway. Get a rock hammer that is heavy enough to smack a rock. You might find a mason's hammer in a hardware store, but these usually  have short handles that don't give you much leverage when you swing it. There is a good description of rock hammers on this wikipedia page: http://en.wikipedia.org/wiki/Geologist%27s_hammer. Pay attention to the discussion of hammer weight. Don't forget the safety glasses! Sharp chips or dust is no fun in your eye. If someone else is swinging that hammer, keep your distance and don't watch them!
Hammers are often placed in outcrop photos to give a sense of scale. So, here's an old geology joke: How do you know a person is a geologist? They have more photos of their rock hammer than the spouse and kids!

Hand lens- By this I mean a jeweler's loupe, not the magnifying glass Sherlock Holme;s uses to read fine print and look for clues. Use your lens to look closely at a rock to see the mineral grains or fine details in the rock. Or pick a sticker out of your hand. These lenses are held right up against your eye, with the rock you are examining very near the lens. You want a 10 power (10x) lens  fro most usage. People who want more detail will find a 14x lens useful.

How to use your hand lens. First, look at the rock without the lens. See something tiny and interesting? Then mark it with your thumb, place the lens over the flashing mineral crystal or whatever you saw, and bring rock and lens to your eye. You will want plenty of light, so this method works best outside in natural light, and with the sun shining over your shoulder- turn your back to the sun.

First, spot what you want to more closely examine...

Guide books- In an earlier post I mentioned some of these. Especially useful to the layperson are the Geology Underfoot and Roadside Geology Series by Mountain Press Publishing Company. There are many other geology field guides written for a popular audience. Search the web to find something in the area you are interested in. Good idea to include 'geology field guide' in your search terms.

Then bring lens and rock up to your eye.

 

Geologic maps remind me the crazy quilts my wife makes.

Geologic map- These take some getting used to. Maybe you can find a local geologist to give you a quick lesson. There are symbols denoting faults, the orientation of rock layers, and colors for each distinct rock unit. You may find these maps for sale from your state's geology agency, or by searching the web for USGS maps.

Road map- get a good one. Your field guide may send you off on minor dirt roads that aren't shown on state-wide maps. There are topographic gazeteers to many states. Check the offerings by DeLorme.

'Small scale' vs. 'large scale' maps- The larger the scale, the more detail. I know this confuses people, it did me once upon a time. Map scales are given in fractions: 1:24,000 means 1 inch on the map = 24,000 inches on the ground, or 2,000 feet. 1:150,000 means there are 150,000 inches on the ground to one on the map ( that's 2.37 miles per inch). So, to remember which is 'small scale' and which is 'large', think of the fraction. Would you rather have 1:2 of pie, or 1:5? I don't know about you, but I'd rather have 1/2 a pie than only a fifth!