Evidence of a Dynamic Earth: Pillow Lavas, Faults, and Plate Tectonics
A Note of Encouragement:
Welcome to the Klamath National Forest Earth Cache program! Most of the Earth Cache locations are located away from established recreational trails and roads. Visitors are strongly encouraged to check with Ranger District offices for information on current road and trial conditions before visiting sites, and be prepared for cross-country hiking.
Layered clothing, water, and appropriate maps are essential when traversing the remote, rugged landscape of the Klamath National Forest. Please be aware of wildlife and poison oak while visiting Earth Cache locations in the forest and use caution when traveling on narrow winding forest roads.
Location Information:
Coordinates: 41° 21' 26.471" N 122° 36' 38.145" W
To get to this Earth Cache from interstate-5, take the exit 766 onto Old Highway 99 S. Go south on Old Hwy 99 S for 9 miles. Then turn right onto Gazelle Callahan road and follow this road for 16.5 miles. You will then turn left onto Forest Service road 41N08 or rail creek road towards Kangaroo Lake for 3.8 miles until you reach Forest Road 41N10. Turn left onto 41N10 and follow it for 2.5 miles. Once you reach the 41N10 and 41N03 intersection, high clearance and 4WD vehicles are suggested due to road conditions (Figure 5). You can choose to walk or drive the rest of the way on 41N10. There is no trail for this Earth Cache, so you will need to use your GPS to locate it.
This Earth Cache does require you to hike up a steep mountain side, so please use caution and make sure you wear boots with plenty of ankle support.
Geological Information:
Pillow structures are not only cool looking, but they are an excellent indicator of tectonics. Pillow structures exclusively form when lava flows under water and rapidly cools. These distinctive features can be found forming today along submarine volcanic vents. When the lava erupts from these vents and comes in contact with the ocean, the outer layer instantly begins to harden. Because this outer layer hardens, heat is trapped inside causing the lava to expand and escape through cracks. This will continue until the lava stops flowing, leaving structures that resemble a pile of pillows or a mound of grapes.
The Cabin Meadows Pillow Lavas formed as a result of this process. The composition of the lavas is basaltic containing minerals such as pyroxene (green/black color) and plagioclase (light color mineral rich in calcium). The Pacific Ocean is around 75 air miles West of Cabin Meadows Pillow Lavas Earth Cache site. These pillow lavas are thousands of feet above sea level. The only way to account for these structures being on this hillside is by plate tectonics and faulting. Therefore, sometime after the pillow structures cooled, a surge of plate movement thrusted the pillow lavas upward and eastward.
Plate tectonics is the concept that plates (pieces of the Earth’s lithosphere) move over the asthenosphere. Plate tectonics is the reason why we have mountains, volcanoes, and faults.
The Klamath Mountains are the result of subduction and accretion, a plate tetonics process. Subduction occurs when one plate (oceanic crust) slides under another plate (continental or oceanic crust). Material scrapes off the top of the oceanic crust as it subducts, causing material to build up, or accrete onto the continent. This material is called an accretionary wedge. Detachment faults form throughout the wedge as it grows because of limited space. Faults uplift this material creating mountians.
When the Farralon oceanic plate subducted under the North American plate, an enourmous amount of material was deposited and thrusted upward.
Pillow lavas aren’t the only indicators of plate tectonics at this site. Evidence of faulting is also present. Faults can cause rocks and structures in rocks to deform; sometimes it can even form fault breccia (Figure 3). Breccia is a rock defined by the angular grains within a fine-grained matrix. Slickenlines are another indicator of faults and movement. These are very small lines or striations on a surface (Figure 4). Offsetting of rocks and layers is another fault indicator.
We know that the Klamath Mountains formed as a result of subduction and accretion because of their composition and structures. Marine fossils in limestone and pillow structures on mountains indicate that subduction and faulting occurred.
When you visit this site make sure you take in the view. On clear days you can see for hundreds of miles. Also notice the landscape surrounding the site; this area has many glacial landforms (see our Earth Cache on Cement Banks).
Logging Requirements and Questions:
- The text “GC41MBA” on the first line
- Number of People in group
- The date or season
- What is the elevation of the Pillow Lavas? Think about the distance these pillow lavas travelled vertically.
- What evidence of faulting do you see?
Contacts:
Salmon/Scott River Ranger District
11263 N. Highway 3
Fort Jones, CA 96032-9702
(530) 468-5351
FAX (530) 468-1290
TDD (530) 468-1298
Klamath National Forest
Supervisor's Office
1711 South Main Street
Yreka, CA 96097-9549
(530) 842-6131
FAX (530) 841-4571
TDD (530) 841-4573
Resources:
Hatcher, R. D. (1995). Structural geology: principles, concepts, and problems. (2 ed.). Englewood Cliffs, New Jersey: Prentice Hall.
Prinz, M., Harlow, G., & Peters, J. (1978). Rocks and minerals. New York, NY: Simon & Schuster.
Winter, J. D. (2010). An introduction to igneous and metamorphic petrology. (2nd ed. ed.). Pearson College Div.
Additional Information:
If you are interested in seeing other geologic sites in the Klamath National Forest please visit us at http://www.fs.usda.gov/attmain/klamath/specialplaces for more information.*
*Not all locations are Earth Caches.
If you would like to suggest a site/location in the Klamath National Forest for Earth Caching please send us an email at knfearthcache@fs.fed.us
