The local San Gabriel Mountains have been created by what we commonly call, earthquakes. Earthquakes as we have come to understand are what we feel beneath us as various plates or segments of continental plates move against one another. Our local mountains are a result of the movement of the Pacific oceanic plate moving under or subducting beneath the North American continental plate. Further, movement, combining, shifting, or separating of land masses happens along fault lines. The plate boundary of the North American and Pacific plate just north and east of this location is well know to us, as the San Andreas Fault. Ice House Canyon lies along the Ice House Canyon fault, which extends east from the San Antonio Fault to the Stoddard Canyon Fault.
As movement occurs along fault lines and subduction zones, friction occurs. Friction generates heat. The greater the amount of pressure and friction, the more heat is created. As heat increases, rocks will change shape and even melt, turning into magma. As below ground pressures increase, this magma or molten (igneous) rock is pushed up towards the surface. Magma that escapes the earth's crust to quickly cool becomes extrusive rock. That magma that slowly cools as it rises forms intrusive rock.
Igneous rocks can be identified by their various characteristics. Extrusive igneous rock are fine grained or glossy, while the intrusive igneous rock have a coarser and larger crystalline structure. Intrusive and extrusive rocks are further characterized by their color, density and composition. Lighter colored rocks tend to be lower in density (weight) and are rich in silica (Si) and aluminum (Al), and contain potassium (K), sodium (Na) or calcium (Ca). These are know as 'felsic' igneous rocks (from feldspar and silica). Examples of felsic rocks include pumice (extrusive) and granite (intrusive) and contain more than 65-75% silica. Felsic igneous rocks are common adjacent to convergent boundaries. Darker colored rocks are higher in density and while still containing silica, are rich in magnesium (Mg) and iron (Fe), and are known as 'mafic' igneous rocks. Examples of mafic rocks include basalt (extrusive) and gabbro (intrusive) and are less than 45-55% silica. Hot mafic magmas produced near subducting slabs may induce partial melting and assimilation of the continental (granitic) crust. Those rocks with silica content of ~55-65% are considered intermediate and include andesite (extrusive) and diorite (intrusive). The silica content of the granite in the Ice House Canyon area is 72%. It is interesting to note that the denser mafic igneous rock weathers, decays or erodes at a faster rate than the lesser dense felsic igneous rock.
Sedimentary rock is created when layer upon layer of finely eroded rock exerts pressure on the lower layers until the heat combines the granules into rock. Examples of sedimentary rock include shale and sandstone. Metamorphic rock is created when sedimentary rock is subject to increasing pressure and temperature. As the temperature of the rocks increases, the grain size increases and further heat and pressure begins to orient the minerals into layers. As the heat continues to rise, those layers within the rock will begin to melt and compress, beginning to fold. Rocks that appear to have folded layers are called migmatite. Eventually, the increasing pressure and heat will melt the rock and turn back into magma to start the process all over again.
To claim a find on this earthcache, please conduct the following observations and answer the related questions. Please email the owner using the link at the top of the page. You may log as a group. Photos are always welcome as long as they do not give away any of the answers. Your log will be deleted if you give away the answers or if your answers are not received within seven days of your posted find.
A. GC6V5Z0 Faulty Magma in Ice House Canyon
B. The number of people in your group and names of all geocachers if submitting as a group.
1. As you are hiking in to the posted coordinates, make some observations of the scree slope to your left (north of the trail). Describe the rock size, type of rock, and where you think the rocks came from/how they got there. Include the coordinates of where you made your observation. You may share these coordinates with others in your post when you log your find.
2. Locate the very largest boulder to the left (north) of the trail at the posted coordinates. GPS signal reception may be difficult in the canyon here, so you may need to look around just a little bit, but you won't need to leave the trail. You'll know the boulder you're being taken to when you see it. Look at the entire boulder, not just one or two sides. Based upon the reading and your observations of this boulder, describe the boulder, type(s) of rock you see in it, measurements of the boulder, type and size of any inclusions, layering, and provide an explanation of how it may have formed and which side/end is the 'bottom' (use compass directions to describe). Include the measurements/dimensions of the boulder.
3. Compare the rocks from the scree slope to those found in the area of the posted coordinates. Describe.
4. Where do you think this boulder and the other similar rocks and boulders came from and how they got here? Is this the same as your answer to Q#1? Why?
I encourage you to print the cache description or at least the images of the rocks if you are unfamiliar with the identification of some of the rocks discussed here.
Resources:
http://blogs.agu.org/georneys/2011/08/28/geology-word-of-the-week-m-is-for-migmatite/
https://en.wikipedia.org/wiki/Mafic
https://en.wikipedia.org/wiki/Felsic
https://en.wikipedia.org/wiki/Igneous_rock
https://en.wikipedia.org/wiki/Metamorphic_rock
https://en.wikipedia.org/wiki/Sedimentary_rock
Google Earth fault line and plate boundaries KML overlays: http://earthquake.usgs.gov/learn/kml.php