This EarthCache involves visiting two stages along Schmidt Lane (the posted coordinates and waypoint "Stage 2") and then sending your answers to the questions on the cache page for credit. The questions are below to facilitate copying and pasting into the website's messaging system. They are also within the description to help guide you through answering them.
Send me a message (preferred) or an email with the answers to these questions:
1. Describe the rock in the outcrop at the posted coordinates focusing on (a) the color and (b) the size of individual grains. (c) Which of (a) or (b) is more helpful in determining that these rocks have undergone metamorphosis?
2. What's one plausible depth and temperature for the formation of the rocks in the outcrop at the posted coordinates? (hint: where does line 5 intersect the dark purple area for P-P facies?)
3. Do you see any large blocks of material in the Hillside Melange, and what type of rock might they be?
4. What's one plausible depth and temperature for the formation of the rocks in the Angel Island Nappe? (hint: where does line 5 intersect the blue area for blueschist facies?)
5. Based on your answers to questions 2 and 4, what kind of fault zone is preserved in the quarry hillside?
6. Please post a photo of either yourself or your signature item with the view of the quarry in the background in your log.
Introduction
Stage 1 will take you an outcrop where you will make a few observations about the rock in front of you. Stage 2 will take you to the area in front of the El Cerrito Recycling Center, where you will make a few observations about the hillside exposed about 500 feet to the northeast. Both locations are open to the public 24/7/365.
Metamorphic facies (pronounced fay-sees)
In geology, a facies is a body of rock with shared, distinctive characteristics. A metamorphic facies, in particular, is a body of rock that was subjected to a specific range of pressure and temperatures. We can identify the metamorphic facies of a body of rock based on the specific minerals within the rock. As rocks move through different pressures and temperatures involved with metamorphosis, the minerals change. Not only do they change in composition, but they also tend to become larger with higher degrees of metamorphosis. In practice, we would identify the minerals in an outcrop to determine its metamorphic facies and then use that facies to determine the pressure and temperature range the body of rock was subjected to.
The diagram below summarizes the possible range of metamorphic facies we can find in nature along with several different paths that bodies of rock can take as they are subjected to specific tectonic conditions (the numbered lines). All rocks start out in the top left of the diagram, where diagenesis simply means the formation of a rock from its constituent parts (sediment for sedimentary rocks and cooling lava/magma for igneous rocks). Depending on the specific tectonic regime, the rock will follow different lines. The most common is line 4, where rocks undergo burial as more material collects on top of them over millions of years.
At this location, we find two metamorphic facies: prehnite-pumpellyite (P-P on the diagram above) and blueschist. If you look at the diagram, the fact that we're examining these two facies should inform you that we're looking at the remnants of an accretionary prism (line 5 in the diagram above), which is part of a subduction zone. Rocks that undergo subduction follow a pressure-temperature pathway that moves through the following facies, in order: zeolite, prehnite-pumpellyite (sometimes classified as a sub-category of zeolite), blueschist, and eclogite. Beyond eclogite, the rocks simply melt, and any information about metamorphism is lost. Subduction metamorphism is unique in that it involves high pressure, low temperature conditions -- this is distinct from the typical burial pathways (3 or 4 on the diagram above), where pressure and temperature rise in tandem. This style of metamorphism is also unique in that involves dehydration of the subducting slab material. This is what drove metamorphism at this location. Ocean water was slowly incorporated into the rocks when they were in the accretionary prism underneath the ocean. As they were subducted beneath the Earth's surface to greater and greater depths, that ocean water was slowly squeezed out of the rocks, which is why we refer to it as "dehydration."
The outcrop at the posted coordinates is metagraywacke, specifically prehnite-pumpellyite facies metagraywacke. That is, the rock initially formed as graywacke and was later subjected to low pressures and temperatures. Graywacke is a type of sandstone characterized by its angular, poorly-sorted grains of sand. It forms from underwater landslides along the ocean floor. Graywacke is so named because it is almost always light to dark gray in color. We use the term metagraywacke to indicate that the graywacke here has undergone metamorphism, specifically due to partial subduction. The prehnite-pumpellyite facies is one of the lowest grades of metamorphism, and it is actually quite difficult to tell that the rocks in the outcrop have undergone metamorphism without a microscope. There is one key indicator, however, based on the initial color of graywacke.
Question 1 (posted coordinates): describe the rock in the outcrop focusing on (a) the color and (b) the size of individual grains. (c) Which of (a) or (b) is more helpful in determining that these rocks have undergone metamorphosis?
Question 2 (posted coordinates): what's one plausible depth and temperature for the formation of the rocks in this outcrop? (hint: where does line 5 intersect the dark purple area for P-P facies?)
Nappes (pronounced naps)
At this site, we can see three distinct bodies of rock:
- Alcatraz Nappe (visible in the outcrop at the posted coordinates): prehnite-pumpellyite facies metagraywacke
- Hillside Melange (visible from Stage 2 at the bottom of the hillside): prehnite-pumpellyite facies shale, sandstone, and basalt matrix
- Angel Island Nappe (visible from Stage 2 at the top of the hillside): blueschist facies metagraywacke
A nappe is a large, sheetlike body of rock that has moved a considerable distance (more than 5 km) from its original position due to a thrust fault. The two nappes here are named after the locations where they were first described, but they occur throughout the North Bay and East Bay. Essentially, a nappe is a sign that older/deeper material has been placed on top of younger/shallower material. Unfortunately, there are no outcrops of the Hillside Melange that we can look at closely, so we can only view it from a distance. You may notice that the melange contains blocks of material. This is a sign that it formed in a highly chaotic environment, where chunks of rock were ripped apart from each other. This is characteristic of an accretionary prism, where rocks are ripped off the subducting plate by the megathrust and churned by the action of the two plates moving past each other. The megathrust is the primary boundary between the two plates -- it's where the largest earthquakes occur. This accretionary prism formed during the subduction of the Farallon Plate beneath the North American Plate around 120 to 40 million years ago.
The diagram below summarizes the processes that occur in an accretionary prism. There is a lot going on in the diagram that is beyond the scope of this EarthCache, but hopefully it can help you envision a wedge scraping material off the subducting plate and churning material along the subduction interface (i.e. the megathrust).
Question 3 (Stage 2 coordinates, lower part of the quarry hillside): do you see any large blocks of material in the Hillside Melange, and what type of rock might they be?
Question 4 (Stage 2 coordinates): what's one plausible depth and temperature for the formation of the rocks in the Angel Island Nappe? (hint: where does line 5 intersect the blue area for blueschist facies?)
Since all of these units of rocks formed at different depths and now lie on top of each other, we know that there are faults between them. They are not active faults, because they preserve the tectonic history of a plate boundary that no longer exists. As a result, we would refer to them as paleofaults. A transform paleofault would be represented by two blocks of the same material that have been horizontally offset. A thrust paleofault would be represented by an older/deeper block of material on top of a younger/shallower block of material. A normal paleofault would be represented by a younger/shallower block of material on top of an older/deeper block of material.
Question 5 (Stage 2 coordinates): based on your answers to questions 2 and 4, what kind of fault zone is preserved in the quarry hillside?
Question 6 (Stage 2 coordinates): please post a photo of either yourself or your signature item with the view of the quarry in the background in your log.
Please Note:
1. You must answer all the questions in your message/email to me.
2. Your logging of this cache and messaging/emailing me should happen at around the same time.
3. I may have to delete your find if I don't have sufficient evidence you visited the site.
The above information was compiled from the following sources:
https://seismo.berkeley.edu/~burgmann/EPS116/labs/Lab_08_Marin/Wakabayashi_IntGeolRev_2015.pdf
https://seismo.berkeley.edu/~burgmann/EPS116/labs/Lab_08_Marin/Wakabayashi_GSA_Field%20Guides_2013_COLOR.pdf
https://en.wikipedia.org/wiki/Subduction_zone_metamorphism
https://en.wikipedia.org/wiki/Metamorphic_facies
https://en.wikipedia.org/wiki/Nappe