This EarthCache involves visiting the posted coordinates 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: (a) Do you see any evidence of flow-banding in the rhyolite at the posted coordinates? (b) Based on your answer to (a), what does that tell you about the viscosity of the rhyolite at GZ relative to other outcrops in the area?
2: I included the location of an outcrop of the Burdell Mountain volcanics as a reference point in the waypoints for this EarthCache. When you're at GZ, how far away is this other waypoint?
3: Assume that motion on the Hayward Fault has been constant and purely horizontal since the formation of the Northbrae rhyolite and Burdell Mountain volcanics 11.1 million years ago. Based on your answer to Question 2, what's the rate of relative motion between these two formations, in inches per year? To convert from miles per million years to inches per year, you can multiply by 0.06336.
4: The Hayward Fault is currently moving at a rate of 0.2 inches per year. Based on your answer to Question 3, has motion on the fault changed significantly over the past 11 million years?
5: Please post a photo of either yourself or your signature item with a view of the bay from GZ in the background in your log.
Introduction
The posted coordinates will take you to an outcrop of the Northbrae rhyolite in a residential neighborhood. This area is open to the public 24/7/365, but please stay on the sidewalk to make your observations. The rocks in front of you are flow-banded rhyolite that formed as the result of volcanic eruptions around 11 million years ago. This rhyolite tends to be light tan to medium brown in color and aphanitic (its grains aren't visible to the naked eye). These eruptions produced felsic-rich rocks, meaning that the viscosity of the erupted lava was very high. As a result, you may notice flow-banding that developed in the solidified rocks. If the viscosity of the erupted lava is too low, flow-banding won't form or will be weak and difficult to recognize. The flow-banding tends to be more pronounced at other outcrops in the Berkeley hills. Some sections of the rhyolite are more tuff-rich, meaning that they formed when volcanic ash solidified rapidly due to the extreme change in temperature after being erupted.
The above image is an example of flow-banding in the Northbrae rhyolite. You can identify it as the linear features going from the bottom left to the top right in the photo.
Question 1: (a) do you see any evidence of flow-banding in the rhyolite at the posted coordinates? (b) Based on your answer to (a), what does that tell you about the viscosity of the rhyolite at GZ relative to other outcrops in the area?
Hayward Fault dynamics
The Northbrae rhyolite is in close proximity to the Hayward Fault -- the fault is about 1000 feet to the northeast of the posted coordinates. What's really interesting about the rhyolite is that the volcanic vent that the lava erupted from has not yet been definitively located. There are no viable candidates for a volcanic vent within the East Bay, leading many geologists to believe that the source of the rhyolite has been separated from these outcrops by lateral motion on the Hayward Fault. In other words, the lava was deposited across the Hayward Fault from the vent, and motion on the fault over millions of years separated the two deposits. A recent study (Henschel et al., 2024) performed geochemical analyses of the Northbrae rhyolite and other volcanic systems in the Marin area and found that the age of the Northbrae rhyolite (11.1 million years old) matches that of the Burdell Mountain volcanics located just north of Novato. The relative abundances of trace elements in both volcanic systems closely match, as well. These observations suggest that the Burdell Mountain volcanics are the source of the Northbrae rhyolite, and that these exposures were separated by the Hayward Fault over the past 11 million years.
Question 2: I included the location of an outcrop of the Burdell Mountain volcanics as a reference point in the waypoints for this EarthCache. When you're at GZ, how far away is this other waypoint?
Question 3: assume that motion on the Hayward Fault has been constant and purely horizontal since the formation of the Northbrae rhyolite and Burdell Mountain volcanics 11.1 million years ago. Based on your answer to Question 2, what's the rate of relative motion between these two formations, in inches per year? To convert from miles per million years to inches per year, you can multiply by 0.06336.
Question 4: the Hayward Fault is currently moving at a rate of 0.2 inches per year. Based on your answer to Question 3, has motion on the fault changed significantly over the past 11 million years?
Question 5: please post a photo of either yourself or your signature item with a view of the bay from GZ 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://www.tandfonline.com/doi/full/10.1080/00206814.2024.2355620#d1e983
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://oaklandgeology.com/2023/05/29/the-el-cerrito-rhyolite/