Here's a breakdown:
* Red Bluff Granite is a large, intrusive body of granite that formed deep underground in the Sierra Nevada batholith. It is older than the Thunderbird Rhyolite, forming roughly 100-90 million years ago during the Late Cretaceous.
* Thunderbird Rhyolite is a volcanic rock, meaning it formed from erupted magma. It is younger than the Red Bluff Granite, and erupted around 85-75 million years ago during the Late Cretaceous.
The critical part of their relationship is that the Thunderbird Rhyolite actually intruded into the Red Bluff Granite.
Here's how that likely happened:
1. The Red Bluff Granite cooled and solidified deep underground.
2. Later, magma rose from below, eventually erupting as the Thunderbird Rhyolite.
3. Some of this magma actually intruded into the pre-existing Red Bluff Granite before reaching the surface.
This intrusion can be seen in outcrops where the Thunderbird Rhyolite can be seen cutting through the Red Bluff Granite. This kind of relationship is common in geology, where younger, more mobile rocks often intrude older, solidified rocks.
The specific location of this intrusion likely influenced the final composition of the Thunderbird Rhyolite. The contact with the granite may have contributed to the presence of certain minerals or the overall chemical makeup of the volcanic rock.
In summary, the Red Bluff Granite and Thunderbird Rhyolite share a close temporal relationship, with the granite serving as the older, intrusive body and the rhyolite as the younger, intrusive rock. This interaction between the two rocks highlights the dynamic nature of the Earth's crust and its continuous evolution through geological time.
