How To Get There
“From the Donner State Park exit on I-80 drive west on Donner Pass Rd 6 miles; turn right at the ‘Vista Point’ sign for the McGlashan Point parking area; walk to the SE end of the overlook wall.” (Geologic Trips: Sierra Nevada)
Short Donner Party History
In May 1846, the Donner party started to California but were delayed in their journey, so when they got to the Sierra Nevadas months later, they were blocked by over 20 feet of snow and had to spend the winter around Donner Lake, which you can see from this overlook. Of the original party of 89, 42 died. Eventually, some of the party were able to make it over the Sierras and bring back help. You should check out the good visitors’ center at Donner Memorial State Park.
Magma and Intrusive Igneous Rock
In general, magma (molten rock) enters the crust from below, rising through the crust because the magma is less dense than the surrounding rock (called “country rock”). If the magma makes it to the surface, a volcano forms.
But not all magma makes it to the surface. A lot of it remains trapped in the surrounding rock where it solidifies into igneous rock. The igneous rock formed within the crust takes a long time to solidify because the surrounding rock insulates the magma, which keeps the heat in, like a Thermos keeps coffee hotter for longer. The longer cooling time allows the minerals forming from the magma to grow bigger to the point that you can see them with your naked eyes. On the other hand, lava (magma that made it to the surface) that forms a volcano, cools quickly when it’s exposed to water or air, and the minerals solidify so quickly that they can’t grow big and cannot be seen with the naked eye.
Magma that rises up into the country rock is called an “intrusion” and the rock formed when it solidifies within the country rock is called “intrusive igneous rock.” Igneous rock formed at volcanoes is called “extrusive igneous rock.” You can tell the difference between intrusive and extrusive rocks by how big the minerals in the rock are, as explained above.
Types of Intrusions
Dikes - Intrusions that cut vertically or diagonally through the country rock.
Sills - Intrusions that run parallel to the rock layers.
Laccoliths - Mountain-sized intrusions where so much magma enters the crust that the thousands of feet of overlying rock layers are domed up. The La Sal Mountains in southeastern Utah is a laccolithic mountain range.
Batholiths - These are the biggest of the intrusions. For an intrusion to be considered a batholith, at least 40 square miles (100 square kilometers) of intrusive rock must be exposed. Batholiths are usually composed of granitic igneous rock.
Tasks
Email me the answers to these questions. Do not put the answers in your log.
At the coordinates look down at the white stripe in the rock at your feet. This white stripe is an intrusion of alaskite, an igneous rock composed almost solely of quartz and feldspar minerals. “It probably represents the last of the magma in the magma chamber and was intruded into the [country rock] that had already [solidified].” (Geologic Trips: Sierra Nevada)
1. On the 1st line of your answers, give the name of this Earthcache.
2. Which of the 4 kinds of intrusion described above is this white stripe?
3. Why do you say that?
4. Measure the width of the intrusion.
5. Optional: What kind of igneous rock surrounds the intrusion?
6. Optional: Post a picture of yourself at this overlook. Look happy to be there!
Sources
- Geologic Trips: Sierra Nevada by Ted Konigsmark. The Trip #5 chapter.
- Wikipedia: the “Donner Party.”