Welcome to Lassen Volcanic National Park, a place full of complex geology. In this EarthCache, we will explore an interesting example of Lassen's eruptive history. Be sure to pack plenty of water, some snacks to eat, and perhaps even a magnifying glass for this adventure.
Visiting Lassen Volcanic National Park requires a $30 per vehicle entrance fee. This fee can also be covered by the America the Beautiful pass (annual, senior, military, 4th grader, or access) so if you have one of those, please remember to bring it.
The location of this cache is car-accessible seasonally, since the park road closes during the winter. Check the Lassen Volcanic National Park website (nps.gov/lavo) for current conditions. Skiing or snowshoeing to the cache along the road in the winter is an option, but it will be a very long trip.
Follow the cache coordinates to a pullout with three large boulders. Let's look at the largest boulder, which is a rich black color with white crystals. This rock is an example of Lassen's black dacite, which was formed on May 14, 1915. Lava welled into the summit's crater and solidified into this dacite, which blocked off the vent. Five days later, on May 19, Lassen erupted again and the dacite shattered and an avalanche carried it downhill to where you are standing now.
Volcanic rocks are determined and named by their silica (SiO2) content. Here is a chart that describes the main types of lava:
Volcanic rocks can also be dominated by different types of minerals, depending on how slowly the lava was able to cool. If a lava runs onto the surface of the Earth, the lava cools quickly and the mineral crystals that grow tend to be small. These rocks are called extrusive, because the lava turns to rock outside of the volcano. However, if the lava is able to cool within the volcano, either in its vent or its plumbing underground, then there is plenty of time for large minerals to grow. These rocks are called intrusive, because the lava turns to rock inside of the volcano.
Let's take a look at what kinds of minerals grow from different types of lava:
Drawing a vertical line down from the dacite heading, we can see that dacite rocks are typically dominated by a mineral called plagioclase feldspar. Look at the rock in front of you. Do you see all of the crystals? Those are plagioclase feldspar!
To earn credit for this EarthCache, please send me your answers to the following questions. Please do not put your answers in your log!
1.) What is the silica content of dacite?
2.) Based on its silica content, does dacite come from a more sticky or a more runny kind of lava?
3.) Is dacite an extrusive or an intrusive rock?
4.) What is the approximate volume percentage of plagioclase feldspar within the largest rock in front of you? You may find this chart that shows what different percentages look like helpful.
5.) What do the feldspar crystals look like? Please describe their shape, color, and anything else you notice.
Pictures are not required for completing this cache, but if you would like to add a picture of your trip to Lassen Volcanic National Park to your log, you are more than welcome to do so! Thank you for visiting!