Opal Cone is one of the interesting geological features in the Diamond head area of Garibaldi Provincial park. It is a one of the few cinder cones in the park, one of which is located on the north side of the Black Tusk. Opal Cone is located at the base of Mt. Garibaldi's south east side. It remains as a reminder of the extreme volcanic activity that occurred in the park thousands of years ago.
Opal Cone is a cinder cone, which is formed over top of an erupting volcano. The unusual cone shape is created by a large layer of cooled volcanic material ejected by the eruption. When the volcano erupts, it shoots up large amounts of lava up into the air, which when it cools, breaks up into small pieces. These pieces, along with a large amount of volcanic ash land on top of the volcano, creating a large pile that ends being strangely symmetrical and smooth. This pile, after a bit of weathering turns into the cone we see today. If you look around, you can even see a small bowl at the top, which is the remnant of the volcanic crater.
Cinder cones commonly occur on the base of a much larger volcano; in this case Opal Cone formed at the base of Mt. Garibaldi, a large stratovolcano which erupted roughly ten thousand years ago. It was this latest eruption that caused Opal Cone to erupt creating the cone we see today. When Opal Cone erupted, it started a a lava flow that followed the ring creek valley. Because this lava flow didn't encounter any glacial ice, it traveled for over fifteen kilometers before finally stopping. Another thing that made this lave flow rather unusual, was that the lava was made up of a type of volcanic rock called dacite, when normally lava flows that large are made up of basalt.
On the trail up to the top, you can see evidence of the glacial activity shaping the land. You will notice that on part of the trail, it will be like walking on top of a wall, with steep slopes on both sides of the trail. This feature is called a moraine. It was caused when a large glacier passed through the valley below. The glacier would push dirt and rock off to the side as it moved creating the large piles we see today. On the base of Opal Cone's western side is an even larger moraine created by the glacier that passed through Ring Creek Valley. Because moraines are not made out of bed rock, they are not geologically stable, and can be destabilized by a sizable earth quake. You can see other evidence of glacial activity nearby; the large boulders that seem to stand all by them selves and seem out of place are called erratics. These are rocks that were being pulled along by a glacier and were left behind when the glacier receded. You can also see glacial striation marks left by the ice scraping the rock as it moved.
Like a moraine, Opal Cone is not made up of bed rock. And just like a moraine, the cone is eroding over time. You can see evidence of this on the southern side of the cone, where the ground has eroded away, creating sheer cliffs that are continuously expanding to this day. Unfortunately, this erosion is being sped up by the presence of hikers going up the trail, as you will see, the trail is rapidly degrading in certain areas, particularly near the cliffs mentioned earlier.
To log this geocache, please proceed to the GZ and then email or message me the answers to the following questions:
1.) Take a look at some of the larger boulders, and describe what makes the boulders unusual from the boulders you would see off of Opal Cone.
2.) Why do you think these rocks look this way?
3.) What colour is the dirt in this area?
4.) Why is the dirt this colour?
Optional: Take a picture of you with your GPS. This is optional, but it is always awesome to take pictures to prove that you were there to share with others.
I will be reviewing the logs of this cache, and if I encounter any logs that did not perform the required tasks, I will be deleting that person's log.