Rocks are like storybooks. They tell a story of heat, time, pressure or a combination of the three. Just as library shelves are full of volumes waiting for people to discover them, rocks sit underfoot waiting patiently for someone to dig into their story a little deeper.
Across the causeway from the town of Marblehead lies what once was an island and is now known as “The Neck”. On Marblehead Neck you will find not only beautiful homes and breathtaking vistas but also several locations where geological formations have some fantastic tales to tell. This earthcache will take you to one of those locations.
Most of the rocks you will discover on this expedition are igneous, which is derived from the Latin word ignis meaning fire. Molten rock called magma is created near the earth’s core. Because it is less dense than the surrounding solids it often makes its way toward the earth’s crust where it emerges through fissures or eruptions as lava. If the molten rock cools and hardens below the surface it is called instrusive igneous. If it emerges through the crust and hardens on the surface it is called extrusive igneous. The faster the molten material cools the finer the grain will be in the hardened rock. Rock that takes longer to cool is coarser and contains more crystalline structures.
A volcano erupts when pressure below the earth’s crust is not allowed to be adequately relieved or vented. The plume or column of gases, lava, ash and debris can reach more than 20 miles up into the atmosphere. The airborne fragments are also called pyroclasts. If the fragments are still hot enough to fuse together they form pyroclastic rock or tuff.
Castle Rock is your destination. According to BostonGeology.com these rocks formed in the Precambrian period about 550 - 750 milliion years ago. That was a period of time when volcanoes were found in our area. The two types of tuff found here are both formed from volcanic ash. The dark welded tuff forms as extremely hot ash piles up - its own heat causes it to weld together into rock. The lighter ashy tuff (also called lithic tuff) forms as cooler ash settles; eventually it is lithified into rock. In both cases is common to see bits of other rock (volcanic cinders and "bombs") in the tuff.
To get credit for this cache please email answers to the following questions. PLEASE NOTE: If you want to claim credit for this cache you must submit answers in your own words. That means each cacher must email his/her own answers. No group responses or parental spokepersons will be accepted.
- NEW LOGGING REQUIREMENTS AS OF 8/2/15. Both types of tuff are present in this location. What would you estimate to be the percentage of each one? You will have to climb to the top and then continue scrambling over the rocks almost to the ocean to view the entire mound in order to answer this. Simply viewing it from the path by the park benches will not suffice. Walking down the stone steps to the beach area will also not suffice. Wear shoes suitable for rock climbing.
- Would you consider the majority of the rock in this location to be intrusive or extrusive and why? No guessing allowed. You must state the reason for your answer.
- Optional: take a photo of yourself and your GPS next to the entrance sign for this park.
UPDATE: 9/9/14 Some logs may have "mysteriously" vanished recently. Don't be surprised if this happens to you if for whatever reason you forget to send answers to the CO. This cache requires a little work (very little), but it's not a "freebie." If you feel your log has vanished by mistake please resend answers prior to posting another log entry.