Rock Story -- Metamorphic rock from Antelope Island similar to those along the county border with Davis County.
Where these rocks are found:
Antelope Island is not in Salt Lake County, but is a short drive away. The island's backbone is mostly these rocks. Salt Lake County has some along the boundary with Davis County.
General observations:
The bedrock sticks out like weird carcasses of rock. There are some huge boulders but not very many tiny pebbles of this material in the vicinity of the bedrock or even downhill from it. It is as though it disappears without much trace.
What you observe:
1. I see vestiges of layering. 2. I see the banding of little black minerals, but the bands are not continuous and they "smush" together. 3. With my hand lens, I see abundant glassy quartz crystals. 4. The quartz and the little black minerals are interlocking, welded together. 5. I don't think if I set this rock in water that the water would percolate into it. 6. There no spaces between crystals. There is no cement between crystals. 7. Along cracks, I see rusty coatings. 8. The rock cracks across the vestiges of layers. It pays no attention to the pattern of the vestiges of layers. It would not be easy to get the rock to split along a band of little black minerals or along a band of quartz. 9. The surface of a fresh face is not really smooth, but it isn't rough. It cracks along crystal faces. It goes right through masses of quartz, not around them. 10. It is a hard rock. It cracks easily along cracks that are there, but it splinters off in wedges from a big piece. It is not easy to control where it splits. If I want to have it split in a given direction, I may not succeed. 11. It has a swirled pattern, a chevron pattern, a welded pattern. 12. It probably was once layered, but I can't tell what it was. It is mostly quartz and little black minerals now. 13. The vestiges of layering is uneven ... some thicker bands than others. 14. In the sunshine, I see small crystal faces glinting at me. 15. The fresh face looks quite different from the weathered face ... The little black minerals aren't so shiny and black on the weathered face ... the quartz crystals look duller.
Where the rock is on the rock cycle when you find it?
Now, it's a chunk of sediment. It came off a bedrock outcrop where Genevieve whacked it off with her rock hammer. So it has been transported by human beings. But if she chucks it, it will just be another stone on the land sUrface. Where was the rock on the rock cycle when it was bedrock? The vestiges of layering are the give-away ... metamorphic rock. It is a high-grade metamorphic rock judging from the welding of the quartz AND the little black minerals that are interlocking and look as if they almost were ready to melt.
Tell me more:
This rock has been subjected to intense heat and pressures. It tells the story of being forced under a continent by tectonic forces, or of the stresses under a mountain range. The vestiges of layers could give a clue about the rock before it was metamorphosed. The quartz layers could have been sand. The little black minerals could have been clays. If this were the case, then this rock, before it was metamorphosed might have been a sedimentary rock. That was a long long time ago. So this rock tells a long history, if we could only understand the clues.
Where is the rock heading on the rock cycle?
It's now a chunk of sediment. It is made up of quartz and little-black-minerals. As it gets busted up, it probably will disintegrate into quartz sand (the quartz) and clays (the little-black-minerals). As sediment it will go one of two ways on the rock cycle: .. The rock, or the sand and clays eroded from it, will get buried by hundreds, probable thousands of feet of sediment. Ground water percolating through it will deposit cement in the voids between the grains. The pressure of the overlying sediments will squeeze out the water from between the clay particles. The mass will become firm and coherent sedimentary bedrock, or, .. The rock, or the sand and clays eroded from it will be buried for a while, but then will be exposed and eroded again and be reworked as sediment. AND then ... if it goes the route of a) sedimentary bedrock ... it then will have two paths it can take ... .. to become a metamorphic rock, or, • to be eroded and become sediment. OR ... if it went the route of b) sediment... it then will have two paths it can take ... • to become a sedimentary rock, or, • to be eroded again and become sediment again.
Sedimentary Example | Metamorphic Example | Ignous Example