***WARNING - DO NOT ATTEMPT TO ENTER THE OCEAN FOR ANY REASON***
IT IS VERY DANGEROUS - WATCH YOUR BACK AND THE WATER
READ THE WARNING SIGNS ON THE WAY TO THE BEACH
The rare sight of a black sand beach is usually caused by eroded volcanic rock, rich in minerals, and lacking silica. In this area of northern California, however, it’s created by the dark shale and sandstone (greywacke) formed by tectonic activity of one continental and two oceanic plates meeting just offshore. The Lost Coast consists of undivided Cretaceous marine metasedimentary and sedimentary rocks of the North American Plate steeply uplifted by the Mendocino Triple Junction in interactions with the Pacific Plate and Gorda Plate (southern extension of the Juan de Fuca Plate). The San Andreas fault runs diagonally through the Shelter Cove peninsula.
Black Sand Beach sits atop the Franciscan Formation, formed during the Cretaceous period between 200 to 80 million years ago, and includes the area along the coast of San Francisco and northward.
An understanding of Franciscan Complex rock types can be gained by considering both the rock and the plate tectonic cycles where processes such as melting, crystallization, weathering, erosion, and burial lithify rocks. These rock-forming processes produce materials with distinctive compositions and textures that provide clues to the environments and conditions of formation that are often most dynamically illustrated at plate boundaries such as those here.
The black sand beach here gets its coloring from altered igneous, metasedimentary, and sedimentary rocks that are exposed nearby. While the beach is black, (pebbles, not sand) the surrounding formations are brown/gray/tan with a small percentage of black in color.
What is an igneous rock?
Igneous rocks (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface.
What is metasedimentary rock?
In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment. Then, the rock was buried underneath the subsequent rock and was subjected to high pressures and temperatures, causing the rock to recrystallize. The overall composition of a metasedimentary rock can be used to identify the original sedimentary rock, even where they have been subject to high-grade metamorphism and intense deformation.
What is a sedimentary rock?
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding, and in the case of the rocks on the beach here, ribbons of marble are compressed limestone embedded in the pebbles and rocks.
See the chart below and notice how the small tip of Point Delgada has different geological makeup than the rest of this part of the coast.
KJf = Franciscan complex: Cretaceous sandstone and Jurassic sandstone with smaller amounts of shale, limestone, chert, and conglomerate.
TK = Sandstone, shale, and minor conglomerate in the coastal belt of NW California. Previously considered Cretaceous, but now known to contain Tertiary microfossils in areas.
Walk out on the beach and check out the rocks and pebbles that make up this part of the coast. In order to claim a "found it" for this earth cache, email or message me the answers to the following questions:
1) Name of the cache, the GC code, and the number in your party.
2) How did black "sand" come to be here when the surrounding area is a different rock?
3) What size are the rocks/pebbles here on average?
4) What is most characteristic about the "sand" here?
5) Why/how do you think the shape of the rocks here was formed?
6) Optional photo of yourself/group exploring the beach.
***Congratulations to BRUHN for FTF***
Map and data courtesy of: https://maps.conservation.ca.gov/cgs/gmc/ California Dept of Conservation
https://en.wikipedia.org/wiki/Lost_Coast
https://ucmp.berkeley.edu/education/calandscape/session3/franciscan.html