Serpentinite Outcrop EarthCache
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The cache is located about 14 miles north of Middletown at a road cut along Big Canyon Road on the public right-of-way. The road is mostly paved with a short, graded-gravel stretch. It can be a bit rough in places. There is ample parking along the road across from the cache.
Serpentinite is most often recognized by its green to blue-green color, as here at this cache. It is a relatively soft rock (hardness 3 – 4.5) with a silky, waxy feel. The curved, polished surface is called slickensides. Serpentinite, often simply called serpentine, is composed of a group of minerals in the serpentine group, chiefly the minerals lizardite (scaly appearance like a lizard’s scales), antigorite and chrysotile. The latter, not present in this formation, is an asbestos, fibrous mineral that has led to asbestos contamination in certain waters fed by run-off from serpentine deposits. The California State Legislature has designated serpentine as the “official state rock and lithologic emblem.” But certain victims of asbestos-related diseases have mounted a “drop the rock” campaign to remove this designation.
Chemically, serpentine is a hydrated iron magnesium silicate. Serpentine rocks are high in iron and magnesium and low in sodium, potassium and calcium. They also may have high concentrations of nickel, cobalt and chromium. It is a metamorphic rock formed at tectonic plate boundaries where one plate is forced below another (subduction). Near the California coast where the Pacific plate dips under the North American plate, serpentine is common. It forms from the common rock peridotite when it comes in contact with seawater at low temperature and pressures.
Serpentine at this location was formed far below the earth’s surface during subduction and later lifted to its present location by tectonic action. Originally formed near the coast, it reached its present location when additional rocks were scraped off the Pacific plate during subduction and added to the coastline. As California’s coast grew to the west by successive scrapings, previous scrapings appeared to move inland. Thus, this serpentine formation arrived at its present location several million years ago.
Soils derived from serpentine are toxic to many plants because of high levels of nickel, chromium, and cobalt; growth of many plants is also inhibited by low levels of potassium and calcium. The grey pines growing at this location thrive in these soils. They are identified by their distinctive long, dusty-looking, grey-green needles. On the other hand, grasses generally do poorly in serpentine soils. A few grasses do grow here, but they are probably supported by soils formed by weathering of the adjacent sandstone.
Logging the Cache
You may post a picture of you or your group, if you wish. But DO NOT SHOW the serpentine formation in the background.
To log this cache you must E-MAIL me the answers to the following TWO questions. DO NOT POST YOUR ANSWERS.
1. The serpentine in this location is mostly broken into smaller pieces by tectonic action or weathering. But a rounded boulder protrudes from the formation about 10 feet above the road surface. What is the diameter of this boulder?
2. Is serpentinite a sedimentary, igneous or metamorphic rock?
Additional Hints
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Treasures
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