Welcome to the San Diego Zoo
In the parking lot of the San Diego Zoo, an intriguing geological feature lies hidden among the parking spaces: a greenstone conglomerate. This EarthCache invites you to explore a rare metamorphic rock known as a metaconglomerate, learning how it formed and what it reveals about the complex geologic history of coastal Southern California.
The Formation of Greenstone Conglomerate
Greenstone is a metamorphic rock that originally formed as basalt or other mafic volcanic rock at Earth’s surface. These volcanic rocks were later altered by heat, pressure, and chemically active fluids during tectonic burial, producing new minerals typical of low grade metamorphism. This transformation commonly occurs in subduction related settings.
Conglomerate begins as a sedimentary rock made of rounded pebbles and cobbles that were transported by water and deposited together. When such a conglomerate is later subjected to metamorphism, it becomes a metaconglomerate. At this site, the outer rock is greenstone that now acts as the metamorphic matrix surrounding clasts derived from several different rock types.
The metaconglomerate you see formed when non volcanic rock fragments were eroded, rounded by water, and deposited as a conglomerate. Later, tectonic forces buried the deposit deeply enough for the surrounding volcanic material to metamorphose into greenstone, locking the older clasts into place while preserving their rounded shapes.
Why Is This Important?
Examining this greenstone metaconglomerate offers insight into:
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Sedimentary Processes – How rock fragments are transported and rounded before burial.
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Metamorphism – How existing rocks are altered by heat, pressure, and fluids without melting.
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Tectonic History – How plate movement can overprint sedimentary rocks with metamorphic features.
Tasks for This EarthCache
To log this EarthCache, visit the site and complete the following tasks. Send me your answers via Geocaching or email.
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Include "Greenstone Conglomerate - San Diego Zoo - GCB6CN9" on the first line of your message.
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Inspect the greenstone conglomerate closely and gently feel its surface. What textures do you observe in the greenstone? Do the clasts feel different in texture from the surrounding matrix? Explain why you think this is.
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The greenstone is mostly green, but you'll notice a variety of colors in the clasts, including white, black, blue, and brown. What do you think causes these color variations? Consider mineral content like iron oxide, quartz, and other trace elements.
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Describe the size and shape of the clasts. Are they rounded or angular? How does this help confirm that this rock began as a conglomerate before metamorphism?
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In your log, attach a photo of yourself or a personal item with the greenstone conglomerate in the background. (Note: photos taken before the publication of this EarthCache will not be accepted.)