Ancient City of Jerusalem Earthcache EarthCache
Ancient City of Jerusalem Earthcache
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At the coordinates you will see rock formations that look like a silhouette of the Ancient City of Jerusalem. Here the formations reveal the earth's history before human records were kept. These unique formations were created millions of years ago by volcanoes and reveal a geologic era of chaos and fury.
Erosion of these rocks continues to shape the earth's landscape just as it has for the last 50 million years. During that time volcanoes were erupting one after another creating a thick, soupy mixture of rock and ash that blanketed the entire region.
This thick, soupy mixture is known as "Breccia." Breccia is formed in many different ways. Geologists usually add a word in front of Breccia to signify which kind of breccia they are talking about. For example, a "sedimentary" breccia forms from things like landslide debris. A "collapse" breccia forms when rocks are partly dissolved, such as limestone or marble. "Tectonic" breccias form where two tectonic plates created a crumbling of the interface, by their relative movements. "Fault" breccias are a result from the grinding action of two fault blocks, as they slide past each other. Subsequent cementation of these broken fragments may occur by means of mineral matter introduced by groundwater. "Igneous" breccia is a term used for a rock composed of angular fragments of igneous rocks. "Pyroclastic" breccia is a term used for a deposit fo igneous rock debris that was ejected by a volcanic blast or pyroclastic flow. "Impact" breccia is a deposit of angular rock debris produced by the impact of an asteroid. "Flow" breccia is a lava testure produced when the crust of a lava flow is broken and jumbled during movement. "Volcanic" breccias are formed during eruptive activities. "Volcanic" pyroclastic rocks are formed by explosive eruption of lava and any rocks which are entrained within the eruptive columns. This may include rocks pulled off the wall of the magma conduit, or physically picked up by the ensuing pyroclastic surge. Lavas, especially rhyolite flows, tend to form clastic volcanic rocks by a process known as autobrecciation. This occurs when the thick, nearly solid lava breaks up into blocks and these blocks are then reincorporated into the lava flow again and mixed in with the remaining liquid magma. The resulting breccia is uniform in the rock type and chemical composition. Lavas may also pick up rock fragments, especially if flowing over unconsolidated rubble on the flanks of a volcano, and these form volcanic breccias, also called pillow breccias.
The unusual formations in front of you were created by "Volcanic" breccia. This volcanic breccia mixture of rock and ash slowly cooled. Runoff from the streams and gullies began carving through the soft breccias, sculpting these unique rock formations.
Erosion continued as the wind blew tiny bits of sand and dust at the rocks, further eroding the formations you see.
To receive credit for this Earthcache, send me an email with answers to the following questioins:
1. What type of breccia formed the formations in front of you?
2. Rhyolite lava flows tend to form clastic volcanic rock by a process known as what?
3. According to the informational sign, volcanoes "here" erupted at a magnitude equal to what?
4. According to the informational sign, what widened the gullies here"
5. Optional. Post a picture of you and your GPS with the Ancient City of Jerusalem in the background. This is not required, but it does show you were here if an answer to a question is not exactly correct.
The above information was compiled from the following sources: NPS Information Pannel About.com Geology Geology.com Wikipedia
Additional Hints
(Decrypt)
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