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Arch Rock is an ashflow tuff with a natural arch and several small rockshelters eroded into the outcrop.
Arch Rock is a natural arch, which is a natural geological formation where a rock arch forms, with an opening underneath. Natural arches commonly form where cliffs are subject to erosion from the sea, rivers or weathering. Waves have eroded the coastal rock of the Oregon Coast for many years creating this arch.
Rock is very solid and is very important in accomplishing this gravity defying feat. Rock hardness varies in differing rock types. The harder the rock, the longer it will resist erosion and weathering processes. Differing hardness and differential weathering creates layering of rock along a coastline. The coastline is composed of many alternating headlands (a point of land, usually high and often with a sheer drop, that extends out into a body of water) and bays. The formation process usually begins when the sea attacks small cracks in a headland and opens them. The cracks then gradually get larger and turn into a small cave. When the cave wears through the headland, an arch forms.
Eventually, with further erosion, the arch will collapse, leaving a pillar of hard rock standing away from the coast - a sea stack, or rauk. Erosion will eventually also cause the stack to collapse, leaving a stump. This stump is usually a small rock island, often small enough to be submerged by high tide.
The formation of a headland and a bay are an integral first steps in sea arch formation. The rock hardness of the headlands is usually harder that the rock found in the bays.
Arch Rock Tuff:
Arch Rock consists of an ashflow tuft which is a type of rock consisting of solidified volcanic ash and pumice ejected from vents during a volcanic eruption.
Tuff can be welded to unwelded. Welded tuff is a pyroclastic rock (rocks composed solely or primarily of volcanic materials), of any origin, that was hot enough at the time of deposition and then cooled slowly to form a dense , relatively unporous mass. If the rock contains scattered pea-sized fragments in it, it is called a welded tuff. During welding, the glass shards and pumice fragments adhere together, deform, and compact together.
Unwelded tuff forms differently than welded tuff and is more porous, which often means it is not as hard as welded tuff.
SEND AN E-MAIL TO THE QUESTIONS BELOW BEFORE YOU POST A LOG!
To log this Earthcache, fulfil the educational requirements by e-mailing me answers to these questions:
1) How many people in your geocaching group?
2) Estimate how long and high the arch in Arch Rock is.
2) What is another name for a sea stack?
3) Do you think Arch Rock is a welded or unwelded tuff? Why?
4) Does Arch Rock have any visible remnants of a headland? What made you decide?
ALSO,(Optional) Post at least one picture of the arch with you and/or your GPS so others can share your adventure.
(No hints available.)
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Coordinates are in the WGS84 datum