The formation of Yap Island EarthCache
Hidden : 2/17/2013
Difficulty:
4 out of 5
Terrain:
4 out of 5

Size: Size: other (other)

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Geocache Description:

This cache consists of a hike of about 4 km and will take more than one our.

Introduction
In general there are three ways to form islands:



a) Volcanic islands
Volcanic islands are formed by growing under-water vulcanos. They consist of basaltic rocks, lava and tuff. In tropical areas they may be surrounded by corals / coral reefs in a secondary biogene process.
Examples are Hawai, Tenerifa.

b) Coral islands
Coral islands are produced by a biogene process: by the growing of corals. Corals are animals. They are colony-building polypes with an outer skeleton of calcium carbonate. Often the inner parts die early and form a ring-lagoon.
Well known examples are the Maledives.

c) Montains of a continent
If a continent sinks under sea level or has high mountains at its outer shelf, they may appear as islands. These islands consist of the rocks that built up the continent and may be sediments, volcanic, plutonic or metamorphic rocks.
Examples are Spitzbergen and South Georgia.

In all cases the islands may arise and sink down several times due to tectonic forces.

Yap belongs to type c). Yap is located on the Philipine plate, at its eastern edge. Here the Philipine plate is liftet up by the sinking Pacific plate (see galery foto 'plates').
The mayority of rocks on Yap belog to the group of green shist.



Green shist consists of an iron-rich silicatic material. It also contains some minerals from the mica famimly. These minerals form flat crystals of usually dark color. Any kind of oriented pressure during the crystallisation process of the mica results in oriented mica crystals. The flat mica crystals are always perpendicular to the direction of pressure. Usually the orientation of the mica crystals is visible as stripes or "layers" in the shist rocks.

What can we see here to proof these facts of the island Yap?

Go to the waypoints and look carefully.

Logging tasks

To log this EarthCache, send us the answers of the following questions/tasks via our profile page. You may log directy, we will contact you if anything is wrong.

1) Describe the rocks you can see at WP1, WP2, WP3 and the cache location. Do you see any differences?
2) At the cache location the rocks show many cracks (see galery foto 'cracks'). What is the orientation of these cracks (north or east or ...)?
3) What do you think causes this orientation?
4) As the Philipine plate lifts up here and Yap was never under sea level, is here any observation possible that indicates this fact?
5) Add a foto from nearby that proofes your visit. Please do not spoil geological features!

Additional information
Green shist contains iron ions in the oxidation state 2+. These ions produce the greenish or blueish color. The main erosion product of this rock type under tropical conditions is laterite where the iron ions are oxidated to the state 3+. Iron 3+ ions are responsible for the dark brownish-red color of this soil.



You can find this soil at many lower parts of the island as it washs out of the mountains.
References:
Geological Environment of Yap Islands, Micronesia
Munemoto Nadachi et al.
Kagoshima University Research Center for the pacific islands
Occasionel Paper No. 34, 69-76, (2001)

Beträge zur Petrographie und Geologie der Deutschen Südsee-Inseln
Erich Kaiser
Jahrbuch der Königlich Preussischen Landesanstalt und Bergakademie für das Jahr 1903 (1907)

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