If you stand at the listing coordinates, you are at Höfn Boulder Park.
This park presents eight massive boulders which characterize the main rocks of Southern Iceland. They were taken from different places in the municipality of Hornafjödur.
As for any earthcache, there are no stashes to be found. Please refer to the logging requirement below.
But, first, let's start with some geological explanations
1. Bedrock of Iceland.
Iceland is a basaltic plateau on the ocean floor across the junction of two oceanic ridges : Kolbeinsey Ridge to the north and Reykjanes Ridge to the south. The ridges are connected by a bifurcated volcanic-zone. Spreading rate across the volcanic zone is 2 cm per year; the oldest rocks being about 16 million years at the east- and west margins of the plateau while new rocks are formed in the volcanic zones.
Tholeiitic basalt is the dominating rock type of the bedrock that differs from the ocean floors due to the formation of acidic rocks in volcanic centres within the rift-zones. The reason may be more extensive magma-formation and higher crustal thickness due to the presence of a mantle plume beneath the Iceland.
2. Formation of the crust.
The icelandic crust is formed within the volcanic zones where basaltic magma from the upper mantle fills in the spreading of the oceanic plaes. Basaltic fissure eruptions and dyke injections form most of the crust. Surface rocks are about 90% basalt and about 10% rhyolite that has evolved within or beneath the crust. The intrusions are the heat source of a geothermal system within the upper crust. Rocks that make up the roots of volcanic centres are extensively altered by hydrothermal fluid.
3. Glacial erosion - Exposure of intrusive rocks
During the Quaternary era( last 2,7 million years ), Iceland was repeatedly covered by glaciers that may have reached thickness up to 3 km over the southern highlands. The enormous erosion-power of the glacier is clearly indicated by fjords and abyssal valleys and, also, by sediments of the coastal plains in South Iceland. During the growth of a glacier, the crust subsides due to the ice load. The Ice Age glaciers vanished some 10 000 years ago and the result was deloading and uplift of the crust. Due to glacial erosion, intrusive rocks formed within the roots of volcanic centres were exposed at the surface.
4. The most common rocks from the rift-zone volcanic centers
The sketch shows the origin of the most common rocks in volcanic centres of the rift-zones. Lithofacies of basalt range from glassy or cryptocrystalline rock to finegrained groundmass, frequently with phenocrysts of plagioclase and olivine. Dolerire is medium-grained dense basaltic rock formed by slow crystallization in dykes and thick lavas. Acidic magma may form as residual liquid on top of crystallizing gabbro intrusions or by partial melting of altered basalt. In lavas the acidic magma forms flow-banded rhyolite. Deep-seated acidic intrusionscrystallize as light-coloured coarse-grained dense granophyre. Gabbro is dense, coarse-grained baat rock formed in intrusions within volcanic centres. The colour of gabbro indicate the relative amount of plagioclase ( clear, white) and clinopyroxene ( dark-green).
| Your task to log this earthcache : |
On site, each boulder is identified by a different number.
Boulder #1 is at the most western location of the park, then the numbering is done clockwise. The numbers appears on a plaque on each boulder.
For reference, below is a top view of the park on which one can note each boulder's number.
Take a close look at Boulder #2 :
This boulder contains many fractures.
1 ) Explain with your words the origin of these fractures
2 ) Are all the fractures looking the same ? What differences can you notice ?
3 ) The rock surface has a particular hue. What color is that hue ? What could be the origin of such a hue ?
Now, take a close look at Boulder# 5
4 ) How is the shape of that boulder compared to the others ? What happened to that boulder that could explain this difference ?
Finally, compare Boulder #3 and Boulder #8
5) What difference can you notice in term of grain size between these two boulders.
Send me your answers to the 5 questions via the message center on geocaching.com page.
I will revert back to you in case of troubles or problems with the answers you provided
As proof of your physical presence on site and as required by earthcache guidelines, please add in your online log, a picture of one of your geocaching items on site ( GPSr, Travel Bugs, GeoCoin, etc ). It is not required that your face is visible on the picture.
Enjoy the visit of Boulder Park and have fun while examining these Icelandic boulders !