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This EarthCache will introduce you to some of the forces that formed the bedrock in the Victoria region. The rock that you will learn about is metamorphosed igneous rock, formed deep below the earths crust, then uplifted to the surface during the collission of various tectonic plates.
WARNING! Stay away from this EarthCache if there are strong winds and/or waves blowing on shore in this area OR if there are icy conditions. The terrain is rough and may be slippery... it is suitable for dogs and children at low tide IF the weather is nice.
| When at the posted coordinates, look around and you will see areas of rock that have been exposed by the ocean waves. Much of the rock has a patchwork appearance made up of light and dark areas. The contrasting light and dark rock in this area is called migmatite. |
Beautiful patterns caused by the metamorphism of igneous rock deep
below the earth's surface.
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When Vancouver Island was first formed it was through a series of submarine volcanos in the tropical latitudes of the pacific about 300 million years ago. These volcanoes formed shallow seas in which lived an amazing array of creatures from ammonites, to giant plesiosaurs. We now call this submarine landmass "Wrangalia". Sediment and fossil evidence of these tropical shallow oceans can be found all over Vancouver Island. This Volcanic region (rocks now called the Sicker Volcanics) slowly moved northeast towards the coast of North America. During its slow migration towards its current location, Wrangalia underwent a number of periods of further volcaic activity, and then crunched into the western coast of North America about 100 million years ago.
The trip up to its current location, along with the volcanic activity and its collision with North America, cause a large amount of turmoil deep within the belly of Wrangalia. Areas of volcanic rock (igneous rock) as well as areas of sediment, underwent transformations through exposure to extreme pressures and temperatures. When a rock type changes its form under these extreme conditions, it is said to have "metamorphosed." Imagine if you will, an area of volcanic rock deep with the earthe's crust. New molten lava, of a different composition, squeezes its way through cracks and fissures into this region breaking off chunks and pushing it apart. After things settle down in the region, this molten mass of rock, being deep beneath the surface, undergoes high pressure and temperatures for a long, long time... long enough for the rock to form into a single mass and cooling slow enough that the various minerals that make it up form crystals. The result is the migmatite you are looking at.
The gabbro-granodiorite make-up of migmatite. Notice the crystaline composition
of both the dark gabbro and the lighter granodiorite. |
Now enter uplift and erosion. The migmatite is uplifted as Wrangalia bends and folds during its (currently ongoing) collision with North America. Uplifted rock is eroded away by multiple glaciations, the most recent of which is the Fraser Glaciation between 30,000 and 10,000 years ago. In the region of the posted coordinates, the migmatite is exposed through the action of waves from Juan de Fuca Strait. Here you can see the metamorpic igneous rock made up of light "grano-diorite" and darker "gabbro." You can see areas where chunks of gabbro have broken off into molten granodiorite, and other areas where molten gabbro has oozed into fisures in the more solid granodiorite. The whole mix has been squeezed and folded and mixed around to form the wonderful patterns that you see here.
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To Claim a find on this Earth Cache:
You must post and correctly label three photos that show that you understand the formation of this rock.
1. Photograph the granodiorite, showing the crystaline structure, with a small amount of the gabbro in the edge of a photograph.
2. Photograph the gabbro, showing a small amount of the granodiorite in the edge of the photo.
3. Photograph yourself, with your GPS at or near the location showing the patterned migmatite behind you in the photograph.
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