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While San Diego is on the other side of the country from the Wissahickon Valley, she will likely find familiar surroundings - geologically speaking - in the mountains above the city.
The rocks that form the Wissahickon Valley were formed the same way as some rocks in California, and that explains the similarity.
In both places, there once existed subduction zones - ocean trenches where oceanic plates dive back into the mantle region deep in the Earth after colliding with lighter continental plates that over-ride the heavier ocean plates.
In the case of California, the now extinct Farallon plate was subducted under California. The Juan de Fuca plate to the north in the Pacific Northwest, and the Coco plate to the south off the coast of Central America are small remnants of the larger Farallon plate, still subducting under the continental plates.
As ocean plates dive under continental plates, sea floor sediments are scraped off the diving plate and accrete to the continental plate.
This process takes place at depth where heat and pressure combine to transform the sediments that are scraped up.
Meanwhile, under the continental plate, magma accumulates and is released as lava from volcanoes, adding to the heat in this active zone of tectonic activity.
If a section of sea floor could be sliced off and examined, there is a series of rocks that is formed, starting with peridotite, or part of the Earth's mantle.
Peridotite is a dark, olive green rock that metamorphoses to serpentine or serpentinite.
There are also pillow lavas formed at spreading centers, the line where oceanic plates form. Pillow lavas are the result of magma oozing into sea water, where it cools rapidly.
As the name suggests, pillow lavas have rounded forms created when the magma is extruded into the cold sea water in a bubble.
Next come ocean sediments - sand and silt that drifts down through the sea and settles on the bottom.
Deposits of clay or silt become shale, and deposits of sand become sandstone.
Shale metamorphoses to schist, and sandstone to quartzite or gneiss.
Thus, sections of sea floor are indeed deposited, metamorphosed, and later uplifted as the continental plate rides up over the oceanic plate.
These sections of sea floor are known as ophiolites (oh'-fee-oh-lites).
Wherever sea floor subducts under a continental plate, ophiolites are scraped off the diving plate and plastered onto the continental plate.
And wherever ophiolites occur, there was once a subducting plate.
Ophiolites occur in the Wissahickon Valley of Philadelphia - indeed in a belt running up and down the East Coast. They occur in California.
Schist forms the summit of New Hampshire's Mount Washington, and ophiolites can be found in the eastern Mediterranean Sea and the Middle East.
Ophiolites are highly interesting assemblages of rocks. Within the general classifications, there are grades and variations caused by variations in temperature or pressure.
For example, garnets, staurolite, and kyanite crystals occur in schist deposits and are formed at different temperatures
For a long time, geologists were puzzled over the origin of ophiolites. It was not until plate tectonics became understood that the way ophiolites form became apparent.
And that was relatively recently in history - within the past 25 years. Prior to that time, the Wissahickon schist was listed as "rocks of indeterminate age."
To be sure, there are variations in the appearance of rocks from one place to another. Some differences are because of age, and some because of the degree of heat and pressure to which the rocks were subjected during the metamorphic process.
And yet schist that is found at Pemaquid Point in Maine, the summit of Mount Washington, along the Wissahickon Creek, and in the mountains above San Diego are all quite obviously schist.
It is also a sign that something went down at that point sometime in the distant past.
