STOP 5:  Layer of amphibole schist in the Wissahickon Formation, evidence of past volcanic activity.  

    Near the top of the hill you will find a dark rock.  Look closely at the black minerals and you should see that the mineral shape is an elongated rectangle.  (You may have to find a small piece of the rock that you can pick up and turn in the light to better see the crystal faces.  A hand lens will also help.)  The mineral is hornblende, a variety of the class of minerals - amphibole.  Because the most important mineral in the rock is an amphibole, the rock is called an amphibolite.
    Amphibolites are metamorphic rocks that were usually originally igneous rocks, most likely a basalt.  Presence of metamorphosed basalts in the Wissahickon Schist supports the inference that the schist originated as sediments deposited on oceanic crust.  While basalts are found in other settings, they are most commonly formed at the mid-ocean ridge or in association with volcanism created by subduction of the oceanic crust at convergent plate boundaries.  Because in this outcrop we see evidence of an inclusion of Wissahickon Schist within the metamorphosed basalt, it appears that the basalt intruded the Wissahickon and is younger than the sandstones and shales that eventually became the Wissahickon Schist.  Because the mid-ocean ridge is unlikely to have a thick sedimentary cover, it seems most likely that the basalt was originated in an a region of plate collision and convergence.

 

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