|
THE MORROS |
|
A dominant landform near the Cuesta College campus is the chain of volcanic necks known as the Morros or the Seven Sisters. Geologists refer to them in scientific literature as the Morro Rock to Islay Hill intrusive complex. They represent the solidified conduits "necks" of ancient volcanoes that erupted approximately 22 to 26 million years ago. The volcanic cones have long ago eroded away and we see only the rock that crystallized in the volcanic neck. They are not active volcanoes! This photo to the left looks west from above Highway 101 near downtown San Luis Obispo. The closest peak is Cerro San Luis with Bishops Peak beyond. |
| Take a closer look.
Here’s Morro Rock, the western-most of the intrusive complex. We’re looking north, towards Cayucos. The Corp of Engineers modified its appearance by blasting a small amount off the southern edge of the rock to provide riprap for the breakwater at Port San Luis. The rock has been tied to the land by a jetty seen east (right) of the rock, north of the channel. |
 |
 |
Take a closer look at Morro Rock.
Here's a close up of the rock. It has a porphyritic texture common to igneous rocks that experienced two-stage cooling. Those white rectangular crystals are plagioclase phenocrysts set in a gray groundmass. The plagioclase formed first during a slow cooling phase. The groundmass was quenched when the magma rose quickly towards the surface. Potassium-argon dating yielded an age of 22.1 +/- 0.9 million years for the crystallization of this rock. |