GEOLOGY OF THE CSUH CAMPUS -- A PIECE OF ANCIENT OCEAN CRUST

by Elwood Brooks

The geology of the California State University, Hayward campus is complex and -- perhaps surprising to you -- poorly known. A geologic map of the campus should have been prepared when the campus was first established in the early 1960's, and the ridges and hills were scraped off clearly exposing the bedrock. Canyons were filled in with materials from the hilltops to produce the flat surfaces you see today covered by buildings, lawns and parking lots. As the buildings were constructed and the lawns planted, most of the rocks and structures exposed by the bulldozers were rendered inaccessible. Now we can see the rocks in only a few places where they are still exposed and in the undeveloped portions of the campus.

OCEAN CRUST, SEA-FLOOR SPREADING AND FORMATION OF OCEANIC PLATES

The rocks underlying the CSUH campus preserve evidence of processes which occurred far out in the Pacific Ocean about 160 million years ago, duing the Middle Jurassic Period of the geologic time scale. They record processes related to the formation of plates and the growth of continents. Most of the campus has been built on rock called gabbro. The campus gabbro is a grayish-green, visibly crystalline igneous rock (which was once molten and cooled at depth where large crystals form). "Fresh", unaltered gabbro consists largely of the minerals plagioclase feldspar and monoclinic pyroxene. But the campus gabbro has been strongly altered, so that the pyroxene crystals have been replaced by crystals of the mineral amphibole. The campus gabbro is approximately 163-169 million years old! In addition to the campus gabbro, there are mudstones and conglomerates (Knoxville Formation), and lavas or shallow intrusive rocks (Leona Rhyolite).

To understand what the rocks on campus represent and how they formed, we must consider briefly the plate tectonic theory. The surface of the earth is made up of about a dozen plates of crust and uppermost, solid and rigid mantle which makes up the uppermost 70 to 100-150 km of the Earth. The plates move exceedingly slowly (a few cm per year) with respect to one another, on a "plastic" layer in the mantle. The plates are either moving toward one another (converging), moving away from one another (diverging) or plates slide past one another (as they do along the San Andreas fault across the Bay). The campus gabbro is thought to have formed at a divergent plate margin. As two plates moved apart, magma (molten rock) generated in the mantle ascended to form a magma chamber. Gabbro crystallizes along the roof and sides of such magma chambers. Considerable magma rises along tension fractures produced as the plates diverge to form dikes which cut across older crust and to erupt on the sea floor as volcanic lava flows. The whole process of plate divergence and the addition of new igneous rocks (gabbro, dikes, lava) to the separating plate margins has been termed sea-floor spreading. New oceanic crust is formed in this way. Once formed, the new oceanic crust moves away from the spreading ridge (where the processes described above occur) and may move toward an adjacent plate. When two plates collide, most of the oceanic crust is consumed (goes down into the mantle or is subducted) under the colliding plate. Some of the oceanic crust may get "scrapped-off" and attach itself to the overriding plate.

The assemblage of igneous rocks (gabbro, dikes, lava), formed on and below the sea floor, if emplaced on the edge of an adjoining continent, is referred to as an ophiolite. The ophiolite emplaced onto the western edge of North America in the Late Jurassic is called the Coast Range Ophiolite for its wide distribution in the California Coast Ranges.

The Leona Rhyolite, until recently thought to be a mere 5 million years old or less, is now known to be at least 99.2 million years old. The radiometric age is from a sample of Alum Rock Rhyolite, collected near San Jose and thought to correlate with (be equivalent to) the Leona Rhyolite. Microscopic examination of paper-thin slices of samples of the Leona Rhyolite reveals that it is not actually rhyolite, but a metamorphosed volcanic rock known as keratophyre, a very finely crystalline rock consisting mostly of the minerals plagioclase feldspar (albite) and quartz. The Leona Rhyolite is probably the uppermost "lava" portion of the Coast Range Ophiolite. Both the campus gabbro and Leona Rhyolite probably were strongly altered -- even metamorphosed -- by circulation of heated sea water through fractures formed in them near the sea-floor spreading ridge axis.

About 145-150 million years ago, during the Late Jurassic Period, the sedimentary rocks of the Knoxville Formation were deposited on the Coast Range Ophiolite (oceanic crust). On the campus, the Knoxville consists largely of yellow-weathered, dark gray (when fresh and unweathered) mudstone containing sandy limestone concretions (hard limy nodules). This assemblage of fine-grained, muddy, limy sedimentary rocks suggests deposition in a fairly deep sea, considerably removed from the continental margin. As the oceanic plate was subducted (consumed) under North America, some rocks were plunged to depths of several miles where they were under considerable pressure and heat. Such subduction of oceanic crustal rocks resulted in metamorphism to a coarse crystalline "high grade" metamorphic rock called amphibolite. Blocks of this amphibolite has been raised to the Earth's surface along narrow, steeply inclined faults or shear zones. Amphibolite occurs as very large blocks enclosed in shale; this odd mixture of sedimentary and metamorphic materials is referred to as melange. On campus, a block of amphibolite is found south of Pioneer Heights.

WHERE TO SEE THESE ROCKS - PIECES OF OCEANIC CRUST

GABBRO IN THE COAST RANGE OPHIOLITE

The campus gabbro, some of the least altered available, is exposed in small excavations along the east side of the sidewalk between the Campus Corner and Orient Express foodstands and the University Union. It is foliated (crystals are in layers) in places, the darker minerals concentrated in parallel streaks, and it is highly fractured. More campus gabbro can be seen (and collected) in the large cut-slopeto the east of the Pioneer Heights student apartments south of Harder Road.

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LEONA RHYOLITE

Across Harder Road, on the southwest side of the Pioneer Heights student apartments, a dirt road leaves the southeast end of the parking lot. Follow the dirt road to the PCB storage building and turn right onto another dirt road which will take you up the hill. This outcrop of Leona Rhyolite is typical in that it does not yield any secrets regarding its mode of formation; was it a lava flow or was it perhaps intruded into or just below the Knoxville mudstones? The variously oriented fractures which cut the outcrop are also typical.

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"HIGH-GRADE" AMPHIBOLITE BLOCK

On the southwest side of the Pioneer Heights student apartments in the grass is a 22 ' block of amphibolite in a dark gray mudstone matrix of the melange. Some rather rare metamorphic minerals can be found in this block. The bluish cast seen in places is due to the presence of crossite, a blue amphiblone. The silvery flakes are of phengitic muscovite mica and the pistachio green areas indicate concentrations of epidote. This forest-green veins of pumpellyite cut the block at one point, and white blades of lawsonite are found on fractures at another place.

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KNOXVILLE FORMATION - MUDSTONES

Continue down the dirt road past the steel gate. Upon rounding the hairpin turn, you will begin to encounter tiny yellow-weathered fragments of gray mudstone in the roadcuts on your left. These characterize the soils developed by weathering of the soft Knoxville Formation. Keep your eyes peeled for scarce, gray-brown chunks of sandy limestone which represents concretions encased in mudstone. Finally, a little sandstone can be found in the roadbed just around the next corner.