A viscera-jouncing drive over dirt roads brought us to Eunice Bluff, our second field area in the vicinage of Kangaroo Lake. Sparse pine growth surrounded our cars, suggesting that peridotite comprised the dirt-covered bedrock, as it can produce highly alkaline soils that hamper the growth of most plants. Peridotite is an ultramafic rock (that is, a rock highly rich in magnesium and iron) and is understood by geologists to comprise most of the Earth’s mantle. As Katrina and Laura have related in their posts, one of our class’s purposes is to assess the geology of the Trinity Ophiolite, of which the rock formations of Kangaroo Lake and Eunice Bluff are a part, to gain greater insight into the geology of the mantle. We visited Eunice Bluff in particular because it features rocks that were also formed in the upper mantle but are uncommon at Kangaroo Lake.
We took a scenic and chilly hike uphill from the cars. Along the way, we passed by two muddy ponds that California’s drought had drastically shallowed. Then, after an hour of swatting at manzanita plants and tripping over roots, we had found it: gorgeous, orange outcrops of lherzolite, dunite, and harzburgite. Geologists group both lherzolite and harzburgite under the rubric of “peridotite,” but the two rock types differ in that lherzolite contains the mineral clinopyroxene, while harzburgite does not.
Both rock types consist of minerals that commonly occur in igneous rocks (rocks that form from magmas) but they have commonalities with classic metamorphic rocks. All harzburgites and lherzolites originated in the mantle and so have had a billions-year long history of high-temperature and high-pressure conditions before ending up on the continental crust, where geologists can map them. For a long time, the genesis of dunites was much murkier than that of peridotite, and, as Professor Warren informed us as we ascended the hand-scratching, boot-gripping ultramafic formations, we would learn how our favorite olivine-rich rock had come to be.
Peridotite generally weathers to an orange color, while dunite weathers to a tan or dun (hence the name “dunite”).
The eight of us hunched over hand samples, squinted through loupes, and pined for lunch as we contemplated the structural relationships we observed among the lithologies described above. Through Professor Warren’s guidance and tutelage, we observed that lherzolite rarely occurred in exposed rock surfaces directly next to dunite, and that dunite seemed to form channel-like features in the harzburgite and lherzolite host rock. We inferred that these channels were the cooled, hardened products of magma flow upward through the upper mantle toward mid-ocean ridges, where volcanic ocean crust forms.
An “island” of harzburgite flanked by two converging dunite melt channels.
While one might interpret the dunite to have crystallized from the magma flowing through these channels, this was not so. We learned that dunite formed due to the reaction of clinopyroxene in lherzolite with two other minerals, not as the result of the crystallization of dunite from a hot magma. As magma flowed upward through the channels, the pyroxene in lherzolite near the channels was effectively leached away, leaving behind harzburgite. We spent the remainder of the afternoon using this model as the basis for interpreting other ultramafic outcrops at Eunice Bluff. Each student chose a feature that he/she wanted to discuss and presented his/her interpretations to the rest of the group.
At the end of the afternoon, bedraggled but geologically inspired, we marched back down the hill toward the cars. To celebrate our great scientific successes, the group supped on pork carnitas and reveled in a refreshing swim at Kangaroo Lake.