Pockets Full of Rocks

This is a story about a young woman whose deeply bedded passion for geology led her on a journey of a lifetime. All her life she’s held an interest for rocks. Perhaps this is because her mother had a business called, Nature’s Treasure Chest, where she would sell agates, rocks, jewelry made out of crystals, and even clocks. Ah, yes, this little girl loved looking at rocks but she didn’t know much about them. Still she collected all the rocks she could at the beach, the lake, and even in parking lots from the planters. At first her parents thought it was cute until soon everything was filled with rocks! What they didn’t know was their little rock collector would find her way to Santorini, Greece collecting more rocks. Cape Mavropetra, Santorini is the place where she’d be. The beach littered with all the rock types seen throughout Santorini. Why is this? Keep reading and you’ll find out!
From Washington D.C. she flew to Denmark then Athens all by herself. Leaving the port from Athens she embarked on a journey that would feed her love for geology and broaden her cultural horizons. Was she nervous? Of course, but that didn’t stop her for she had so much force. After nearly a month on Santorini her journal filled with content of the island’s volcanic history. It was as if she was reading a book, each new day brought one more new page to Santorini.

Cape Mavropetra, located on the north shore of Thera, and one of the last pieces to the puzzle. She wrote down her notes as fast as she could, worried that the humidity would ruin the pages in her journal or that she might miss something the professor said. “Phase 4” she wrote down, the last phase of the Minoan eruption. It finally clicked to her what had caused all the destruction. “A phase represents different eruption characteristics. Changes in the vent of a volcano cause changes on the landscape.” She wrote. Phase 4 of the Minoan eruption represents the caldera collapse. Excitement filled her notebook as she wrote down every word. The phases leading up to this one are all unique, but for this girl, phase 4 hit the peak.

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Fig. 1. A map of Santorini with Cape Mavropetra located on a northern shore of Thera. (1)
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Fig. 2. Cape Mavropetra, a lithic lag covered with all the different rock types that are seen throughout Santorini. Photo credit: Natasha Voss.

To be more specific phase 4 is split into two. There are lumpers and splitters in geology, don’t ask why, it’s just what they do. Phase 4a contains more lithic fragments than any other phase. This is an indicator that the volcanic vent was being excavated and was on the verge of collapsing. All the different rock types found on the island can be pinpointed on north eastern shores of Thera. At first the young woman didn’t quite understand. How can plutonic rocks (rocks at considerable depth) be brought up to the surface? She was told to think of a soda bottle. If shaken vigorously the carbonation will fizz immensely once the cap is removed. Most of the soda is lost unfortunately but this analogy proved to be true. “Ah-ha!” She exclaimed. The reasoning for all the rock types found at Cape Mavropetra finally became clear. She was so thrilled that she might have even shed a tear. But the story wasn’t complete, there was much more to learn, for phase 4b took quite the turn.

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Fig. 3. An outcrop of Cape Mavropetra showing phase 4 deposited directly above phase 3. A few of the lithic fragments are circled in orange out of the abundant amount seen in phase 4 which indicates excavation of the vent.

In phase 4b, the caldera collapses sending an eruption column of volcanic material presumably 50 km into the atmosphere. This was it, “the grand finale of the Minoan eruption,” she wrote. The hair on her arms sprung up as if she were in defense mode. Learning about phase 4b was like watching a suspenseful film. Her penmanship was legible only for the fact that she was sitting down while writing them. The explosion was caused by magmatic gases being fully exposed to atmospheric pressures due to the overlying material being blown out in phase 4a. This eruption, approximately 3600 years ago, is the second largest eruption in human history following Krakatoa in 1883.

Rocks are no boring subject to this young woman. Once she learned the volcanic history of Santorini her whole perception of the Hellenic arc changed. Upon arriving at Cape Mavropetra she was given the task to collect all the different rock types that she could. Of course to her, there couldn’t have been a better task. Cape Mavropetra isn’t just any ordinary beach. The rocks littering this beach are full of meaning and significance to the Minoan eruption. As mentioned earlier, the rocks found at Cape Mavropetra are a representation of all the rocks throughout Santorini. This is why Santorini has many types of beaches, such as Red Beach which is made out of cinders. Without the collapse of the caldera the lithic lag forming Cape Mavropetra wouldn’t exist. “Each rock represents a phase and/or time of Santorini.” From the basement rock of limestone to the youngest basalt, it is all seen at Cape Mavropetra.

Once asked to collect the different rock types there was no hesitation from her. Each step she took she bent down to pick up another rock. “Oh, look at this one!” She exclaimed at almost every rock. Not that she was talking to anyone but rather just thinking out loud. The look of amazement draped over her face as she picked up rocks that she had never seen outside of the classroom. After about 10 minutes of collecting the different rock types the class gathered and compared all the rocks. Scoria, schist, pumice, pumiceous rhyolite, limestone, andesite, bombs, breccia, conglomerates, basalt, and dacite were the rocks that the class came up with. What does this mean? What is so fascinating about this variety of rocks? Well, as any geologist or future geologist is born to say, rocks matter! Each one of these rocks has specific conditions in which it forms under such as pressure and temperature. When the caldera collapsed it forced out all of these rocks from those specific conditions and previous volcanic events.

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Fig. 4. Rocks on rocks! This picture is the variety of rocks gathered from Cape Mavropetra.

“Limestone is the basement rock of Santorini. In other words, it was the first rock to comprise Santorini before volcanism.” She wrote in her journal noting that it was one of her favorite rocks collected at Cape Mavropetra. Limestone forms in shallow marine environments and is preserved in the rock record. “The reason we see limestone at the top of Mt. Profitas Ilias, the highest point on Santorini, is because of the normal faults forming grabens and horsts.” Grabens and horsts can be thought of by three blocks undergoing extensional stress causing the two outer blocks to slip downward relative to the center block, Fig. 5. The sample of limestone that caught the young woman’s eye is angularly shaped with a mix of grey, white, and reddish pink hues. Fascinated by the limestone she made sure to set it aside for further examination.

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Fig. 5. A cartoon of grabens and horsts. Normal faulting is caused by extensional stresses. As these blocks are pulled apart the outer two blocks (hanging walls) slip downward relative to the center block (foot wall). Limestone is found on top of Mt. Profitas Ilias due to the normal faulting creating horsts and grabens.
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Fig. 6. Limestone collected from Cape Mavropetra. It is angularly shaped with a mix of grey, white, and reddish pink hues.

As she continued to hurry with her notes to not miss anything she emphasized on the definitions of breccia and conglomerate. Seeing how these two rocks are similar in many ways she didn’t want to get them mixed up. “Breccia, angular clasts within a matrix and conglomerate, rounded clasts within a matrix.” Seems simple but what else sets these two rocks apart from one another other than the shape of their clasts? Origins. Both Breccia and conglomerates are clastic sedimentary rocks but breccia can also have originated from igneous processes.

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Fig. 7. Breccia from Cape Mavropetra. Angular clasts embedded within a mafic matrix.

Lastly, one rock she was having trouble identifying was green schist. As soon as she realized what rock specimen she was holding in her hand a large grin began to take over her face. Green schist is a low grade metamorphic rock meaning that it forms under low pressures and temperatures. On Santorini schist can be found under the Cape Riva Tuff. A steep yet rewarding hike down to Cape Plaka is an excellent place to view the schist.

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Fig. 8. Green schist also from Cape Mavropetra. This rock was broken open to expose a fresh surface for further identification.

At the end of the day when the class departed Cape Mavropetra she couldn’t help but think how this was her favorite location on Santorini. To any other person this could have just been another ordinary beach, though it isn’t meant for swimming or tanning. There was a lithic lag outcrop measuring approximately 7.5 m high and a cobble beach. As the saying goes, beauty is in the eye of the beholder. The beauty of this beach is definitely in the eye of the young woman who measured its outcrop and gazed out to sea on the rocks representing all of Santorini’s history. “What force must have been fueling the Minoan eruption?” She thought. In ways it’s almost unimaginable to think of what it was like here on Santorini about 3600 years ago on the day of the Minoan eruption. The journey for her has come to an end now. Not sure of when or if she’ll return, one thing is certain, she’s got her journal and pockets full of rocks to bring back with her.

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Fig. 9. The young woman standing at Cape Mavropetra after she finished collecting all the different rock types. She has both her arms and pockets full of rocks!

One thought on “Pockets Full of Rocks

  1. Hi Leah – By now you are enjoying Athens for one more night before you head home. Thanks for being so persistent in posting this blog (internet woes).

    Its funny how, as geologists, the most beautiful things on this planet don’t often align with what is most popular or typically accepted as beautiful. That is something that really struck me from this post. Although there are literally millions of people who come here every year, I doubt any of them have seen Cape Mavropetra. In fact, its likely only know to fisherman and other boatmen.

    For us though, this cape hold some of the secrets of the end of the Minoan eruption. I am wondering why you chose to write in third person here? I think it takes away a little from your own excitement and also requires more words (writing for the web should be as concise as possible).

    What made you chose just those three rocks to focus on? I think this blog would have benefited from a brief explanation of all the rocks you found (with pictures), perhaps in a gallery format rather than inserted directly into the text.

    One small thing is that the basement schist here is not only under the Cape Riva Tuff, but all other volcanic material as well (remember Cape Riva is ~21 ka).

    Have a safe trip home and let us know you are there safe! Lisa

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