Rock Record: Santa Cruz Natural History in the Palm of your Hand

By Gavin Piccione and Graham Edwards

Even the smallest and most seemingly ordinary rock tells a story about the processes that have shaped the area in which it is found. From this point of view, beaches are a massive archive of the geologic history of a landscape that contains tiny samples of all the rocks that make up the surrounding area.

In this installment of Rock Record, the Geology Gents (Graham and Gavin) spend a day at the beach exploring the natural history of Santa Cruz looking only at the sand and rocks that fit in the palm of their hand.

Join the “conversation” below, as the Geology Gents chat about what they see when they look at the sand.

Building Beaches through Erosion and Transport

Graham: For all of the attention that beaches get as beautiful places to go lay in the Sun, they really are interesting geologically.

Gavin: We usually think of beaches as a geomorphic landscape: a product of the movement of wind and water and how it shapes and transports geologic material. We don’t usually talk about them as geologic.

Graham: We talk about sea cliff erosion and longshore currents that move sands from these cliffs, as well as nearby rivers and streams. The longshore currents carry their sediment load along the coastlines, and pile them up in certain “low-energy” places, that we call beaches.

Gavin: Ah, yes, I remember we spent some time chatting about this in one of our Rockin’ Pop-ups about Coastal Geology.

A Far-Reaching Collection of Rocks

Graham: But for all our talk of beaches as products of weathering (the breakdown of geologic material) and erosion (the transport of geologic material from one place to another), are they not, my friend, still rocks and something of lithologic interest?

Gavin: Lithology: the study of the characteristics of different rocks and rock types. Well, this sand is made up of rocks, and if we can identify the rock types and note their other properties, then we might be able to determine some interesting things about the geologic origins, or provenance, of these sands, as we often do with larger rocks.

Graham: Let’s take a handful then and look more closely at these sand grains.

Gavin: The sand grains are mostly round. Many are clear and glassy looking, some with a smoky dark grey color. We are familiar with this mineral of course. Quartz!

Graham: Some are blocky looking and cream-white to pinkish in color. Another favorite! Feldspar!

Gavin: Those light-colored feldspars and quartz make up most of the sand, but there are a few other darker minerals. Dark flaky sheets of biotite, black grains of other mafic minerals like amphibole or pyroxene, and some greenish looking minerals – these could be pieces of serpentinite (a Central Coast favorite!) or glauconite (fossilized ocean muck).

Graham: You know, the relative abundances of these minerals really tell you something about how certain rock and mineral types survive the weathering and erosion process. The feldspar and the quartz, especially, are relatively rugged minerals that resist being broken down by the chemical actions of water and the mechanical actions of tumbling around in rivers and waves. But the greenish and darker minerals, which are less resilient to weathering, are very rare. Most of them have been crushed or dissolved to far smaller sizes and have been swept out to sea.

Gavin: So a lot of this is geomorphology? This crushing and grinding of sediments down as they tumble through rivers and waves crash over them along the coast?

Graham: Without question, the erosive forces that shape beaches matter a great deal. If many of these rocks come from the Santa Cruz mountains, they have travelled a great distance and been worn down along the way. Their small size and rounded, almost smooth shapes speak to this long journey.

Gavin: And these pebbles? They are much larger and have sharper angles. There are pieces of mudstone, limestone, and fine and coarse sandstones. Common rocks found along the coast here. Likely these came from sea cliffs nearby that were worn by waves. Since these have not had to travel so far, they have not been weathered quite as much.

Graham: Yes, only the closest rocks can survive in pebble form. 

A Hint at the Tectonic History of California

Gavin: And yet, there is one pebble type here that is not familiar from our coastal outcrops: these granite pebbles. Where could these come from?

Graham: Well, granite, made up mostly of quartz and feldspar with large interlocking mineral crystals, is particularly durable when it comes to weathering. Perhaps it has survived a longer journey than the other pebbles.

Gavin: But from where? These granites remind me of the granite of the Sierra Nevada.

Graham: That must be granite from the Salinian Block! Carried up from the region of today’s southern California by the San Andreas fault. These granites formed along with the rocks of today’s Sierra Nevada over 100 million years ago in huge magma chambers. Over the last 30 million years, the San Andreas fault has carried the Salinian block up to the northwest, pulling rocks from the southern extents of the Sierra Nevada along with it. 

Gavin: Ahh, and so the Santa Cruz mountains are full of this granite! So that’s where all this quartz and feldspar are coming from?

Graham: That’s right! The “basement” rocks of the Santa Cruz mountains are these Salinian Block granites overlain by sedimentary rocks and metamorphic rocks of the last 30 million years or so. Quartz and feldspar sands come mostly from the granites, though some may be recycled from sandstones.

Gavin: How efficient, that sands from ancient beaches are weathered out of sandstones and returned to beaches today.

Graham: And bits of serpentinite and mafic minerals are also weathered out of other rock types and incorporated into the sands.

Gavin: So the sands at these beaches are not just the result of wind, rivers, and waves. They’re the whole of the Santa Cruz mountains and even nearby coastal cliffs, ground to fine grains and mixed well.

Graham: When we take a trip to the beach and sit in the sand, we really do find ourselves in the rocks of the Santa Cruz mountains. And while it’s a little bit harder to see and recognize each individual rock, when you look closely you can see each little bit of it.

Rock Record is a monthly blog featuring musings on the mineral world from Gavin Piccione and Graham Edwards.

Graham Edwards and Gavin Piccione are PhD candidates in geochronology with the Department of Earth and Planetary Sciences at UC Santa Cruz. They also host our monthly Rockin’ Pop-Ups as “The Geology Gents”.

Rock Record: Caverns of Time

By Gavin Piccione and Graham Edwards

Caves are an intersection between natural destruction and creation, where existing rocks are eroded away, and new rocks are continuously formed. Descend into one of the many caves in Santa Cruz and you’ll get a unique viewpoint of the striped marble walls that were built offshore of the area’s ancient shorelines and thrust up onto the continent over the past million years.

This geologic flux also means that one can never enter the same cave twice, since the competing geologic processes of erosion and precipitation are constantly shaping the interior. When you visit a cave, you’ll be a first-hand witness to the geologic processes that shape the Earth.

Gavin and Graham explore speleological features in Empire Cave on the UCSC campus.

Image of marble eroded by water.
Evidence of dissolution in marble in Empire Cave.

Solutional Caves and Karst

The caves of Santa Cruz are called solutional caves because they form when groundwater dissolves pathways through rock, just as sugar or salt dissolve in warm water. However, to dissolve rock, it takes more than just plain old water. Rainwater and groundwater carry dissolved carbonic acid and organic acids that are pulled out of the soil as water percolates down from the surface. In Santa Cruz the bedrock (the solid rock beneath the soil) is mostly marble and made of the mineral calcite which is very susceptible to being dissolved by weak acids like these. Thus, the slightly acidic water can easily dissolve its way into the rock.

You can try this at home or in the field! Find a piece of marble or limestone, scratch at the surface a little bit with a paperclip, key, or coin to make a powder. Drip some vinegar (which is also a weak acid) on the powder and watch it fizz as the calcite dissolves!

As rainwater and groundwater slowly dissolve their way through the marble bedrock over thousands to millions of years little cracks turn into large caves, where the many dissolved minerals such as calcite precipitate, or become solid and separate out of the liquid, forming speleothems. These water-sculpted caves have a distinctive structure and shape: smoothed and rounded marble walls filled with holes and tunnels, covered with speleothems giving most surfaces a ropey, grooved, and nodular texture. In karst systems caves can develop as we’ve described, but if they grow close enough to the surface, the caves can’t support the weight of overlying rock and they collapse forming sinkholes, a familiar feature in the Santa Cruz area.

Since karst caves are formed by flowing water, water often continues to flow into them. Especially in the rainy wintertime, caves can partially or completely fill with water, so it’s important to be very careful whenever entering or exploring caves and postpone your spelunking if it’s rainy!

Sinks of Gandy, a karst cave in West Virginia.

Below the Surface

One of the most scientifically alluring aspects of caves are the preservation of rocks and artifacts that their sheltered interiors provide. Materials within caves are not subject to erosion or weathering to the extent that those exposed to the atmosphere are. Caves also stay the same temperature year-round because they are not affected by atmospheric temperature fluctuation, and instead have temperatures regulated by the ability of the surrounding rock to hold heat. These mild conditions help artifacts like the Dead Sea scrolls and cave paintings to survive for millennia. Similar to archeological preservation, rocks that would normally erode quickly at the surface of the Earth are remarkably well preserved in caves.

Ancient cave painting from Lascaux Cave, France (Wikipedia)

Caves are treasure troves of samples for geologists, biologists, hydrologists, and countless other kinds of scientists. Among the most exciting (at least for us geochronologists) are records of changing climate found in speleothems. As this calcite builds thicker and thicker speleothems, it captures the chemical signature of the climate at the time of its formation. By studying these speleothems, scientists can reconstruct the timeline of global climate change over the past 650,000 years.

Lava tube and lava stalactite at Craters of the Moon National Monument.

Other Types of Caves

Some caves form at the same moment the rock forms, usually from cooling lava. Some examples of these volcanic caves are lava tubes — an open tunnel left behind when a subterranean conduit of lava drains. Caves can also form in the deep chasms left behind by rifts, where volcanoes split the land apart as they swell. Since lava flows, volcanoes can leave all sorts of other cave-like voids behind from the flow, storage, or drainage of lava.

Sea caves, or littoral caves, form where waves carve out deep caverns into sea cliffs, usually into weaker parts of rock. We even see a few shallow sea caves along the Santa Cruz coastline!

Anchialine caves are caverns that connect inland pools, called anchialine pools, to the ocean where the cave ends underwater. The water levels in these pools often rise and fall with the tides, and these are popular sites for scuba-spelunking!

Caves can even form above ground! Talus caves form in the spaces between large boulders at the base of rocky cliffs, and if water flows into or under a glacier, it can melt out glacial caves!

Critters in Caves

Meta dollof spider photo from our exhibit Crystals, Caves, and Kilns (2013).

While we most often associate caves with bats, caves are home to all sorts of organisms, including fungi, arthropods (insects, spiders, scorpions, and the like), salamanders, and fish. These creatures are usually adapted to live in these perennially dark, often flooded environments. Because caves are often isolated and disconnected habitats, many creatures that live in caves are endemic, meaning they are found in that cave network and nowhere else on Earth!

For example, the Cave Gulch cave network, just North of Santa Cruz, is home to two endemic species: the spider Meta dolloff and the pseudoscorpion Fissilicreagris imperialis.

Be very conscientious when you visit caves since these are the only homes for many of the creatures that live there. Never leave trash, burn fires, or damage the interiors of caves. If you do go into caves, be sure to pack trash out, or even better, enjoy the cave from the outside and leave its subterranean residents in peace.

Rock Record is a monthly blog featuring musings on the mineral world from Gavin Piccione and Graham Edwards.

Graham Edwards and Gavin Piccione are PhD candidates in geochronology with the Department of Earth and Planetary Sciences at UC Santa Cruz. They also host our monthly Rockin’ Pop-Ups as “The Geology Gents”.