Green Pages

If Rocks Could Talk . . .

Their story would be fascinating.

By Carmen Hoyt, Waterfront Assistant, The School for Field Studies

After how many birthdays do you stop keeping track? If it’s any consolation, the Earth is 4.54 billion years old and still going strong.

4.54 billion years . . . think about it. A billion is difficult to grasp, not to mention four times over. If every second of our day counted for a year of Earth’s life, it would take about 144 years, far beyond the current capacity for a human lifespan.

The history of our planet is so incredibly long that it’s easier to conceptualize as a calendar year. If our planet’s first day on the job was January 1, modern humans didn’t evolve until December 31 at 11:38 PM. Twenty-one minutes later, at 11:59 PM, began the Holocene Epoch, the equivalent of 12,000 years ago when the most recent ice age—the Paleolithic Ice Age—came to an end and the Earth as we know it began.

So, what happened in that metaphorical year before humans existed? If the planet could talk, what stories would it tell us? Unfortunately, our Earth is vocally inhibited, but it tells us stories in different ways. Geology, the study of rocks and the processes that impacted them, is an excellent method of communication, especially if you are interested in a little history lesson. I decided to listen to a chapter about the Turks & Caicos Islands, and here’s what I learned.

In the context of the calendar year analogy, the Turks & Caicos have their origin somewhere around December 15, after the fourth mass extinction marked the end of the Triassic period, the beginning of the Jurassic period, and the slow break-up of the supercontinent Pangea. It was during this time that dinosaurs ruled and the climate across the planet didn’t stray far from hot and dry. The fourth mass extinction failed to wipe out the dinosaurs, but their luck would be tried again all too soon.

Pangea’s demise began when Gondwana (the agglomeration of the African, South American, Antarctic, Indian and Australian continents) drifted away from Laurasia (Eurasia and North America). The foundation for the Turks & Caicos, along with the Bahamas, grew from the continental crust that North America pulled away from Africa during the split of Gondwana and Laurasia 200 million years ago. The crust on most of the planet, including the Turks & Caicos, is composed of basalt, a dark, dense rock that results from the cooling of lava or magma. 

This huge cave on the beach at Mudjin Harbor in Middle Caicos is known as a flank margin cave.

The basaltic crust was buried deep under layers of limestone that formed during the Jurassic, Cretaceous and the Tertiary periods (a time span of nearly 197 million years) from biological sources in warm, shallow seas typical of the Caribbean we know and love. Less typical, however, was the marine life responsible for such limestone deposition. Limestone is made from the mineral calcite, which is derived from the calcium carbonate structures found in reef-building species of prehistoric hard corals or the boxy shells of conical, clam-like organisms called rudists. Calcite can also be deposited from the ocean water itself, though this process is not quite as common.

During this time, reefs were growing and thriving, and about 3 million years ago, the closure of the Central American Seaway with the surfacing of Panama definitively sealed off the Caribbean from the Pacific, separating species indefinitely. The Tertiary period lasted up until the ice ages of the Pleistocene, a three-and-a-half-hour time period on December 31 in our grand analogy just prior to the start of the modern Holocene at 11:59 PM. It was during these chilly hours that reef development (and limestone deposition) in the Caribbean and elsewhere came to a halt with cooler temperatures and falling sea levels.

Conch Bar Caves in Middle Caicos are one of the largest cave systems in the Caribbean.

Two interesting features of the limestone-dominated geology of both the Turks & Caicos and the Bahamas are an intricate cave system and blue holes, both products of what is called a Karst landscape. The Turks & Caicos Islands boast one of the largest cave systems in the Caribbean: the Conch Bar Caves located in Middle Caicos. Caves like these form when rainwater (which is slightly acidic through interactions with carbon dioxide in the atmosphere) percolates through depressions and cracks of the limestone, collecting in cavities that grow in volume as weaker parts of the limestone dissolve, eventually connecting as underground caves.

In South Caicos, students studying at The School for Field Studies’ Center for Marine Resource Studies are always oriented to their new home at the beginning of every session with a town tour. One feature we always point out is the “Boiling Hole” at the center of the decommissioned salinas, where valuable salt was produced during the 1700s. The Boiling Hole is an entrance to an underground cave system that is connected to the ocean, and it was used to control the saltwater entering the salinas.

Blue holes, in my humble opinion, are even more mysterious and alluring. They are appropriately named; even if you’ve never seen one, you can imagine what it would be like. They’re spectacular geological features with an unrivaled appreciation for geometry. Like deep, circular, underwater sinkholes with sheer walls, they look more like a relic of plugs taken by extraterrestrial life than a story of Earthly geology.

One of the world’s most famous blue holes is in Belize, but the Bahamas’ best kept secret is that there are quite a few around the archipelago, with guest appearances in the Turks & Caicos Islands. I once noticed one on a flight from Providenciales to South Caicos. Turns out, if you look at a satellite map of Middle Caicos, its deep blue color stands out among the light sand of the Caicos Bank just off the coast of the land.

So, what is responsible? Limestone is the necessary ingredient, but a few key steps must take place. First, the stone must be exposed to the atmosphere to start the weathering process. It is during cooler periods in the Earth’s climate that ice is created, absorbing some of the ocean water and lowering sea levels. On limestone-based islands, the less-dense freshwater source floats atop the more-dense marine ground water.

The initial forms of blue holes resemble small freshwater ponds. Similar to the way rain water dissolves rock by intruding along cracks and seams to form caves, the freshwater source exposed to the atmosphere dissolves carbon dioxide and forms a weak acid. However, there is an additional acidic solution created from the interaction of the fresh water with the salt water at the mixing zone below. Picture a liquid plunger of sorts, drilling deep into the Earth over millions of years. These two processes carve the vertical walls of the blue hole as sea level rises and falls with changes in the climate.

Such changes were characteristic of the Quaternary period, starting with the ice ages of the Pleistocene. It was during this three-and-a-half-hour time period that the sea level was 300–400 feet lower than it is today, exposing the Caicos Bank as a cliff-fringed plateau where these caves and holes started forming. 

It’s hard to imagine the Turks & Caicos as anything other than the warm, tropical islands they are today, but they have endured quite the journey through time. What started off as dense, volcanic rock deep below the surface of the sea became a thriving coral reef, built up by many layers until exposed to the atmosphere and intricately carved by rainwater. Of course this is a vast oversimplification of many hundreds of millions of years of change, but it’s a lesson in patience and metamorphosis. Any exposed rock formation can give you a clue as to how it ended up the way it is if you’re willing to look and listen, carefully.

For additional information about The School for Field Studies, visit www.fieldstudies.org or contact us on South Caicos at hhertler@fieldstudies.org.



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On the Cover

Agile LeVin—photographer, explorer and chronicler of everything TCI on his website www.visittci.com—took this drone photo of the multi-textured wetlands of West Caicos. He was part of the expedition that investigated the site of the historic pirate attack in the area. For more information and photos, go to page 48.

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