Scientists Study El Yunque’s Exceptionally Slow Erosion
February 28, 2013

Puerto Rico’s El Yunque Withstands The Test Of Time

Alan McStravick for — Your Universe Online

The lush tropical island of Puerto Rico is completely covered with plants and fauna. The exception to this rule resides on the large, majestic, flat-topped promontory known as El Yunque. El Yunque, Spanish for “the anvil,” rises high into the sky above the rivers and streams that run below, and has been a Puerto Rican icon since pre-Columbian times.

Researchers from the National Science Foundation´s (NSF) Luquillo Critical Zone Observatory (CZO) wanted to learn why, in this humid climate, El Yunque has escaped being taken over by the native plant life. The striking absence of which has led to a slower erosion rate for this rock face. The most recent study was undertaken to measure the current rate of the rock´s erosion.

When you venture out to El Yunque, you typically find it stretching its full 3,412 feet into the sky where it reaches high up into the mist from the low lying clouds. The bald-faced rock, often buffeted by hurricanes brought in by the trade winds, stands as a high spire amidst the miles of rainforest that surround it on all sides.

As a result of the rains that shower El Yunque up to three times daily, the area immediately around El Yunque is subject to an average greater than 13 feet of rain each year. These rains flow off of El Yunque, flowing down through the Luquillo watershed, eventually joining the nearby rivers and rivulets.

The NSF, which oversees the Luquillo CZO, also monitors five other CZO´s in watersheds across the US. Other CZO´s are located in the Southern Sierra Nevada, Christina River Basin on the border of Delaware and Pennsylvania, Susquehanna Shale Hills in Pennsylvania, Boulder Creek located in the Colorado Rockies, and the Jemez River and Santa Catalina Mountains in New Mexico and Arizona.

The NSF and its scientists believe this oft-misunderstood critical geologic zone, represented by the smallest extension of Earth that rises above a forest canopy all the way down to the base of the structure, is ripe for examination. They hope their work will foster a new understanding of these critical zones and how they affect the surrounding ecosystems.

The researchers claim the importance of these CZO´s reach into several individual and important components. From affecting the water cycle to the breakdown of rocks that will eventually play a role in soil formation to patterns of plant growth and landforms, these CZO´s are responsible for each of these processes occurring within the immediate area. CZO´s even play a role in the evolution of nearby rivers and valleys.

"The critical zone is our living environment," says Enriqueta Barrera, program director in NSF's Division of Earth Sciences, which funds the CZO network. "The CZOs offer us new knowledge about this important zone and its response to climate and land-use change."

Barrera goes on to state that the CZO´s are the first systems-based observatories with a primary dedication to understanding just how the Earth´s surface processes are coupled. “They will help us predict how the critical zone affects the ecosystem services on which society depends.”

El Yunque, with its slow erosion rate, required a new model developed by scientists Jane Willenbring, Gilles Brocard and the late Frederick Scatena of the University of Pennsylvania, to calculate exactly how this iconic rock has changed through time.

In order to do this, they developed a method that involved counting isotopes, or variants, of chemical elements that are known to accumulate in rocks after they have been subjected to cosmic rays originating in space.

These variants, known as cosmogenic nuclides, aided the team in confirming soils in forests that are undisturbed by human activity will erode at a rate of 250 to 500 feet every one million years.

Using Puerto Rico as an example, we learn the undisturbed forest areas on this tropical island have, since the first Europeans stepped foot in the new land in 1498, have eroded 1.6 to 3.2 inches. Additionally, the research team found the presence of forests, even in steep, hurricane prone environments, can aid in greatly reducing erosion.

One thing that is known about the Luquillo critical zone is that, chemically, it is weathering at a wide range of rates. “But its thick weave of matted roots and vegetation holds in the soil and stabilizes the hill slopes such that they erode more slowly than one would expect,” says Willenbring.

Also located within the Luquillo critical zone are areas that experience an escalated erosion rate. As an example, the well-known La Coca Falls are eroding comparatively quickly. This is caused, in no small part, by the fast rush of the waters through steep canyons and gullies where it is known there is a large transference of both gravel and boulders due to the fast moving water.

"A wave of erosion--whether fast or slow--affects all parts of the critical zone," says Willenbring. "It sets the tempo for how quickly minerals and nutrients are ferried to the surface, which in turn feed the forest above.

"We were surprised by how connected the landscape is. It seems as though even the trees understand geomorphology."

The research team found they were encountered by all new questions as each new piece of information came to light. They wanted to determine the passivity of soil microbes and trees. They also wanted to learn if these organisms actively sought to position themselves in the most fruitful environment or if they worked to actually change the environment they were already in.

It was the examination of the above mentioned cosmogenic nuclides that factored into their being able to address those questions and more. This was the first study that allowed researchers to make the first measurements of the erosion rate of the peak of El Yunque.

Much like the lush soil and its slow erosion, the researchers were able to determine the surface of El Yunque has, since the arrival of the first Europeans, experienced erosion equal to .08 inches. That equates to eroding about 13 feet for every million years.

According to the team, it is this relatively slow rate of erosion that provides explanation as to why El Yunque still soars above the forest.

"The texture and composition of the rocks that form El Yunque are more erosion-resistant than those of the surrounding landscape," says Willenbring.

But what is it about El Yunque that gives it this uncharacteristically slow rate of erosion? As it turns out, El Yunque is a remnant of an ancient supervolcano. This volcano, known as Hato Puerco, was one of the largest and most active volcanoes for the region. Hato Puerco existed during the Cretaceous period some 145-66 million years ago.

"El Yunque's hardness and chemical properties came from being 'cooked' in the chamber of the volcano," says Willenbring. Other rocks are not typically subjected to these same heat conditions. Therefore, other rocks are “softer”, geologically speaking, and are less resistant to chemical breakdown and erosion.

Willenbring goes on to describe El Yunque, this Puerto Rican icon, as a hard-headed cap atop the island. This cap is one that managed to escape the same geologic fate of all other rocks on this tropical isle.