Last updateFri, 17 Jun 2016 12pm

Canadian initiates first geological study of Jalisco’s obsidian deposits

In 2013 I paid my first visit to Selva Negra, the popular name for the Ahuisculco Flora and Fauna Sanctuary, located 30 kilometers southwest of Guadalajara.

This is a biological corridor linking Bosque la Primavera and the Sierra de Ahuisculco. Selva Negra is actually the title of a song by Guadalajara rock group Maná which finances the restoration and conservation of the corridor in cooperation with the local ejido.

I was introduced to this bosque by Franky Alvarez, who summed it up as “two big cerros, one composed of black obsidian and the other of red.” Indeed, anyone who takes a stroll along the paths of Selva Negra can’t help be impressed by the huge quantities of obsidian underfoot, obsidian of great purity and quality.

But something else impressed me on that first visit three years ago. We had parked our car in a quarry before entering the park on foot. It was obvious they were mining jal, which consists of pumice and other solids ejected during a volcanic explosion, but a close examination of the quarry wall revealed something I had never seen before. In the jal there were literally hundreds of hard, round balls, obviously not chunks of pumice. What were they?

That’s the question I asked Canadian geologist Chris Lloyd and it triggered his own geological study of the Ahuisculco area, a study which is still ongoing today.

As for the naturally formed stone balls, Lloyd learned that they were spherulites, formed by crystalization and frequently associated with deposits of obsidian. Occasionally these round formations can grow to enormous sizes, in which case they are called megaspherulites, an example being the Great Stone Balls or Piedras Bola of Ahualulco, which sometimes reach a diameter of three meters.

In February, archaeologist and obsidian specialist Dr. Rodrigo Esparza visited the quarry and surrounding obsidian deposits. He discovered that the area was dotted with the remains of ancient obsidian mines and workshops and pointed out to Lloyd that no geological study of these or any of Jalisco’s other obsidian deposits (which are the fourth largest in the world) had ever been undertaken.

All of this piqued Lloyd’s curiosity and for the last year he has been engaged in figuring out the mechanisms which produced the Ahuisculco flows and in mapping their extent.

“So far I’ve mapped 14 square kilometers,” he told me, “and I’ve found 69 individual flows spread over an area of 359 hectares.”

With the help of several “sherpas” Lloyd collected around 200 samples and delivered them to Esparza for trace geochemistry analysis, which produces an extremely detailed list of all the elements present in a piece of obsidian. “We need this data to connect all these flows together,” said Lloyd.

Lloyd has found that most of the obsidian in the Ahuisculco flows is pure black or black streaked with gray, with occasional intrusions of “Indian Blood” obsidian which is a mixture of red and black. 

“I also discovered that there have been some notable eruptive phases in the area. The obsidian was not just flowing on the surface, but there was at least one big bang, which left tuft deposits atleast two or three meters deep.”

Detailed mapping of obsidian deposits has been carried out in very few places around the world.

“In Mexico, there have been a few geologic studies of the Sierra de Pachuca – the most famous obsidian in Mesoamerica,” said Lloyd.  “The only problem there is that most of the actual obsidian is buried 20 to 40 meters under an avalanche deposit, so there’s not much detail as to the original obsidian flow.” 

As for Jalisco, nothing of the sort has ever been attempted and the size of the Ahuisculco flow was unsuspected. “It could turn out to be the biggest flow in western Mexico,” said Lloyd, noting that in the course of his explorations he has located over 300 obsidian mines and workshops. (A typical mine may often appear as a low depression, typically surrounded by thousands of “rejects” among which one might find broken or incomplete knives and spearheads.)

Finishing the study is turning into a long-term project, Lloyd recognizes. The sierra contains 100 square kilometers and he has covered 14 in a year. 

“Unless I step up my visits – I’ve been going there once a week – I’m looking at another three years, at least,” he said.

If you would like to visit Selva Negra, check chapter five of my book “Outdoors in Western Mexico Volume Two” or see Wikiloc.com under GuadHikes – Ahuisculco to Selva Negra Woods.


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