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“Jal” in Jalisco: the history and geology of the Primavera Forest

If you have spent time in Guadalajara, you have seen the sandy-colored hills and oak and pine forests of the Primavera Forest (Bosque la Primavera). Perhaps you have biked or hiked through this semi-desert terrain, weaving around dry shrubs and into the woods to catch sweeping city views. Maybe you've lazed in the naturally hot water of Rio Caliente. But do you know about the history of this place and why the Primavera Forest is so important?

The Primavera Forest, at 30,500 hectares, wraps around the western part of Guadalajara. As nearly the same size as Guadalajara, the second largest city in Mexico with a population of over 5 million, the Primavera Forest is crucial in mitigating carbon. This place is also home to many species, such as the puma, fox and tequila bat.

We spoke with Canadian geologist Chris Lloyd to understand the phenomena behind Guadalajara’s largest protected natural area, wildlife refuge and a UNESCO Biosphere Reserve.

A Dive into History

About 95,000 years ago, a huge volcanic eruption called Tala Tuff unearthed what is now Guadalajara, creating the Primavera Caldera, a large depression in the land. A series of smaller explosions continued as magma pushed out of the Earth, and these plumes of ash and sediment settled to create the hills that now make up the Primavera Forest.

Much of the sediment now found in the Primavera is a lightweight rock called pumice mixed with ash, in Spanish, this mixture is called jal. This porous rock can be seen all over the Primavera and surrounding areas of the state. The overwhelming presence of jal is linked to the name of the state of Jalisco, which translates to “over a sandy surface” in the indigenous language of Náhuatl.

While there haven’t been recent eruptions in the area, the aliveness of the magma chamber beneath the Earth’s crust can still be noticed at Rio Caliente in the Northern part of the Primavera, where natural springs reach up to 70C or 158F. Furthermore, earthquakes are not infrequent because the Primavera Forest is uniquely located at the crux of three fault systems (Colima, Chapala and Tepic).

The Primavera Today

Living on the edge of the Primavera Forest, geologist Chris Lloyd hikes through the pumice-walled canyons and around the forested hills three times a week. Since moving here in 1994, Lloyd has uncovered historical information about the area and noted recent changes due to wildfires, heavy rains and human use.

Showing us around the canyons with some of the world's tallest walls of pumice rock, Lloyd excitedly ran his hands over the layers of ash, pumice and other sediments, pointing out the markers of various magma eruptions. “I like finding out new things that haven’t been discovered,” said Lloyd. “Geology is like creating a puzzle of history without having all the pieces.”

Despite a lack of new eruptions, Lloyd has seen the changes change significantly in the past few decades due to wildfires and flash floods. Due to the long dry seasons in Jalisco and the frequent wildfires in the area (some naturally occurring, others from planned agricultural burns or picnics gone wrong), many trees are burned, weakening root systems and rendering the land less able to hold water. According to Nasa Earth Observatory, there have been 491 forest fires in 2023 in Jalisco alone. Then, when big rain storms come in every summer, the topsoil is easily swept away, which causes the canyon walls to crumble and change drastically.

Lloyd, resting his hand on a massive boulder, looked around the canyon bend to where the boulder sat before rolling more than 10 meters during a flash flood. These extreme natural phenomena also impact the habitat of native species.

Why Does it Matter?

Understanding the explosive history behind the Guadalajara area and the Primavera Forest gives us insight into the unique formations of the area. This area is important in maintaining wildlife populations, keeping the air clean and healthy, and furthermore, when we learn about the history, significance and beautiful aspects of a place, it makes us more invested in protecting it. “The Primavera is a big producer of oxygen, it is about 17 square kilometers, a comparable size to the city of Guadalajara, so for people that breathe, this forest carries a lot of importance,” said Lloyd. We need to care for the land so the land, in turn, can care for us.

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