A Landscape of Fire and Water
The Garrotxa volcanic field is the best-preserved volcanic landscape on the Iberian Peninsula. Volcanic activity in this region began around 700,000 years ago, creating a landscape of over 40 distinct volcanic cones and more than 20 basaltic lava flows. The field's monogenetic nature means each volcano erupted only once, over a period of days to months. The most recent confirmed eruption occurred approximately 11,500 years ago, involving a complex event that formed both the Croscat and Santa Margarida volcanoes.
Two main eruption styles define Garrotxa's geology. The first is Strombolian, characterized by the explosive ejection of gas and pyroclasts (scoria, lapilli, and volcanic bombs)which build up classic cinder cones. The Croscat volcano is a prime example, rising 160 meters above the plain. The second, more explosive style is phreatomagmatic. This occurs when rising magma at over 1000°C encounters underground water, causing violent steam explosions. This activity creates wide, circular craters called maars. Santa Margarida volcano, with its 500-meter-wide crater, resulted from such a phreatomagmatic phase. This interaction between magma and water shaped much of the region's topography.
A Timeline Written in Magnetism
The volcanic field was not continuously active but experienced bursts of eruptions separated by long periods of dormancy. Scientists unravel this timeline using techniques like paleomagnetic dating. As lava cools, magnetic minerals within the basalt align with the Earth's magnetic field at that specific moment in time. Since the magnetic field's orientation and intensity shift over millennia, this "frozen" magnetic signature dates the lava flow. By comparing these signatures from different flows to a known timeline of Earth's magnetic field changes, geologists can constrain the age of eruptions.
This method, combined with advanced radiometric dating, has refined the history of the Garrotxa field. For instance, the two lava flows that form the dramatic 50-meter-high cliff upon which the town of Castellfollit de la Roca is built have been dated to approximately 217,000 and 192,000 years ago, respectively. More recent studies have dated the last eruption of the Croscat volcano to between 13,000 and 15,000 years ago. While the field is currently quiet, the pattern of clustered activity suggests that the underlying geodynamic conditions that allow magma to reach the surface have not changed, making future eruptions a possibility.