SUBDUCTION OF TECTONIC PLATES
Subduction—the process by which Earth swallows lithospheric plates—is a unique attribute of our planet. Among the terrestrial planets, only Earth exhibits active subduction zones and plate tectonics. Plate tectonics is driven by subduction, a continuous recycling system that helps keep our planet cool. Remarkably, it also sustains the conditions for planetary habitability and evolution. Earth benefits from subduction in countless ways: through the carbonate–silicate cycle, volcanism, magma generation, continental crustal evolution, and rock diversity.
We, as human beings, benefit from living among one of subduction's most magnificent surface expressions: mountains. Subduction zones are intrinsically linked to mountain-building mechanisms. Stretching for nearly 7,000 km along the western margin of South America, the Andes stand as one of Earth's greatest mountain ranges, associated with the Chile-Peru subduction zone. Volcanism has occurred along most of their length from Jurassic to Neogene times. The Central Andes are characterized by the generation of highly silicic magmas. These well-known siliceous volcanic deposits are referred to as ignimbrites. Ignimbrites result from pyroclastic density currents that combine interrelated pyroclastic, effusive, and caldera-collapse processes. The ignimbrites of the Central Andes result from the interplay between accelerated subduction rates and high rates of crustal melting.
Reconstructing the folding of the Andean mountain chain is a remarkable geological story to tell.
-Written by PhD(c) Belén Muñoz
Playa-Lake
“Playa-lake” is a term that combines two geological environments: playa (beach or coast in Spanish) and lake. Although the geological history of a playa lake is neither linear nor literal as the term might suggest, this landscape tells the story of two constantly evolving environments that form the ruins of an ancient saline lake.
The geological history of a playa lake begins with the formation of a lake. This lake results from the accumulation of surface waters that become trapped in semiarid continental regions (endorheic basins). Eventually, this lake becomes saline, as its waters are enriched by salts, sulfates, and carbonates from the substrate. Due to the intermittency of rainfall periods, these lakes are ephemeral. They flood and desiccate intermittently, taking on the appearance of a shallow rather than a deep lake.
What happens when the lake evaporates?
The geological history evolves from a lake to a playa. The playa landscape is arid and saline. As the water level drops, shorelines form, marked by zoned evaporite deposits from the margins toward the interior. This leaves a mixture of mud, clays, gypsum, salt crusts, and polygonal desiccation cracks (mudcracks).
The oldest ruins of the saline lake (pre-dating the Holocene) leave a deposit of thinly laminated and lithified rocks with alternating light and dark colors. These laminations evidence the dissolution of salts and carbonates during the periods of flooding and desiccation.
The geological history of a playa lake begins with the formation of a lake. This lake results from the accumulation of surface waters that become trapped in semiarid continental regions (endorheic basins). Eventually, this lake becomes saline, as its waters are enriched by salts, sulfates, and carbonates from the substrate. Due to the intermittency of rainfall periods, these lakes are ephemeral. They flood and desiccate intermittently, taking on the appearance of a shallow rather than a deep lake.
What happens when the lake evaporates?
The geological history evolves from a lake to a playa. The playa landscape is arid and saline. As the water level drops, shorelines form, marked by zoned evaporite deposits from the margins toward the interior. This leaves a mixture of mud, clays, gypsum, salt crusts, and polygonal desiccation cracks (mudcracks).
The oldest ruins of the saline lake (pre-dating the Holocene) leave a deposit of thinly laminated and lithified rocks with alternating light and dark colors. These laminations evidence the dissolution of salts and carbonates during the periods of flooding and desiccation.
-Written by PhD(c) Belén Muñoz