Geology · what lies beneathOld rocks, young mountains
The forest grows on rock more than 500 million years old, but the mountains that hold it up rose only a few million years ago. That uplift put a wall in front of the moist air from the east, and that is where the Yungas began.
Geological cross-sectionFrom Santa María to San Miguel de Tucumán
A simplified west–east slice at the latitude of Tafí del Valle. Tap a layer to see what it is.
Late Precambrian – Early CambrianMetamorphic basementSea-floor sands and muds, folded and turned into phyllite and schist. The schists of the Sierra de San Javier have given ages of 565 to 537 million years. It is the rock of almost every range.
Simplified sketch · c. 110 km across · vertical exaggeration ×5 A story in five chapters
Not to scale. - c. 565–537 MaA sea floor is folded
The basement sediments are deformed and turn into schist.
- OrdovicianGranite intrudes
Magma rises and cools inside the crust.
- Late Cretaceous – PaleogeneA rift and red beds
The crust stretches and fills with red sandstone.
- Late Miocene – PlioceneThe ranges rise
Basement blocks rise between faults. Behind the Aconquija the climate turns drier.
- Last 165,000+ yearsDust rains down
Wind covers valleys and slopes with loess, and soils form on it.
Why it matters for the forest
1The mountain makes the rain.Moist air from the east hits the ranges, rises, cools and drops rain and fog on the eastern slope. Behind lies the rain shadow of the dry valleys.
2Steep, fragile slopes.Schist splits into flakes and forms thin soils. With summer rain, cleared slopes slide easily.
3Soils that came on the wind.The loess of the foothills and plain makes deep, fertile soil. That is why the edge of the foothill forest was the first to be cleared for crops.
4Ground that is still moving.The faults that raised the ranges are still active, and earthquakes and erosion keep shaping the ravines.
SourcesGonzález, O. E., Viruel de Ramírez, M. E., Mon, R., Tchilinguirian, P. y Barber, E. (2000). Hoja Geológica 2766-II San Miguel de Tucumán, 1:250.000. SEGEMAR, Boletín 245. · Kemp, R. A. et al. (2004). Pedosedimentary development of part of a Late Quaternary loess–palaeosol sequence in northwest Argentina. J. Quaternary Sci. 19: 567–576. · Kleinert, K. y Strecker, M. R. (2001). Climate change in response to orographic barrier uplift. GSA Bulletin 113: 728–742. · La Formación Puncoviscana y unidades estratigráficas vinculadas en el Neoproterozoico – Cámbrico temprano del noroeste argentino. Latin American Journal of Sedimentology and Basin Analysis.