Excellent question. The movement of gravel-sized (typically 2-64 mm) gypsum crystals in deserts is a fascinating puzzle, as gypsum is a relatively soft and soluble mineral. Unlike quartz sand, which is moved by wind, larger gypsum pieces require more specialized mechanisms.
The primary forces moving gravel-sized gypsum crystals around in deserts are:
1. Water (The Dominant Force)
Despite the arid environment, deserts experience intense, episodic rainfall.
- Sheetwash & Flash Floods: Sudden storms create sheets of water that flow across the surface, capable of transporting gravel-sized material for considerable distances. These flows can concentrate crystals in alluvial fans, playa margins, and arroyo beds.
- Solution & Re-precipitation: Gypsum is soluble. Water can dissolve crystals in one location and re-precipitate them as new crystals elsewhere, effectively “moving” the gypsum in solution. Over time, this can form large crystal clusters (like those in Mexico’s Cave of the Crystals) or pervasive surface crusts.
2. Wind (But with a Twist)
Wind alone rarely moves gravel-sized material. However, it plays a crucial indirect role:
- Deflation & Lag Deposits: Wind erodes and removes finer sand and silt, leaving behind a concentrated layer of larger gypsum crystals and other debris as a desert pavement or lag deposit. This makes it appear as if the gravel has been moved horizontally, when instead, the surrounding material has been removed.
- Salation-Driven Creep: High winds can drive sand grains that saltate (bounce). These impacting sand grains can slowly push or rotate larger gypsum crystals across the surface in a process called creep. This is a slow, incremental movement.
- Dust & Powder Transport: Gypsum can be eroded into a fine powder by abrasion and transported far as dust, later cementing into new forms.
3. Hydrological & Groundwater Processes
This is key for explaining large, pristine crystal formations.
- Capillary Action & Evaporation: Groundwater saturated with calcium sulfate moves upward through the soil by capillary action. At the surface, rapid evaporation causes the gypsum to crystallize, often forming desert roses (sand inclusions) or interlocking crystals that can be gravel-sized. Fluctuating water tables can grow and re-dissolve crystals over cycles.
- Playas & Sabkhas: In dry lake beds (playas) and coastal salt flats (sabkhas), groundwater chemistry and evaporation lead to the in-situ growth and disruption of gypsum crystals. Crystal growth itself can exert pressure, moving adjacent material.
4. Slope Processes
- Gravity & Slumping: On hillslopes, especially after water has lubricated the sediment, gypsum gravel can move downslope via creep or small slumps.
- Biogenic Activity: In rare cases, animal burrowing or root growth can displace crystals.
A Famous Example: White Sands National Park, New Mexico
The iconic white dunes are composed of sand-sized gypsum grains. However, the process illustrates the system:
- Gypsum dissolves from surrounding mountains into the trapped basin of Lake Lucero.
- The lake dries, and gypsum precipitates into selenite crystals (which can be gravel-sized or much larger).
- These surface crystals are broken down by freeze-thaw and wet-dry cycles into sand grains.
- Wind then moves the sand-sized grains into massive dunes.
In summary: For gravel-sized crystals, the major transporter is episodic flowing water. Wind mainly acts to expose them or move them slowly via sand bombardment. The most significant “movement” in place is often chemical—through dissolution and re-precipitation via groundwater—which can completely re-locate the gypsum over time.
