The correct option is 2 and 3 only.
Explanation
Sand dunes are depositional landforms created by the wind in arid and semi-arid environments. The specific shape of a dune is determined by factors such as the direction and strength of the wind, the abundance of sand supply, and the presence of vegetation.
- Statement 1 is Incorrect: Barchans are crescent-shaped dunes that form in areas with constant wind direction and limited sand supply. Their defining characteristic is that the wings (or points) are directed away from the wind direction (downwind). The windward side is convex and gently sloping, while the leeward side is concave and steep.
- Statement 2 is Correct: Parabolic dunes are often referred to as reversed barchans. They typically form when sandy surfaces are partially covered by vegetation. The vegetation anchors the arms of the dune, causing the wings to point towards the wind direction (upwind), while the loose sand in the center is blown forward.
- Statement 3 is Correct: Seif dunes are similar to barchans but are characterized by having only one wing or point. This morphology results from a shift in wind conditions, which causes one wing of a barchan to elongate significantly while the other remains underdeveloped or is modified.
Key Takeaway: The crucial difference between Barchans and Parabolic dunes is the orientation of their wings: Barchan wings point downwind (away from the wind), whereas Parabolic dune wings point upwind (towards the wind).