Which deposit is associated with swelling in the presence of water and is formed from the alteration of volcanic ash?

• A. Bentonite
• B. Gypsum
• C. Halite
• D. Sandstone

Answer: (A)

Swelling clays formed by alteration of volcanic ash, known as bentonite, are characterized by their ability to expand significantly when they come into contact with water. This happens because bentonite is typically rich in smectite minerals (like montmorillonite) which have a layered 2:1 structure with weak interlayer bonds. When water and hydrated exchangeable cations enter between these layers, the layers separate and hydrate, causing substantial swelling and high plasticity. This combination—formation from volcanic ash and pronounced swelling in water—makes bentonite the correct choice.

Gypsum and halite are evaporites that form from drying environments and do not exhibit the same swelling behavior in the presence of water. Sandstone is a clastic rock formed from cemented sand grains and does not arise from volcanic ash alteration or swell in water.