Which best defines alluvial deposits?

Which best defines alluvial deposits?

• A. A mineral-rich rock formed by crystallization deep underground
• B. A deposit formed by meteorite impacts
• C. A deposit of volcanic glass near lava flows
• D. A deposit where gems are eroded from their source rock, and then transported away from the source and further concentrated

Answer: (D)

Alluvial deposits are sediments laid down by running water after rocks are weathered and eroded. As a river or stream moves material away from its source, heavier minerals—such as gems—are carried downstream and eventually settle out and concentrate where the water slows, creating placer-like deposits. This is exactly the idea of gems being eroded from their source rock, transported away, and then concentrated in a new location. The other scenarios describe processes that aren’t driven by surface water transport and deposition: crystallization underground forms mineral-rich rocks, meteorite impacts produce impact-related materials, and volcanic glass forms from cooling lava near eruptions.

Alluvial deposits are sediments laid down by running water after rocks are weathered and eroded. As a river or stream moves material away from its source, heavier minerals—such as gems—are carried downstream and eventually settle out and concentrate where the water slows, creating placer-like deposits. This is exactly the idea of gems being eroded from their source rock, transported away, and then concentrated in a new location.

The other scenarios describe processes that aren’t driven by surface water transport and deposition: crystallization underground forms mineral-rich rocks, meteorite impacts produce impact-related materials, and volcanic glass forms from cooling lava near eruptions.