An isthmus made of sand is known by which geographic term?

An isthmus made of sand is referred to as a tombolo. This term specifically describes a landform that connects an island to the mainland, often formed by the accumulation of sediment, such as sand, deposited by waves and currents. Tombolos are typically characterized by their narrow, sandy connections and can create unique ecosystems and landscapes.

The understanding of this term is crucial in geography, as it highlights the interaction between marine processes and landform development. In contrast, the other terms do not describe an isthmus made of sand: an erg is a large desert area with sand dunes, karst refers to landscapes formed by the dissolution of soluble rocks, leading to features like sinkholes, and a polder is a tract of low land that has been reclaimed from a body of water, often seen in the Netherlands. Thus, the choice of tombolo accurately aligns with the definition of a sandy isthmus.