Final answer:
Younger islands typically have black sand beaches due to fresh volcanic activity, while older islands have white sand beaches composed of coral and shell materials. The color change from black to white sand indicates the island's geologic and biologic evolution over time.
Step-by-step explanation:
The composition of sand on beaches is influenced by the geological activity of the surrounding area. Younger islands, often formed by recent volcanic activity, display sand that is derived from lava and is characteristically black due to the presence of basalt and other dark, volcanic minerals. As islands age and coral ecosystems develop, erosion and weathering processes break down coral skeletons and other marine organisms, resulting in white sand beaches comprised of biogenic materials like coral and shells. This transition reflects the different stages of island evolution, with younger islands boasting black sand beaches and older islands typically showcasing white sand beaches.
Beaches on youthful islands, with their fresh basaltic substrates, do not typically contain the bioclastic components found on more mature islands where coral sands, often white or pinkish due to the coral and shell material, prevail. Additionally, the description of the sand in various locations on older islands, noting features such as medium to fine grain size and the presence of shell fragments, aligns with the typical characteristics of coral sand beaches.