Answer to Question 1
Answer: Many white-sand beaches contain mostly quartz. Quartz is abundant in river sediment delivered to the sea because it is the weathering-resistant mineral that is most abundant in continental rocks. Quartz also lacks cleavage, so continuous movement and abrasion by waves do not easily break it. Black-sand beaches are common on basaltic volcanic islands. Basalt does not contain quartz to produce white sand, but the fine-grained igneous rock is crushed into small sand-size grains by wave erosion along the shore.
Answer to Question 2
Answer: By building groins, which are walls built perpendicular to the shoreline, and trap sediment on the upcurrent side to widen a beach. The longshore current erodes the beach, however, on the downcurrent side of the groin.
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