Answer to Question 1
As a volcanic island subsides, the fringing reef grows upward, keeping pace with the seafloor subsidence, becoming a barrier reef when a lagoon develops and finally an atoll when the only solid material is the outer ring of coral.
Answer to Question 2
Spring tides occur every two weeks when the alignment of the Earth, Moon, and Sun increases the tidal range. During a neap tide, the Moon's gravitational pull is exerted at a 90 degree angle to that of the Sun, and so the pull of the two detract from each other.