To find lots of details for yourself, look at the point in Chapter 2 when Mafatu first sees the island. Then look at the point in Chapter 3 right after he wakes up on the island and finds that he's alive.
The major features of the island are its volcanic cone, its many trees, and its barrier reef. The island has one great mountain in its middle with slopes going up to it. Its an old volcano. The whole island is covered with trees. We are told what kinds of trees some of them are in Ch. 3. There are coconuts, pandanus, breadfruit, guava, and mangoes. The trees are better than on the atoll where Mafatu lives because this is a high, volcanic island. We know there are two major rivers because there are only two openings in the barrier reef.
These are the major features of the island.