The passage about islands

Thousands of islands are scattered in the Pacific Ocean. It is thought that the islands near the continental shores, such as the Japanese islands, the Philippines, the Marianas, New Guinea, the Solamons, New Hebrides, most of the Fiji Islands, and New Zealand, may once have been part of the Asia and Australian continents. For this reason, most of the islands of the western Pacific are called Continental Islands.

The islands of the central Pacific are known as Oceanic Islands, for they are of volcanic coral origin. They are the tops of undersea mountains, which rise from the ocean floor. Among the many oceanic islands are the Hawaiian Islands, the Marshalls, the Carolines, the Cook Islands, the Tuamotus, and the Marquesas Islands. Some of them rise from depths of almost 5,500 meters (18,000 feet). Coral islands are formed from the skeletons of tiny sea animals called coral polyps. The coral builds up on the slopes of undersea mountains. The floor of the eastern Pacific is fairly level at a depth of nearly 5,500 meters (18,000 feet). For this reason, there are very few islands in the eastern Pacific. The western Pacific is heavily dissected by a complicated pattern of ridges and trenches. The ridges in this region support most of the Pacific’s oceanic islands. The trenches, with depths greater than 7,500 meters (24,500 feet) are the lowest parts of the Earth’s surface. In one of them, the Marianas, a depth of 11,034 meters (36,200 feet) has been measured near the island of Guam. One surprising fact about the ocean is that four-fifths of the world’s active volcanoes are in the Pacific or in the lands bordering it. These areas of volcanic activity are also the regions where earthquakes are commonest.

The passage is mainly about ………. .

1) islands and ocean floor

2) islands in the eastern pacific

3) coral and oceanic islands

4) active volcanoes in the Pacific

One of my students asked me this question. I chose #3 because more space is devoted to coral and oceanic islands than to anything else. However, answer key shows #1.

Can you help me here? How do you justify the correct answer?

Thanks.