Andesite line

The andesite line is an important feature in the Pacific Ocean basin. It shows the difference between two kinds of volcanic rocks. Inside the andesite line are rocks called basaltic, found in the Central Pacific Basin. Outside the line, the rocks are more like andesite, found near the edges of continents.

The andesite line runs close to areas where the Earth’s plates are moving under each other, called subduction zones, and deep ocean trenches. It follows the western edge of islands off California, passes south of the Aleutian Arc, and goes along the eastern edge of places like the Kamchatka Peninsula, the Kuril Islands, Japan, the Mariana Islands, Yap, Palau, the Solomon Islands, Fiji, Tonga, and New Zealand’s North Island. It continues along the western edge of the Andes mountains in South America and back to the islands off California.

Inside the andesite line, you find deep ocean trenches, underwater volcanoes, and volcanic islands. Here, basaltic lava flows slowly to form large volcanic mountains. Outside the line, volcanic eruptions tend to be more explosive. The Pacific Ring of Fire runs close to the andesite line and is known for its powerful volcanic activity.

The name “andesite line” was first used in 1912 by a geologist from New Zealand named Patrick Marshall. He noticed a clear boundary in the rocks and volcanic activity stretching from east of New Zealand to places like Fiji and the Solomon Islands.