Kilauea is a shield volcano in the Hawaiian Islands, the most active of the five volcanoes that together form the island of Hawaiʻi. Located along the southern shore of the island, the volcano is 300,000 to 600,000 years old. Because it lacks topographic prominence and its activities historically coincided with those of Mauna Loa, Kilauea was once thought to be a satellite of its much larger neighbor. Structurally, Kilauea has a large, fairly recently formed caldera at its summit and two active rift zones, one extending 125 km (78 mi) east and the other 35 km (22 mi) west, as an active fault line of unknown depth moving vertically an average of 2 to 20 mm (0.1 to 0.8 in) per year. Kilauea's high state of activity has a major impact on its mountainside ecology where plant growth is often interrupted by fresh tephra, drifting volcanic sulfur dioxide and producing acid rains. Nonetheless, wildlife flourishes where left undisturbed elsewhere on the volcano.