Wanderlust World Wanderlust World The age of the lithosphere at the Tonga trench varies by location along the trench. The Tonga trench is a convergent boundary where the Pacific Plate subducts beneath the Indo-Australian Plate. The Pacific Plate moves westward at about 15 centimeters per year. The age of the Pacific Plate at the Tonga trench ranges from about 50 million years near the northern end to about 120 million years near the southern end.