At 27km long and up to 4km wide, the Tasman is the largest of NZ's glaciers, but it's melting fast, losing hundreds of metres of length each year. It is also melting from the surface, shrinking around 150m in depth since it was first surveyed in 1891. Despite this considerable shrinkage, at its thickest point the ice is still estimated to be over 600m deep.
In its lower section the melts have exposed rocks, stones and boulders, which form a solid unsightly mass on top of the ice.
Tasman Lake, at the foot of the glacier, started to form only in the early 1970s and now stretches to 7km. The ongoing effects of climate change are expected to extend it much further in the next decade. The lake is covered by a maze of huge icebergs, which are continuously being sheared off the glacier's terminal face. On 22 February 2011 the Christchurch earthquake caused a 1.3km-long, 300m-high, 30-million-tonne chunk of ice to break off, causing 3.5m waves to roll into the tourist boats on the lake at the time (no one was injured). You can kayak on Tasman Lake with Glacier Kayaking.
In the glacier's last major advance (17,000 years ago), the glacier crept south far enough to carve out Lake Pukaki. A later advance did not reach out to the valley sides, so there's a gap between the outer valley walls and the lateral moraines of this later advance. The unsealed Tasman Valley Rd, which branches off Mt Cook Rd 800m south of Mt Cook Village, travels through this gap. From the Blue Lakes shelter, 8km along the road, the Tasman Glacier View Track (30 minutes return) climbs interminable steps to an aptly rewarding viewpoint on the moraine wall, with a side trip to Blue Lakes on the way. Now only replenished by rainfall, the Blue Lakes have reduced in size and are now a muddy green colour.