The Dome

This dome is a smoothed part of a volcanic crater rim curved around the now flattish area to the north and east.. This crater is the northern one of two main locations of eruptions over the long term at Ruapehu - the other is to the south of the dome ridge in the southern summit area, now occupied by Crater Lake.
This northern crater is called Iwikau (with Summit Plateau in the middle), and it formed in a giant collapse scar when the northern top of the mountain fell off 10,500 years ago (see The Mounds GeoTrip for details).
The collapse was of a wider cone that had stretched from Paretetaitonga peak (west above you) to the farthest peak northeast of you (Te Heuheu). We think it collapsed because ice support melted away as the climate warmed out of the last ice age.
Whakapapa valley to the west directly below you has lost its glacier to melting. The ice filled Summit Plateau gets lower every year, and the Mangatoetoenui glacier is now actually cut off from its cirque (original source area) which was Summit Plateau. Whangaehu glacier is diminishing but 'still hangs in there'.

What happened to Dome Shelter? It was removed! Volcano monitoring is done from instruments at other places on the mountain now.
What are Cathedral Rocks? They are a stack of spattered lava mostly from Iwikau Crater.
Crater Lake is in the youngest southern crater, sitting in an old collapse scarp. Another old cone fell away to the east prior to 10,000 years ago, leaving the high surrounding rim including the highest point on Ruapehu (Tahurangi peak). The vent then rebuilt as Pyramid Cone with Crater Lake inside it. Around your feet are scattered loose rocks (blocks and bombs) from eruptions of Crater Lake including those in 1995 /96 and 2007, as well as eroded pieces of the underlying lava that makes up the ridge bedrock.