Meads Wall

Ruapehu is a volcano built over more than 200,000 years. Meads wall is a sheet of frozen magma that was squeezed up within the oldest part of the volcano. It's since been exposed through erosion and is butted up against by much younger lava that flowed from the direction of the ski area.

Wide (about 1m across, 6-sided) prismatic joints are typical of magma that cooled slowly to rock, insulated inside a crack deep in the old volcano.
Why do you think the prisms point horizontally (ie. like holes in the honey-comb on a beehive)? Because they are the tops of columns, and columns in cooled magma always point toward the cooling surface. In this case that was the crack wall, which has now eroded away leaving the dike as an impressive wall.
The hard magmatic rock, and vertical nature of exposed dikes makes them very popular with rock climbers. the prismatic joints act as great (but challenging) hand and foot holds.
Can you see the youngest lavas? How old do you think these are? They flowed from a new cone (Summit Plateau) that was growing and filling in a deep glacial valley where the ski area is now. This all happened in the couple of thousand years after the top of Ruapehu collapsed away about 10,000 years ago. The remnants of the collapse are The Mounds down below Whakapapa Village near the State Highway. (see that trip).