(http://pubs.usgs.gov/gip/dynamic/graphics/hot_spot.gif)
The reason that this particular mountain is no longer active is because it is no longer connected to the Hot Spot, which feeds the lava through the island. This is also the way each island was formed and created. The Hot Spot released lava onto the ocean floor and builds up over time creating each mountain. As the Pacific tectonic plate shifts Northwest, the location of where the lava flows changes. Creating each individual mountain.
(http://www.hawaiiforvisitors.com/images/oahu/attractions/oahu-koolau-mountains-hvcb-301x200.jpg)
Thankfully, Ko'olau range on Oahu no longer has the ability to erupt because it has shifted away from the Hot Spot. But the creation that was left behind is truly beautiful.

