Studies of the volcano's lava accumulation rate have put the volcano's age at somewhere near one million years. It is believed to have breached sea level more than 500,000 years ago. Beginning 300,000 years ago, the rate of eruption slowly decreased over a long period of time to a point where erosion wore the volcano down faster than Kohala rebuilt itself through volcanic activity. Decreasing eruptions combined with increasing erosion caused Kohala to slowly weather away and subside into the ocean. The oldest lava flows that are still exposed at the surface are just under 780,000 years old. It is difficult to determine the original size and shape of the volcano, as the southeast flank has been buried by volcanic lava flows from nearby Mauna Kea and Mauna Loa. However, based on a sharp increase of incline on the submarine slope of the volcano at a depth of , scientists estimate that in its prime the portion of Kohala that was above the water was more than wide. Geologists estimate that the volcano was once about twice its current size.