Scientists have identified four distinct phases of volcanic activity in the formation of Mount Fuji. The first phase, called "Sen-komitake", is composed of an andesite core recently discovered deep within the mountain. "Sen-komitake" was followed by the ""Komitake" Fuji", a basalt layer believed to be formed several hundred thousand years ago. Approximately 100,000 years ago, "Old Fuji" was formed over the top of "Komitake" Fuji. The modern, "New Fuji" is believed to have formed over the top of Old Fuji around 10,000 years ago.