Mt Fuji has a complex geologic origin. The large stratovolcano has a base diameter of almost 50 km and culminates in a 500 m wide and 250 m deep summit crater. The volcano overlies several older volcanoes, whose remnants form irregularities on Fuji's symmetrical profile, inlcuding Komitake and Ko-Fuji (Older Fuji) which was active 100,000 - 10,000 years ago.
The present-day, mainly basaltic edifice started to grow about 11-8,000 years ago when large lava flows were erupted that still form 25% of the volume of the edifice today.
From 8000 to 4500 years ago, Fuji's activity was mainly explosive before another effusive cycle took place between 4500 to 3000 years ago. In the past 3000 years, large explosive eruptions occurred in between phases of milder effusive activity. From 3000 to 2000 years ago, most eruptions took place at the summit, while a large number of flank eruptions occurred during the past 2000 years, forming more than 100 flank cones.
The last confirmed eruption of Mt Fuji took place in 1707 and was Fuji's largest during historical time. It deposited ash as far as present-day Tokyo and formed a large new crater on the east flank.
1707 eruption of Mt Fuji
On 26th October 1707, a new eruption announced itself with a large 8.4 magnitude earthquake devastating Honshu island, followed by several smaller earthquakes felt near Mt Fuji.
The eruption started on 16th December 1707 from a new vent on the SE flank of the volcano erupting a sub-plinian column of ash and pumice, turning into basaltic lava fountaining after 6 hours into the eruption. On the first day of the eruption, 72 houses and 3 Buddhist temples were destroyed in Subassiri town 10 km from the volcano.
Ash fell all over the south Kanto plain, Tokyo, and on areas of the NW Pacific ocean 280 km from the volcano. The total volume erupted over 16 days was estimated to 0.68 cubic km of magma.
Violent explosions were recorded until 25-27 December, before the eruption calmed down and ended on 1st January 1708.
Taken from http://www.volcanodiscovery.com/fuji.html