Sakurajima is one of the most active volcanoes on the planet -- it produces hundreds of ash-laden explosions each year. Normally, these eruptions are small, reaching only a few hundred meters above the restless Showa crater. Occasionally, large explosions occur and this weekend, Sakurajima produced one of its largest explosions in decades (possibly the largest since the VEI 4 eruption in 1914; see video above). The eruption plume (see below) from Sunday's eruption reached 5 km (~16,500 feet) and ash from the eruption spread across nearby Kagoshima (the host of the recent IAVCEI Scientific Assembly), causing some limited visibility and train delays due to the ash fall in the city only 8 km from the summit of Sakurajima. The ash was enough to prompt people to wear dust masks to prevent breathing the hazard volcanic glass shards -- remember, volcanic ash is really just pieces of volcanic glass that are shattered by the explosive expansion of bubbles in an erupting magma. Some of the news footage of the eruption shows what might be small pyroclastic flows generated by the explosion as well (thanks to James Reynolds for noting that). You can also check out a great collection of images from the eruption that were on Twitter as well (thanks to Boris Behncke for that link). UPDATE: Seems that we have another example of information about an eruption being lost in translation. This Washington Post article on the eruption also mentions a 1-km "lava flow". However, the Japanese Meteorological Agency report of the eruption (in Japanese) clearly states that it was a pyroclastic flow (or pyroclastic density current if you want the technical parlance) that travelled ~1 km from the vent. CultureVolcans has a nice timelapse that captures the flows generated during the eruption.
The eruption plume from Sakurajima on August 18, 2013. Image: 1011Nuko / Twitter. No word on what caused this eruption, but some speculation might be a new slug of gas-charged magma in the system or a small plug in the Showa crater being destroyed due to overpressuring in the volcanic conduit. As I mentioned, explosions are common at Sakurajima as this footage by James Reynolds from 2010-11. You can also see how these explosions look in HD in this video by Mike Lyvers as well. If you want a lot of detail on the history of the recent activity at Sakurajima, check out Boris Behncke's comment below. Oddly enough, this was also Sakurajima's 500th eruption of 2013. With this constant activity, Sakurajima is always worth watching on the multitude of webcams pointed at the volcano.
Video: FNNnewsCH / YouTube