The Ijen volcano complex at the eastern end of Java consists of a group of small stratovolcanoes constructed within the large 20-km-wide Ijen (Kendeng) caldera.  The north caldera wall forms a prominent arcuate ridge, but elsewhere the caldera rim is buried by post-caldera volcanoes, including Gunung Merapi stratovolcano, which forms the 2799 m high point of the Ijen complex.  Immediately west of Gunung Merapi is the renowned historically active Kawah Ijen volcano, which contains a nearly 1-km-wide, turquoise-colored, acid crater lake.  Picturesque Kawah Ijen is the world's largest highly acidic lake and is the site of a labor-intensive sulfur mining operation in which sulfur-laden baskets are hand-carried from the crater floor.  Many other post-caldera cones and craters are located within the caldera or along its rim.  The largest concentration of post-caldera cones forms an E-W-trending zone across the southern side of the caldera.  Coffee plantations cover much of the Ijen caldera floor, and tourists are drawn to its waterfalls, hot springs, and dramatic volcanic scenery.

The relatively inconspicuous, 1731-m-high Kelud stratovolcano contains a summit crater lake that has been the source of some of Indonesia's most deadly eruptions.  A cluster of summit lava domes cut by numerous craters has given the summit a very irregular profile.  Satellitic cones and lava domes are also located low on the eastern, western, and SSW flanks.  Eruptive activity has in general migrated in a clockwise direction around the summit vent complex.  More than 30 eruptions have been recorded from Gunung Kelud since 1000 AD.  The ejection of water from the crater lake during Kelud's typically short, but violent eruptions has created pyroclastic flows and lahars that have caused widespread fatalities and destruction.  After more than 5000 persons were killed during an eruption in 1919, an ambitious engineering project sought to drain the crater lake.  This initial effort lowered the lake by more than 50 m, but the 1951 eruption deepened the crater by 70 m, leaving 50 million cubic meters of water after repair of the damaged drainage tunnels.  After more than 200 deaths in the 1966 eruption, a new deeper tunnel was constructed, and the lake's volume before the 1990 eruption was only about 1 million cubic meters.