Literally “Mountain of Fire”, Mount Merapi is the most active volcano in all Indonesia — no mean feat — and it has erupted at least 68 times since 1548. The volcano is thought to be largely responsible for the downfall of the Hindu Mataram kingdom in 1006 and the desertion of the huge temple complex at nearby Prambanan. Recent eruptions include 1994 (64 killed), 2006 (no immediate deaths, but a prelude to an earthquake that killed over 5000) and 2010 (30 dead).
Anyone thinking of climbing the mountain must first check if it is safe to do so. While climbing the mountain with a qualified guide is recommended it is not necessary or mandatory, as the track to the summit is wide and clearly marked. But this is not the case at night. Many routes are invisible since visibility is low as it’s dangerous.
Do avoid the rainy season which spans Late October to Early April. The slopes become slippery and the high altitude, night climate and rain makes the hike unbearably cold, especially if you are waiting for the rain to pass at one of a handful of sheds on the route up. Waterproof winter wear is necessary if you intend to brave the rains in these months and especially at night. The rainy season also means cloudy days and reduces the prospects of viewing any sunrise. Do pack a set of dry clothes to change into, since you are bound to get wet, and good hiking shoes with sufficient friction to mitigate the wet roads. The trip to base camp by car can also get hazardous as fog builds up after a downpour and visibility degrades to less than 1 m.
Trekking on and around Merapi is a popular activity (at least when the volcano isn’t spewing out hot gas and ash). From Selo, the nearest village on the north slope, it is 3 hr hike to the summit for the fit and healthy. For others it will take 4+ hr to climb, and nearly the same time to get back down again.
Carry a small pack with some food, a good flash light, with spare batteries (available for purchase in Selo village), and at least 2 litres of water per person.
The track to the top is a reasonable grade and is mostly the mountain’s natural water drains, a guide is recommended, can get muddy and slippery when it rains. Once through the tree line the track is less defined and the final climb to the summit (after the memorial plaque) is a very steep scramble over loose and broken rock. While not in itself dangerous, do take your time on the last section. There is a 200 m section of ash which is the hardest once surpassed the rocky section to the summit is reasonably steep but is like any other for regular hikers.
Most climbers start at about 1.00 am to be sure of reaching the summit by sunrise at 5.30 am. Night climbing is wise in any case, as the upper reaches of the mountain are completely treeless and the sun can fry you to a crisp. The sunrise views across East Java are quite stunning.
The way back can be more tricky that the rise, since the ash and the inclination of the route, make it almost impossible to stand up, so pay particular attention.
ref : http://wikitravel.org