Haleakala Volcano:Hawaiian Islands Mauii
World volcano Profiles >North America >Haleakala Mauii
Haleakala Volcanois on the island of Mauii (3055m / 10023ft) and is a huge shield volcano which covers 75% of the island; in Hawaiian its name means "house of the sun" and arriving at the top for sunrise is a popular tourist pursuit. It emerged from the sea around is around 1.25 million years old and last erupted in the 1790 (ish). Look out for the Silversword plant it only grows here.
The Geology Bit
The summit area of Haleakala consists of an enormous crater like depression 11km by 3km and nearly 800m deep. It is an array of reds, oranges and black loose slopes with scattered cindercones. The colours come from the weathering of the iron in the basalt and ash. The depression is thought to be a product of erosion rather than eruption.There are two rift rift zones; southwest trending and eastward trending. The volcanoes potential to erupt again is in dispute as it is thought to have moved off the hot spot but may yet have some secondary volcanism left.
What to see and How to see it?
There is a paved road up to the summit of the volcano which is accessible to to normal cars. National Park HQ is at 2100m(7000ft) where you can information.The road twists and turns until you are above the clouds . Finally you arrive at The Crater Rim Visitor Centre which is open from dawn until 3 in the afternoon. The centre is at 2970m or 9744ft so you may experience some light headiness or breathlessness as you ascended from sea level very quickly. It will be cool and windy at the top so pack warm clothes.
There is a magnificent view of the crater area from here and you can shelter in the visitor centre if its a bit too breezy.
The most popular trail is the Sliding Sands trail which descends into the crater. Remember if returning to the carpark the way down is easy but you have to climb back out too! The full distance of the trail is 21km or 13miles and takes about 7-8 hours to reach the Holua cabin. The path descends a red cinder covered track on the side of the originally named Red Hill Cinder Cone. At 3km there is the a cinder cone to climb before rejoining the trail. The way to the hut is colourful,and littered with cinder cones and lava flows to explore. Once you reach the cabin you can walk to the Haleakala road but you will need picking up. Don't assume you will find space at the cabin unannounced it is wise to check with the park authorities how to reserve a space well before you arrive in Hawaii.
Many people choose to walk to the first crater before returning to the carpark.
Take plenty of water,food and warm clothing. It is a dusty trail and you will get covered in red volcano dust - looks like a good sun tan till you wash it off! 
A popular thing to do is to freewheel by bycycle from the summit down to sea level. you don't have to bike up as a van will take your bike to the top and then its downhill all the way.There are lots of companies who run trips to do this on Mauii.
Links
BikingMauii.com: Descend the volcano on a bike
Maui-downhill.com:Descend the volcano by bycycle
Haleakala: Hiking Guide
National Park Service:Haleakala
USGS:Haleakala
Global Volcanism project