Volcanic activity created the entire state of Hawaii. If you're island hopping, check out these hot spots:
Leahi (or Diamond Head) is Hawaii's most famous landmark, just south of Waikiki. Take a flashlight on your hike up to the top (a 175-step staircase takes you 231 metres) to explore underground tunnels and old military bunkers. hawaiistateparks.org
The Punchbowl Crater in Honolulu is about 100,000 years old. Early Hawaiians used the crater as an altar where unlucky humans were sacrificed to appease the gods. After the Second World War, the area became a military cemetery and national memorial. Visitors can stroll up to an overlook on the crater's rim for a grand view of Honolulu.
Go hiking or horseback riding around Haleakala, one of the world's largest dormant volcanoes. There may be no lava, but the sunrise and sunsets from atop this 3,055-metre volcano are not to be missed. There are many bus and bike tours, but you can drive up as well. nps.gov/hale
Head to Hawaii Volcanoes National Park ( nps.gov/havo) to explore old and new eruptions from Mauna Loa and Kileau. If lava is flowing, go straight to the viewing platform outside the park in Puna County at the end of Highway 130 to watch the lava spurt into the air or sizzle into the ocean. Another way to enjoy the show is a boat tour with Lava Ocean Adventures (808-966-4200; lavaocean.com).
Off the south coast of the Big Island another active volcano – Loihi – is also erupting (and is expected to become the state's next island in about 250,000 years).
Mauna Kea's summit, at 4,200 metres, is the highest point in the state and a sacred spot for many Hawaiians. There are several observatories at the summit. Mauna Kea Visitor Information Station and Subaru Telescope Tour offer free tours, but you've got to get yourself up there. It's a rough road to the top, so don't tackle it in a rental car. Mauna Kea Summit Adventures is a good place to book for sunset and night-sky tours (808-322-2366; maunakea.com). Adventurous types have been known to ski Mauna Kea, but this is guerrilla skiing and not recommended, even by the Hawaii Ski Club (yes there is one!). There's no lift, no facilities and the snow cover is patchy.
For more information, check out VolcanoTrek.com, which offers geologist- and volcanologist-led tours around the world.