Mauna Loa from Saddle Road
|Elevation||13,679 ft (4,169.4 m)|
|Prominence||7,100 ft (2,164.1 m) |
|Topo map||USGS Mauna Loa|
|Age of rock||less than 200 thousand years|
|Volcanic arc/belt||Hawaiian-Emperor seamount chain|
|Easiest route||jeep trail|
Mauna Loa (pronounced /ˌmɔːnə ˈloʊ.ə/ or /ˌmaʊnə ˈloʊ.ə/ in English, [ˈmounə ˈloə] in Hawaiian) is the largest volcano on Earth in terms of volume and area covered and one of five volcanoes that form the Island of Hawaii in the U.S. state of Hawaiʻi in the Pacific Ocean. It is an active shield volcano, with a volume estimated at approximately 18,000 cubic miles (75,000 km3), although its peak is about 120 feet (37 m) lower than that of its neighbor, Mauna Kea. The Hawaiian name "Mauna Loa" means "Long Mountain". Lava eruptions from Mauna Loa are silica-poor, thus very fluid: and as a result eruptions tend to be non-explosive and the volcano has relatively shallow slopes.
The volcano has probably been erupting for at least 700,000 years and may have emerged above sea level about 400,000 years ago, although the oldest-known dated rocks do not extend beyond 200,000 years. Its magma comes from the Hawaii hotspot, which has been responsible for the creation of the Hawaiian island chain for tens of millions of years. The slow drift of the Pacific Plate will eventually carry the volcano away from the hotspot, and the volcano will then become extinct within 500,000 to one million years from now.
Mauna Loa's most recent eruption occurred from March 24, 1984, to April 15, 1984. No recent eruptions of the volcano have caused fatalities, but eruptions in 1926 and 1950 destroyed villages, and the city of Hilo is partly built on lava flows from the late nineteenth century. In view of the hazards it poses to population centers, Mauna Loa is part of the Decade Volcanoes program, which encourages studies of the most dangerous volcanoes. Mauna Loa has been intensively monitored by the Hawaiian Volcano Observatory since 1912. Observations of the atmosphere are undertaken at the Mauna Loa Observatory, and of the Sun at the Mauna Loa Solar Observatory, both located near its summit. Hawaii Volcanoes National Park covers the summit and the southeastern flank of the volcano, including a separate volcano, Kīlauea.
Mauna Loa is the world's largest shield volcano in terms of area covered. Mauna Loa is shaped like a shield, because its lava is extremely fluid (it has low viscosity), and its slopes are not steep. Eruptions are rarely violent, and the most common form is in the Hawaiian style, which involves lava fountains feeding lava flows. Typically, at the start of an eruption, a rift up to several kilometers long opens, with lava fountains occurring along its length in a so-called "curtain of fire." After a few days, activity normally becomes concentrated at one vent.
Eruptions generally occur in three regions on the mountain: at the summit and in two rift zones extending northeast and southwest of the summit. About 38% of eruptions in the last two hundred years have occurred at the summit, 31% in the northeast rift zone, and 25% in the southwest rift zone. The remaining 6% have occurred from vents to the northwest of the summit, away from the rift zones. Its summit caldera is called Mokuʻāweoweo; it is 1.75 to 3 miles (3–5 km) in diameter. The caldera probably formed 1,000–1,500 years ago when a very large eruption from the northeast rift zone emptied out the shallow magma chamber beneath the summit, which then collapsed.
Seismic data can reveal the locations of the magma chambers beneath the volcano which feed activity. Some types of seismic waves, known as "S-waves," cannot travel through liquid rock, so magma chambers cast 'shadows' in seismic data. Seismic shadows reveal a magma chamber about 1.75 miles (3 km) beneath the summit and smaller magma bodies beneath the rift zones.
Trade winds blow from east to west across the Hawaiian islands, and the presence of Mauna Loa strongly affects the local climate. At low elevations, the eastern (windward) side of the volcano receives heavy rain, and the city of Hilo is the wettest in the United States. The rainfall supports extensive forestation. The western (leeward) side has a much drier climate. At higher elevations, the amount of precipitation decreases, and skies are very often clear. Very low temperatures mean that precipitation often occurs in the form of snow, and the summit of Mauna Loa is described as a periglacial region, where freezing and thawing play a significant role in shaping the landscape.
Mokuʻāweoweo is the summit crater (caldera) of Mauna Loa.
Although a trail was developed in ancient times, Mokuaweoweo proved a difficult goal for the first outsiders who underestimated the difficulty of the journey. An American visit in 1840 turned into a trip of almost a month. Historically very active, Mokuʻāweoweo has been quiet since 1984.
The name of the summit crater is derived from the ʻāweoweo, a red Hawaiian big-eyed fish, Priacanthus meeki, purportedly due to resemblance of the eruptive fires to the color of the fish. The main oval depression is about 3 miles (4,800 m) long and 1.5 miles (2,400 m) wide. Two side craters, partially fused with the main one, are known as North Pit and South Pit. Southwest of the summit caldera are two smaller pit craters, Lua Hou (New Pit) and Lua Hohonu (Deep Pit).
Mauna Loa began erupting between 700,000 and 1,000,000 years ago and has grown steadily since then. Like all of the Hawaiian islands, Mauna Loa has its origins in the Hawaii hotspot—a plume of magma rising from deep in the Earth's mantle. The hotspot remains in a fixed position, while the Pacific Plate drifts over it at a rate of about 4 inches (10 cm) per year. The upwelling of the hot magma creates volcanoes, and each individual volcano erupts for a few million years before the movement of the plate carries it away from the rising magma.
The hotspot has existed for at least 80 million years, and the Emperor Seamounts chain of old volcanoes stretches almost 3,600 miles (5,800 km) away from the hotspot. Currently, the hotspot feeds activity at five volcanoes: Mauna Loa, Kīlauea, and Hualālai on the Big Island, Haleakalā on Maui, and Loʻihi, a submarine volcano south of the Big Island and the youngest Hawaiian volcano. Mauna Loa is the largest of these, although Kīlauea is currently the site of the most intense volcanic activity.
Prehistoric eruptions of Mauna Loa have been extensively analyzed by carrying out radiocarbon dating on fragments of charcoal found beneath lava flows. The mountain's prehistoric activity is probably the best known of any volcano. Studies have shown that a cycle occurs in which volcanic activity at the summit is dominant for several hundred years, after which activity shifts to the rift zones for several more centuries, and then back to the summit again. Two cycles have been clearly identified, each lasting 1,500–2,000 years. This cyclical behavior is unique to Mauna Loa among the Hawaiian volcanoes.
Records show that between about 7,000 and 6,000 years ago Mauna Loa was largely inactive. The cause of this cessation in activity is not known, and no known similar hiatus has been found at other Hawaiian volcanoes except for those currently in the post-shield stage. Between 11,000 and 8,000 years ago, activity was more intense than it is today. However, Mauna Loa's overall rate of growth has probably begun to slow over the last 100,000 years, and the volcano may in fact be nearing the end of its tholeiitic basalt shield-building phase.
The ʻAinapō Trail was established in prehistoric times, rising from the village of Kapāpala over 11,200 feet (3,400 m) in about 35 miles (56 km). Although the more active and accessible Kīlauea caldera was the usual site to honor the fire goddess Pele, offerings and prayers were also made on Mokuʻāweoweo during its eruptions. Journeys up the trail extended over many days and required many porters. Several established camps along the way supplied water and food.
John Ledyard led a team to attempt an ascent on the third voyage of Captain James Cook in January 1779. They struggled to go directly up from Kealakekua Bay, but turned back after an estimated 24 miles, thinking they had about 11 miles more to go. The summit was actually only a total of 20 miles from the coast.
Archibald Menzies led a party from the Vancouver Expedition in the first successful ascent to Mokuʻāweoweo with a written record. He also first tried a direct route from the west and turned back in February 1793. After arriving again the next year, he climbed to the summit of Hualālai, on the northwest part of the island. He then tried to cross the interior plateau to Mauna Loa, but had to turn back. He consulted King Kamehameha I and was astonished to learn he could take canoes to the south and follow the route established by Ancient Hawaiians known as the ʻAinapō Trail.
On his third attempt on February 16, Menzies reached the summit and used a barometer to estimate the height as 13,634 feet (4,156 m) compared to its currently known height of 13,679 feet (4,169 m). He was surprised to find heavy snow and morning temperatures of 26 °F (−3 °C). He again underestimated the length of the climb and exhausted food rations, arriving with only three coconuts left for the summit party. The sheer mass of Mauna Loa is deceptive. Slopes are generally not very steep (always less than 12°) which makes it very hard to judge progress on the trail.
It is sometimes reported that missionary Joseph Goodrich reached the summit around this time, but he never claimed this himself. He climbed Mauna Kea, and described the sight of Mokuʻāweoweo from Mauna Kea through a telescope.
Isidor Löwenstern successfully climbed Mauna Loa in February 1839, only the third successful climb in 60 years.
|U.S. National Register of Historic Places|
|Nearest city:||Hilo, Hawaii|
|Architectural style(s):||Stone shelter|
|Governing body:||National Park Service|
|Added to NRHP:||July 24, 1974|
The United States Exploring Expedition led by Lieutenant Charles Wilkes was tasked with a vast survey of the Pacific Ocean starting in 1838. In September 1840 they arrived in Honolulu, where repairs to the ships took longer than expected. He decided to spend the winter in Hawaii and take the opportunity to explore its volcanoes while waiting for better weather to continue the expedition. King Kamehameha III assigned American medical missionary Dr. Gerrit P. Judd to the expedition as translator.
Wilkes sailed to Hilo on the island of Hawaiʻi and decided to climb Mauna Loa first, since it looked easier than Mauna Kea. On December 14 he hired about 200 porters, but after he left he realized only about half the equipment had been taken, so had to hire more Hawaiians at higher pay. When they reached Kīlauea after two days, their guide Puhano headed off to the established ʻAinapō Trail. Wilkes did not want to head back downhill so he blazed his own way through dense forest directed by a compass. The Hawaiians were offended by the waste of sacred trees, which did not help morale. At about 6,000 feet (1,800 m) elevation they established a camp called "Sunday Station" at the edge of the forest.
Two guides joined them at Sunday Station: Keaweehu, "the bird-catcher" and another whose Hawaiian name is not recorded, called "ragsdale". Although Wilkes thought he was almost to the summit, the guides knew they were less than half way up. Since there was no water at Sunday Station, porters had to be sent back ten miles to a lava tube on ʻAinapō Tail which had a known supply. After an entire day replenishing stocks, they continued up to a second camp they called "Recruiting Station" at about 9,000 feet (2,700 m) elevation. After another full day's hike they established "Flag Station" on December 22, and by this time were on the ʻAinapō Trail. Most of the porters were sent back down to get another load.
At the Flag Station Wilkes and his eight remaining men built a circular wall of lava rocks and covered the shelter with a canvas tent. A snowstorm was in progress and several suffered from altitude sickness. That night (December 23), the snow on the canvas roof caused it to collapse. At daylight some of the group went down the trail to retrieve firewood and the gear abandoned on the trail the day before. After another day's climb, nine men reached the rim of Mokuʻāweoweo. They could not find a way down its steep sides so chose a smooth place on the rim for the camp site, at coordinates . Their tent was pitched within 60 feet of the crater's edge, secured by lava blocks.
The next morning they were unable to start a fire using friction due to the thin air at that altitude, and sent for matches. By this time, the naval officers and Hawaiians could not agree on terms to continue hiring porters, so sailors and marines were ordered from the ships. Dr. Judd traveled between the summit and the Recruiting Station to tend the many who suffered from altitude sickness or had worn out their shoes on the rough rock. Christmas Day was spent building rock walls around the camp to give some protection from the high winds and blowing snow. It took another week to bring all the equipment to the summit, including a pendulum designed for measuring slight variations in gravity.
On December 31, 1840 the pre-fabricated pendulum house was assembled. Axes and chisels cut away the rock surface for the pendulum's base. It took another three days to adjust the clock to the point where the experiments could begin. However, the high winds made so much noise that the ticks could often not be heard, and varied the temperature to make measurements inaccurate. Grass had to be painstakingly brought from the lowest elevations for insulation to get accurate measurements.
On Monday, January 11, Wilkes hiked around the summit crater. Using an optical method, he estimated Mauna Kea was only 193 feet (59 m) higher (modern measurements are 104 feet (32 m) feet). On January 13, 1841, he had "Pendulum Peak, January 1841 U.S. Ex, Ex." cut into a rock at the site. The tents were dismantled and Hawaiians carried the gear down over the next three days, while Wilkes enjoyed a lomilomi Hawaiian massage. He continued his measurements at lower elevations and left the island on March 5. For all the effort he did not obtain any significant results, attributing gravity discrepancies to "the tides".
The Wilkes expedition's camp site's ruins are the only known physical evidence in the Pacific of the U. S. Exploring Expedition. The camp site was listed on the National Register of Historic Places on July 24, 1974 as site 74000295, and is state historic site 10-52-5507.
A summit shelter was built with some of the stones from Wilkes' camp site and mortar in 1934. In 1916 Mokuʻāweoweo was included in Hawaii Volcanoes National Park, and a new trail was built directly from park headquarters at Kīlauea, an even more direct route than the one taken by Wilkes. This trail, arriving at the summit from the east via Red Hill, became the preferred route due to its easier access and gentler slope. After falling into disuse, the historic ʻAinapō Trail was reopened in the 1990s. A third modern route to the summit is from the Saddle Road, up to the Mauna Loa Observatory which is located at the elevation of 11,135 feet (3,394 m) a few miles north of Mokuʻāweoweo, and the North Pit trail.
Although Ancient Hawaiians had witnessed eruptions for many centuries, written records exist only for eruptions that have occurred since the early 1800s. The first historical eruption occurred in 1832, and since then 32 eruptions have been documented.
The typical pattern is for an eruption to start at Mokuʻāweoweo, then move to a focal vent lower on one of the rift zones. Often, a major rift eruption is preceded by a smaller event that is limited to Mokuʻāweoweo, a year or more before the larger one. The latter occurred in 1880, 1940, and 1949, before the massive flank eruptions of 1881, 1942, and 1950 In total, these eruptions have covered over 310 square miles (800 km2) of the volcano's flanks with lava flows. Typically, eruptions have been brief but intense, with 0.06 to 0.12 cubic miles (0.25–0.5 km³) of lava erupted over a few weeks.
The most prominent historic eruptions of Mauna Loa, with extensive lava flows in forest and agricultural areas, were in 1855, 1881, 1935, 1942, and 1950 on the northeast rift zone; 1887, 1907, 1916, 1919, 1926, and 1950 on the southwest rift zone; and the 1859 radial vent eruption.
An eruption in 1935 that headed toward the city of Hilo led to an unusual employment of air power. Five bombers of the 23d and 72d Bombardment Squadrons of the United States Air Force dropped bombs in the path of the lava in order to divert it away from Hilo. Although it was deemed successful, the eruption stopped six days later, so the effectiveness of the bombing is unknown.
The 1950 eruption produced lava flows along a 20 kilometers (12 mi) fissure from Lua Hou to the southwest. The first flow extended 24 kilometers (15 mi) from the vent at 2,800 meters (9,200 ft) elevation to the ocean in only three hours during the night, and a second flow before dawn cut off the escape route for people in the village of Hoʻokena. The eruption eventually focused on a flow further to the south, which also reached the ocean.
On March 25, 1984 an eruption started: a small summit eruption was followed by a larger flank lava flow that threatened Hilo. Fissures opened to the northwest and southeast, from the summit down to 9,500 feet (2,900 m) above sea level. Flows from this eruption headed rapidly towards Hilo again, but stopped about 2.5 miles (4.0 km) from the outskirts when the eruption ended after three weeks.
In 2002 seismic activity, which had been low, increased, and inflation suddenly started, and the caldera walls started to move apart at 2 inches (5 cm) per year. This is thought to indicate that magma is filling a reservoir about 3.1 miles (5.0 km) beneath the summit. The inflation has been intermittent, sometimes slowing, and sometimes stopping for several weeks. Thus far, though, it has always restarted, and this is likely to indicate an increased probability of an eruption in the next few years.
In July 2004 a swarm of deep earthquakes began, and continued until the end of the year. Earthquakes were detected at a rate of one per day for the first three weeks, increasing steadily over subsequent months to 15 or so per day by the end of the year. The swarm ended in December 2004, and earthquake levels have been only moderately elevated since then.
Kīlauea lies on the southern flank of Mauna Loa and was originally thought to be a satellite vent of Mauna Loa. However, chemical differences between the lavas from the two volcanoes show that they have separate shallow magma chambers. They are now considered separate volcanoes. Nevertheless, activity patterns at the two volcanoes do appear to be correlated.
The most apparent relation between the two mountains is that, generally, periods of frequent activity at one volcano coincide with periods of low activity at the other. For example, between 1934 and 1952, Kīlauea was dormant and only Mauna Loa was active, while from 1952 to 1974, only Kīlauea was active while Mauna Loa lay dormant.
The 1984 eruption of Mauna Loa started during an eruption at Kīlauea, but had no discernible effect on the Kīlauea eruption. Occasionally, though, eruptions at one volcano do seem to influence activity at the other. The recent inflation of Mauna Loa's summit began on the same day as a new large lava flow broke out at Kīlauea's Puʻu Ōʻō crater. Geologists have suggested that a "pulse" of magma entering Mauna Loa's deep plumbing system could have increased pressure inside Kīlauea and triggered the eruption.
Volcanic eruptions in Hawaiʻi rarely cause fatalities—the only fatality due to volcanic activity there in the last century occurred at Kīlauea in 1924, when an unusually explosive eruption hurled rocks at onlookers, killing one. However, property damage is common. Mauna Loa is a Decade Volcano, which means it has been identified as worthy of particular research in light of its frequent eruptions and proximity to populated areas. Many towns and villages near the volcano are built on lava which has erupted in the last two hundred years, and there is a very strong likelihood that future eruptions will cause damage to populated areas.
The main volcanic hazard at Mauna Loa is lava flows. Most flows advance at about walking pace and present little danger to human life, but eruptions at Mauna Loa can be more intense than those at Kīlauea; for example, the 1984 eruption emitted as much lava in three weeks as Kīlauea's current eruption produces in three years. Such high emission rates can generate comparatively fast-moving flows.
Two eruptions of Mauna Loa have destroyed villages. In 1926, the village of Hoʻōpūloa Makai was overrun by lava flows. In 1950, the most voluminous eruption ever seen at Mauna Loa sent lava flows racing towards the sea. The village of Hoʻokena Mauka was destroyed on June 2, 1950 by the advancing flows. Hilo is partly built on lava from the 1880-81 eruption and is at risk from further lava flows. The brief but intense 1984 eruption saw lava flow towards Hilo, but it had not reached any buildings when the eruption stopped.
A rarer but potentially greater hazard at Mauna Loa is the possibility of a sudden massive collapse of the volcano's flanks. Deep faults allow large portions of the sides of Hawaiian mountains to slide gradually downwards, the best known example being the Hilina Slump. (There is also the more ancient example of the Ninole Hills.) Occasionally, a large earthquake can trigger a collapse of the flank, creating a massive landslide which may trigger a tsunami. Kealakekua Bay, on the western slope of Mauna Loa, was created by such an event. Undersea surveying has revealed numerous landslides along the Hawaiian chain and two giant tsunamis are known to have occurred: 200,000 years ago, Molokaʻi experienced a 246-foot (75 m) tsunami, and 100,000 years ago a megatsunami 1,066 feet (325 m) high struck Lānaʻi.
A recent example of the risks associated with slumps occurred in 1975, when the Hilina Slump suddenly moved forward by several yards. A magnitude 7.2 earthquake resulted which triggered a small tsunami with a wave height of a few yards, killing several campers at Halape.
Mauna Loa is an intensively monitored volcano. The Hawaiian Volcano Observatory (HVO) was established in 1912 to observe the Hawaiian volcanoes, and the HVO has developed many techniques to help predict when eruptions at Mauna Loa and other volcanoes are imminent.
One of the most important tools is seismometry. More than 60 seismometers around the Big Island enable scientists to measure the intensities and locations of hundreds of small earthquakes every week. Earthquakes can begin to increase years before an eruption actually starts: The 1975 and 1984 eruptions were both preceded by one to two years of increased seismic activity at depths of less than 8 miles (13 km).
Another type of seismic activity occurs in the hours preceding an eruption. So-called harmonic tremor is a continuous "rumble" which contrasts with the normal seismic activity of sudden shocks and is believed to be caused by the rapid movement of magma underground. Volcanic tremor normally indicates an imminent eruption, although it may also be caused by shallow intrusions of magma which do not reach the surface.
Another important indicator of what is happening underground is the shape of the mountain. Tiltmeters measure very small changes in the profile of the mountain, and sensitive equipment measures distances between points on the mountain. As magma fills the shallow reservoirs below the summit and rift zones, the mountain inflates. A survey line across the caldera measured a 3-inch (76 mm) increase in its width over the year preceding the 1975 eruption and a similar increase before the 1984 eruption.
The location of Mauna Loa has made it an important location for atmospheric monitoring by the Global Atmosphere Watch and other scientific observations. The Mauna Loa Solar Observatory (MLSO), located at 11,155 feet (3,400 m) on the northern slope of the mountain, has long been prominent in observations of the Sun. The NOAA Mauna Loa Observatory (MLO) is located close by. From its location well above local human-generated influences, the MLO monitors the global atmosphere, including the greenhouse gas carbon dioxide. Measurements are adjusted to account for local degassing of CO2 from the volcano.
Since October 2006, the Array for Microwave Background Anisotropy (AMIBA) has been exploring the origin of the universe.
The island of Hawai'i comprises over half of the area of the state of Hawaii in the United States of America. To avoid confusion with the state, it is almost universally called the Big Island. It is home to the most active volcano in the world, located in Hawaii Volcanoes National Park, as well as the largest mountain in the world in volume (Mauna Loa) and the tallest mountain in the world as measured from its base on the sea floor to its peak (Mauna Kea).
There are two major airports if you are flying into the Big Island, Kona International Airport  and Hilo International Airport . There are a few direct flights from the mainland, mostly from California and Seattle, but it is more common to arrive via Honolulu or Kahului. You should try to get a flight direct from the mainland to Kona to save time waiting (and walking) around the Honolulu airport. There are no direct flights to Hilo from the mainland.
If you can't find a direct flight, consider that Kona's airport is by far busier and requires a lot of time to pass all checkpoints. Hilo's airport has fewer flights, is smaller, so the time between rental drop-off and boarding is much shorter.
Inter-island "hoppers" arrive from all the other islands several times a day. Local flights are available through three main airlines, Hawaiian , Island Air  and go!  to the two major airports. These airlines provide frequent service between the islands, largely connecting through Honolulu, although there are some direct flights from Kona and Hilo to Kahului Maui. Daily round-trip service is also available between Hilo and Kona. Pacific Wings,  provides fun flights in small prop planes between Hilo, Kona and Kamuela. Keep in mind that an inter-island flight could use up almost an entire day, due to the fact that you must pack, check out of hotel, get to the airport, return rental car, go through all the airport procedures, fly, wait for luggage, get rental car, check in, unpack. It's tempting to try and see as many of the islands as you can on one trip, but it's best to spend no less than three nights on an island. In the case of the Big Island, think of the two sides of the island as separate. It takes about 2 hours 15 minutes to drive from Kailua-Kona to Hilo, and about six hours to circle the island.
Although several cruise ship lines operate in Hawaii, there is currently no dedicated inter-island boat service. Hawaii Superferry, a private company supported by the Hawaii state government, which current runs high-capacity catamaran ferry services between O‘ahu and Maui, was planning to open a route between Maui and the Big Island but that has been buried due to a recent Supreme Court decision.
Renting a car is a good way to see the island, however, renting a 4WD is best. Many of the Big Island's finest sights are only accessable by 4WD.
Getting around by local bus, bikes, or on foot work well if you're staying in one area. Many budget travelers are unpleasantly surprised by the extremely limited public transport on the Big Island. The county's Hele-On bus is free, but the schedules are mainly intended for commuting . There are some bus companies offering excursions from Hilo to destinations like Volcano, but they require reservations.
Hitchhiking is extremely easy & convenient on the Big Island. As most places are near the Mamalahoa Highway, you're never too far from a good hitching spot. As traffic generally flows from the Hilo side to resorts of Waikoloa Resorts in the morning, you can easily make it to the beaches without waking up at by 5:45 to catch the Hele-On bus. As a large amount of locals take the spirit of Aloha seriously, hitching is the best way to travel for free throughout the Island. It's not just the usual crowd of young people picking up hitchers either, I've been picked up by families, grandmas, and even the mayor of Honoka'a! As a note, the district of Kau in the south of the island is very sparsely populated with low traffic levels, so hitching isn't near as good here as it is on the northern half of island.
If you're thinking about renting a Jeep, Alamo doesn't disable the four wheel drive option in their Jeeps as some other rental agencies do. However, keep in mind that you are violating the rental contract by driving on "unpaved roads" (let alone some of the four wheel drive only roads). As long as you are careful and sensible about it though, you'll be fine. Just keep in mind that you are "on your own" if you get in trouble while violating the rental agreement. If you're going to violate the rental contract by driving on unpaved roads, you have less of a chance of getting stuck if you have the ability to shift to four wheel drive.
Harper Rentals  has four wheel drive vehicles that are allowed by contract to drive on unpaved roads. You will pay more for that right.
The Big Island has the usual array of sub-tropical island activities. While the Kona side has a number of white sand beaches, the coastline on the Hilo side tends to be rocky. This is due to the relative age of the coastline.
Due to its altitude, latitude, and the lack of interference from city lights, Mauna Kea provides among the best sites on earth for telescopes. You may notice the strange orange and pink hues put out by street lights on the Big Island. They are sodium lights used to ensure that the views from Mauna Kea are unpolluted.
Stop at the Visitor Information Station of the Onizuka Center for International Astronomy. They set up several telescopes nightly for the public to enjoy. Volunteers provide programs daily at 6PM, and will answer any questions you have as you look at the unbelievable number of visible stars. Remember to bring your jacket, as the elevation is over 9200 feet at the Visitor Center.
The beaches of the Big Island, especially on the Kona side, have been consistently voted amongst the best beaches in the world. Some (like Mauna Kea Beach) front hotel resorts, while others (like Makalawena) remained unencumbered by modern tourism. Hapuna Beach is reputed to be one of the best, consistent with the picture many outsiders have in their head of what a Hawaiian beach should be.
The island has one of the few green sand beaches in the world (see above), and several black sand beaches.
The Big Island has some fantastic snorkeling. Go to Kona Boys  to get your gear and some guidance on the best places to jump in. The Kona side has most of the best snorkeling, but Puna also has some excellent sites. Go in the morning on the Kona side, and in the afternoon in Puna, for clear and calm conditions.
Hilo Surfboard Company: Is the Big Island's most ‘authentic’ surf shop. People travel all the way form Kona to check out boards as they REALLY DO have the largest selection of boards. And unless you want a Hilo Surfboard Company T Shirt or shirts from a couple ‘locals. Like Moku Nui or KRU, better go to the mall. This is a real core surf shop! Owner Scott Murray will be stoked to see you and talk story! 84 Ponahawai St. Hilo. 808.934.0925
There are also hiking and camping tours too. Very convenient since humping camping equipment on a plane is difficult.
The Big Island has a tour company for every possible tourist endeavor. If you don't see it covered here, search for it. Chances are there will be a tour guide for what you want to do.
Companies to go with to see the mantas: Manta Ray Dives of Hawaii or Big Island Divers
Dance, culture, and traditional crafts classes are available for long or short term students. Many resorts offer 1-3 day classes in hula or lei making.
Hawaii's unemployment rates are among the lowest in the nation, and thus it is impressive that the Big Island boasts one of the lowest unemployment rates in the state. While tourism, military, and agriculture have typically been the largest employers, recent new job growth has resulted primarily from a residential building boom. The astronomical observatories are another important group of employers.
The Big Island has 2 resort areas on the Kohala coast - Mauna Lani and Waikoloa Beach resort. The two resorts neighbor each other and feature hotels and condo/townhome rentals. There are also great Bed and Breakfast type places, vacation rental homes and small operator hotels too. The Big Island is the only county in Hawaii that has no restrictions on the operation of vacation rentals. Before making reservations it's best to review a map of the island and plan ahead. Think about the activities and sightseeing you'll want to do.
Note that even solidified lava flows can still be very dangerous, as there are hidden flows of molten lava with only an overlying thin crust of rock in many places between Pu'u O'o and the shoreline. And of course there's the threat of methane explosions and lava bench collapses, so do not walk to the edge of the lava bench unless the rangers say it is safe to do so.
The usual disclaimers about the more active things to do apply. If you have a tour guide, they often have insurance premiums to be beholden to and as such make them more aware of safety issues. But otherwise the island is mostly remote and help can be far away. Know your physical limits.
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Mauna Loa is an active volcano in the Hawaiian Islands of the United States. Measured from sea level Mauna Loa is 4,169 meters high. It is also one of the largest volcanoes measuring 60 miles long and 30 miles wide. It is about 40,000 cubic meters in volume and over 4 kilometers above sea level. Measured from its base on the ocean floor it reaches above 8 kilometers making it the tallest mountain on Earth, but not the highest. That distinction is reserved for Mount Everest.