Questions and answers were provided by Master Volunteer Ranger Ed Bonsey.
1. How many volcanoes are there on the Big Island? Which ones are extinct, dormant, or active?
Five volcanoes make up the island of Hawai`i: Kohala, Mauna Kea, Hualalai, Mauna Loa, and Kilauea.
Volcanoes that will never erupt again are considered extinct. Dormant volcanoes have not erupted in historic time (the last 200 years in Hawai`i) but probably will erupt again. Active volcanoes have erupted in historical time (the last 200 years in Hawai`i).
Kohala, the oldest volcano on this island, last erupted about 60,000 years ago and is considered extinct.
Mauna Kea last erupted 3,600 years ago and is dormant.
Hualalai, Mauna Loa, and Kilauea are active.
Hualalai erupted seven times in the last 2,100 years. The only historic eruptions were in 1800 and 1801. Mauna Loa last erupted in 1984 and sent flows towards Hilo. Kilauea has been erupting since 1983.
Loihi, a submarine volcano, is 15 miles (24 km) southeast of the island and 3,178 feet (969 m) below sea level. Loihi will probably not reach sea level before 250,000 years or more. Seismicity, geothermal vents, and fresh lava indicate Loihi is active.
2. Where does the lava come from?
Rocks that are moving upward in the mantle beneath Hawai`i begin to melt about 40 to 60 miles (60 to 100 km) depth. The molten rock, called magma , rises because of its relatively low density. The magma "ponds" in a reservoir 1 to 4 miles (2 to 6 km) beneath the summit. The magma can follow fractures up to the crater and produce a summit eruption. During the current eruption, the magma has followed a zone of weakness, the East Rift Zone. Magma reaches the surface at a vent, an opening at the surface through which volcanic material is extruded. There have been several vents during the last 10 years. The currently active vent is 15 miles (24 km) from the summit and 6 miles (9.6 km) above the coast.
3. What is the difference between a caldera and a pit crater?
A caldera is a large, basin-shaped volcanic depression, more or less circular, the diameter of which is many times greater than that of the included vents. A pit crater is a crater formed by sinking in of the surface; not primarily a vent for lava.
4. How much lava is erupted everyday from Kilauea? How deep is the lava from the current eruption? How many new acres of land have been added to the island? How many homes have been destroyed by the current eruption?
The current eruption of Kilauea has destroyed 187 structures including the park's Wahaula Visitors Center.
The current eruption rate of Kilauea volcano is 250,000-650,000 cubic yards/day (200,000-500,000 cubic meters/day). That is enough to cover the floor of the caldera with a thin layer of lava every day or resurface a 20-mile-long two-lane road, like Chain of Craters Road, every day. Remember, the eruption has been continuous since 1983, that would make the stack of lava on the road about 20 miles tall!
Fortunately, most of the lava is transported by lava tubes to the ocean, where it fragments, and adds layers of rubble to the submarine flank of the volcano. Lava in the Kamoamoa area is about 15 feet (5 m) deep where it first crossed the road. Lava from the Kupaianaha eruption is about 75 feet (25 m) deep at Queens Bath, 50-75 feet (16-25 m) feet deep in Kalapana, and 45 feet (15 m) near the sea cliff at mid-flow.
Since 1983, about 500 acres of new land has been added to the island. In addition to 181 homes, the Wahaula visitors center, the Royal Gardens Community Center, the Mauna Kea Congregational Church, and the Kalapana Drive-in have been destroyed.
5. Where does the lava come from? How long did it take for the lava to move down the lava tube system from the vent to the ocean? How much heat is lost as the lava moves from the vent to the ocean? How large are the lava tubes feeding the coastal flow (how long and what diameter)? How thick is the crust over the lava tube (and under my feet)?
Lava is supplied in lava tubes that extend from vents on the flank of the Pu`u `O`o cone down to the ocean. A skylight (collapsed roof of the tube) provides a view into a lava tube.
The lava originates by melting in the mantle 40 miles (60 km) below the surface. Liquid rock below the surface is called magma. The magma rises through conduits to a reservoir a couple of miles beneath the summit of Kilauea. From there, the magma travels through a dike that extends from the summit down into the flank of the volcano. The magma rises from the dike up to the vents at the surface. From the vents down to the coast the lava is transported in tubes. It takes about 3 hours for the lava to move down the lava tube from the vent to the ocean. The temperature of the lava drops about 14F (8C) as it moves from the vent to the ocean. The first crust forms on the lava at about 930F (500C). The lava tubes are about 6 miles (9.6 km) long and have a diameter of 12 to 15 feet (4 to 5 m)(up on the pali). Tubes on the coast are 3 to 6 feet (1-2 m) below the surface, about 3 feet (1 m) in height, and up to 30 feet (10 m) in width. The tubes are usually a few feet below the surface.
6. When will the eruption stop?
There is no way of knowing when the eruption will stop. The current eruption is the most long-lived in historical time. Some geologists think that the main vent for Kilauea has shifted from the summit, where there was over one hundred years of activity that ended in 1924, to the East Rift Zone, where most eruptions have been located since 1955. If this model is correct, activity on the East Rift Zone may continue for a century or more.
7. How long after a lava flow does it take for an area to revegetate?
That depends on several factors: rainfall, type of lava (or ash), and proximity to the caldera (a source of sulfur that makes acid rain). Sword ferns can be found on flows after about one year. Pahoehoe lava from the 1973 fissure eruption near Mauna Ulu has several species of native plants (most are a few feet tall). Lava from the 1955 eruption on the lower East Rift Zone has abundant tall (20-30 feet, 6-10 m) `ohi`a trees.
8. What is the composition of the lava?
The lava is basalt. Hawaiian basalts contain about 50% silica, 10% each of iron, magnesium, calcium, about 15% aluminum, 2% titanium and 2% sodium.
9. If magma is removed from great depths and used to make the volcanoes, is there a void (hole) beneath the island?
No. There is no "hole" beneath the island due to magma withdrawl. Yes, liquid rock is removed. Magma is derived from a layer in the earth called the mantle. Around the Pacific Ocean basin is a ring of subduction zones, called the Ring of Fire, where oceanic plates are pushed beneath continents. The oceanic plates are assimilated back into the mantle and flow or convect laterally and vertically. Some mantle probably resupplies the plume beneath Hawai`i. Think of it as a global system of supply and demand. Also the "flow" is of a solid and is at a very gradual rate, maybe millimeters per year. The concept of a solid that flows might seem strange. Think of stacking bricks on an asphalt driveway on a hot summer day. If you return hours later the bricks will have sunk down an inch (few cm) or so. In actuality the asphalt flows laterally from beneath the bricks in response to their weight. If you don't want to damage your driveway, watch toothpaste as it comes out of the tube. It is another solid that flows.
10. Will Kilauea ever explode like Mount St. Helens? Have there ever been explosive eruptions of Kilauea?
Although there have been explosive eruptions of Kilauea, it is not the same kind of volcano as Mount St. Helens and does not erupt in the same manner.
Mount St. Helens magma is more viscous. Therefore gas cannot escape as readily, resulting in explosive eruptions. One index of explosivity is volume of eruption. Since the start of the current Kilauea eruption more than 1,400 million cubic meters of lava have been erupted. Mount St. Helens erupted 1 cubic kilometer of ash (about 10 times greater than the current Kilauea eruption).
There have been explosive eruptions at Kilauea. The Uwekahuna Ash was erupted about 1,500 years ago and may be related to an older caldera that filled with lava prior to the development of the present caldera. The Keanakakoi Ash was erupted during weeks or months of activity in 1790. The early activity was driven by degassing magma that interacted with groundwater. Later eruptions were driven by steam explosions. The warriors were killed near the end of the eruption by what geologists call a base surge. Surges are gas-rich and have little volcanic material. This "steam blast" was not enough to burn their skin but did cause them to suffocate.
The 1924 eruption was caused by a 660 feet (200 m) drop in the lava lake that allowed groundwater to enter the conduit. The resulting steam-driven explosions shot rocks weighing several tons as far as 2,600 feet (800 m). At the end of the eruption Halema`uma`u was 1,320 feet (400 m deep) (deep enough that the Empire State building would fit inside) and had doubled in width from 1,400 to 3,000 feet (430 to 920 m). Note that these explosive eruptions are infrequent.