Volcano Watch — From a puddle to Kīlauea: development of the world's most active volcano

All of the events that we experience at Kīlauea—eruptions at the summit and on the rift zones, intrusions of magma that don't reach the surface, earthquake swarms that accompany eruptions and intrusion, and the large earthquakes that accompany growth of the volcano and trigger landslides—are responses by the pile of layered lava to the unrelenting pressure of magma rising from below.

The story of Kīlauea's development from a small puddle of lava on the ocean floor more than 100,000 years ago to the world's most active volcano will be covered in this and next week's "Volcano Watch" column.

An important control on volcano growth is the composition of the magma and the rate at which it is supplied. Hawaiian volcanoes are built of basalt, which is supplied from a zone in the Earth's mantle that is hot enough to melt the surrounding rocks. Seismic studies establish that this zone is deeper than the deepest earthquakes (35 miles), and probably extends as deep as 65 miles from the surface.

Each volcano in the Hawaiian-Emperor Chain of islands and seamounts, which extends from the inhabited islands of our State in the southern part of the chain through Midway and all the way to deeply submerged seamounts near the Aleutian Islands, has been fed from the same mantle source. This source has been located beneath the present site of the Big Island for more than 70 million years, beginning just before the dinosaurs became extinct. The zone in which melting takes place is termed the "Hawaiian hot spot." The northwestward motion of the Pacific Plate causes each volcano to grow and then slowly become extinct as it moves away from the hot spot.

Lo‘ihi seamount, forming off the southern shore of Hawai‘i, is the youngest volcano in the chain. We know from samples dredged from Lo‘ihi's surface that its magma differs in chemical composition from that supplying Kīlauea and Mauna Loa. The composition indicates that the supply of magma to Lo‘ihi is relatively low, consistent with its location over the very southern edge of the hot spot. By the time a Hawaiian volcano has reached its shield-building stage, as have Kīlauea and Mauna Loa, the volcano has moved well within the perimeters of the hot spot, and magma is supplied at a higher and more uniform rate than in the pre-shield stage. Several studies of recent magma supply to Kīlauea have shown a rate of about 1 cubic mile every 40 years. The relatively constant magma supply guarantees that there will be a large upward-directed pressure being applied to the volcano at all times.

At the beginning of Kīlauea's history, when magma first erupted on the sea floor, volcanic glass fragments and bulbous lava "pillows" were deposited on a sedimentary layer that overlays older lava layers making up the Pacific Plate. Because of the intrinsic instability of lavas fragmented by the interaction of magma and seawater, the growing volcano was never firmly attached to the underlying ocean floor. Thus, as the mass of the volcano increased, gravity tended to spread the volcanic edifice laterally, much as a pile of sand widens at its base as more sand is added to the top. The difference between sand and layers of fragmented lava is that sand is free to flow as individual particles while the lava layers will break.

Eventually, this fracturing in response to gravitationally-controlled spreading produced long, linear rift zones on the volcano. Experiments using gelatin models injected with colored dyes have demonstrated that the orientation of rift zones is determined by pre-existing volcanic structures. Because Kīlauea is built on the southern flank of Mauna Loa, its two rift zones parallel the rift zones of Mauna Loa. The great bulk of Mauna Loa has prevented the formation of a third rift zone, which would have been oriented to the northwest.

Kīlauea's rift zones are important to the volcano's development because they allow magma to intrude the flanks of the edifice, which contributes to lateral spreading of the volcano. Recent work by the Hawaiian Volcano Observatory staff and others has shown the rift zones to consist of a highly fractured upper layer (where magma intrusions and earthquake swarms occur), a fluid magmatic core, and a deep, mostly solid core composed of dense accumulations of the mineral olivine, which has crystallized and settled out of the fluid core. The weight of the deep core is currently considered to be the principle force causing the volcano edifice to slide on its interface with the sea floor.

The greatest impact of the formation and development of rift zones it that the spreading of the volcano is now directed by the volcano's internal magmatic system and not solely by gravity acting on the growing accumulation of lava.

The formation of a magma storage reservoir beneath the volcano's summit also occurred early in Kīlauea's history, in response to neutral bouyancy forces within the growing volcano, i.e., magma rises because it is lighter than the solidified lava at the base of the volcano, which is made dense by the weight of the growing edifice. The rising magma stops where its density equals that of the less dense lavas that make up the upper part of the volcano. The principle of neutral buoyancy also determines the structure of the magma system in the rift zones.

Kīlauea has grown to its present size through a complex interaction of magma moving upward, and fractured intrusive and extrusive rocks that resist this upward movement. Magma will move laterally when it can no longer rise, making the rift zones preferred sites of secondary magma storage. The growth of the volcanic edifice by intrusion of magma beneath the volcano's surface further complicates the underground structure, forcing magma to seek new pathways after each eruption, intrusion, or large earthquake.

Magma, unlike water, finds it difficult to maintain continuous flow, as its properties change with small changes in temperature (cooling) or pressure (resistance to upward movement or shock waves produced by large earthquakes). Cooling and increasing pressure tend to slow magma flow and eventually close magma conduits. Thus, long-continued eruption, such as Kīlauea's current activity, is rarely achieved, because it requires an unusual combination of circumstances to keep magma conduits open.

Next week's "Volcano Watch" will summarize the recent history of Kīlauea, offering an explanation as to why we are currently experiencing a long-lived eruption. The topics covered are the subject of a seminar to be given by Tom Wright, former Scientist-in-Charge of the Hawaiian Volcano Observatory, on Tuesday, 7:00 p.m., at Kīlauea Visitor Center.