June 14, 2001
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Heat is deep and magma is shallow in a hot-spot system
The hot-spot theory is the well-known modern explanation for the origin of the Hawaiian island chain. In it, the Pacific plate drifts northwestward over a relatively fixed hot spot. Magma generated by the heat then penetrates the plate and rises to the surface, leaving a string of volcanoes. The active volcanoes Kilauea, Mauna Loa, and adjacent submarine Lo`ihi are the Hawaiian hot spot's latest children.
Confusion commonly surrounds the distinction between the site of the hot spot and the site where magma is generated, judging from the questions we receive. Let's start with the source of heat, which has two basic causes. Primordial heat was created when the planet first coalesced from cosmic debris. Additional heat has been created by the decay of radioactive elements in the Earth. The Earth is cooling as this second source of heat slowly diminishes.
Many geophysicists believe that hot spots originate as perturbations in a zone between the Earth's core and overlying mantle. This zone, which lies about 2900 km (1700 mi) deep, might develop a small bump that protrudes slightly outward into the mantle from the core. The bump transmits the intense heat of the core into adjacent mantle, which in turn is surrounded by cooler mantle. Although solid, the hotter mantle material will rise because of its greater heat. Convection is the process by which heated material rises and cooler material sinks. We see convection every day; for example, the currents that swirl in a pot of liquid on a stove or the warm air that rises over the surface of sun-heated lava. Solid materials also convect, although at considerably slower rates-perhaps only a few centimeters per year in the case of the mantle. Within the Earth, heated blobs of mobile yet solid mantle rise within a solid cooler mantle. Though heat is being transferred by these rising blobs, no magma is created because nothing has melted.
Individual blobs probably don't traverse the entire mantle. As each one stalls, its heat is transferred to adjacent rock, provoking continued convection. It's a "Pony Express," in which the horses will traverse only one part of the mantle, yet the message, the heat, continues through.
Heated rocks remain solid at the great pressures deep in the mantle. Perhaps about 100-120 km (60-70 mi) deep, however, the pressure is sufficiently low that melting can take place. Beads of magma sweat from the rocks and rise. Our convection system now comprises liquid rising within a solid mesh. The beads coalesce to create a braided stream that penetrates the crust of the Pacific plate. This intricate column of magma is broad enough to fuel Kilauea, Mauna Loa, and Lo`ihi. Ultimately the magma accumulates in complex reservoirs lying 3-5 km (2-3 mi) beneath each active volcano.
In summary, a hot spot originates at relatively great depth, the magma at relatively shallow depth. The hot spot is merely an anomalous concentration of the heat that is constantly being transferred from the Earth's interior to its surface. In our current thinking, a spot begins as a perturbation at the core-mantle boundary, deep within the Earth. If the perturbation is sufficiently large, it takes on a life of its own, feeding a thermal plume that may last at least 80 million years, as in the case of the Hawaiian-Emperor volcanic chain. Heat is transferred upward by convecting solids. Magma forms when relatively shallow, solid mantle is torched by this heat. Lava is magma after it reaches the Earth's surface.
Eruptive activity of Kilauea Volcano persisted at the Pu`u `O`o vent during the past week. Lava moves away from the vent area toward the ocean in a network of tubes. Small breakouts occur from the tube system above Pulama pali, but most of the lava descends the pali in two separate flows. The small eastern flow, near the Royal Gardens subdivision, continues to be weakly active. A tube in the western flow carries lava down Pulama pali and then turns southeastward across the coastal flat, eventually feeding the active ocean entry 400-900 m (1/4-1/2 miles) northeast of Kupapa`u. A more westerly stream in the western flow travels down the pali and breaks out from the tube system at several places between the pali and the shoreline, 4-5 km (2.5-3 miles) from the end of Chain of Craters Road. Lava from this part of the flow is not entering the ocean.
Updated: June 18, 2001 (pnf)