Volcanoes are also prodigious land builders as they have created the Hawaiian Island chain. Kilauea and Mauna Loa, two of the world's most active volcanoes, are still adding land to the island of Hawaii Hawaii's Volcanoes Revealed

Subduction of oceanic crust produces a Magma Plume

Hawaiian volcanoes typically evolve in four stages as volcanism waxes and wanes: (1) early alkalic, when volcanism originates on the deep sea floor; (2) shield, when roughly 95 percent of a volcano's volume is emplaced; (3) post-shield alkalic, when small-volume eruptions build scattered cones that thinly cap the shield-stage lavas; and (4) rejuvenated, when lavas of distinct chemistry erupt following a lengthy period of erosion and volcanic quiescence. During the early alkalic and shield stages, two or more elongate rift zones may develop as flanks of the volcano separate. Mantle-derived magma rises through a vertical conduit and is temporarily stored in a shallow summit reservoir from which magma may erupt within the summit region or be injected laterally into the rift zones. The ongoing activity at Kilauea's Pu‘u ‘O‘o cone that began in January 1983 is one such rift-zone eruption. The rift zones commonly extend deep underwater, producing submarine eruptions of bulbous pillow lava.

Origin of the Hawaiian Isles Volcanism Hidden deep beneath the Earth's surface on the Pacific Ocean’s sea floor lie one of the most constructive and yet least-understood natural phenomena in the world – Volcanoes of the Hawaiian Island Chain. It extends for 2,400 km (1,800 miles) from the extinct seamounts and atolls in Northwestern Hawaiian Isles to active volcanoes in southeast in Hawaii’s Big Island. The Pacific Oceanic plate is moving northwesterly. Hawaiian volcanoes are formed by a column of magma - molten rock - rising from deep within the Earth on the seafloor, erupting on the surface, and hardening in layers down the sides.. This forms the familiar shield shaped mountain we associate with volcanoes. The mechanism is termed as the Hotspot Volcanism, due to convection in the Upper mantle. The hotspot on the Pacific Oceanic Plate has been active for past 43 million years. Hawaiian Volcanic Isles are also flanked by the South Arch Volcanic field and North Arch Volcanic Field. The Hotspot is not fixed as new data reveals and the formation of the islands is influenced by subduction of shallow oceanic lithosphere. There are two geologic trends of the Hawaiian volcanism, Kea-Kilauea Trend and Loa- Loihi Trend.

Life-cycle of Hawaiian hot spot volcanoes: The Hawaiian Islands volcanoes are the product of a mantle hot spot in the middle of the Pacific Plate. The hot spot's current activity is underneath the southern end of the island of Hawaii and the next volcano in the chain, Lo'ihi Seamount, is forming on the sea floor just to the south of Hawaii and should emerge in another 200,000 years. The volcanoes undergo a progression of eruption styles and chemistries as they age, from pre- shield stage (Lo'ihi), through the major shield-building stage (Kilauea), to post-shield (Haleakala) and rejuvenated stages (such as Diamond Head on Oahu when it erupted). As the enormous mountains build on top of the ocean crust, the crust flexes downward and the islands slowly sink. Erosion takes its toll on the islands: giant landslides have occurred off all the islands, and some of the debris has traveled hundreds of kilometers offshore. Most of the Hawaiian Islands have irregular shapes, not like the round volcanoes. This is because their sides tend to collapse in gigantic landslides, leaving chunks the size of cities scattered around the deep sea floor near Hawaii. If such a landslide happened today it would be devastating to the islands and due to tsunamis – Pacific Ring of Fire

Once a volcano has grown above sea level, subaerial eruptions produce lava flows of jagged, clinkery ‘a‘a or smooth, ropy pahoehoe. If the flows reach the ocean they are rapidly quenched by seawater and shatter, producing a steep blanket of unstable volcanic sediment that mantles the upper submarine slopes. Above sea level then, the volcanoes develop the classic shield profile of gentle lava-flow slopes, whereas below sea level slopes are substantially steeper. While the volcanoes grow rapidly during the shield stage, they may also collapse catastrophically, generating giant landslides and tsunami, or fail more gradually, forming slumps on seafloor. Deformation and seismicity along Kilauea's south flank indicate that slumping is occurring there today. Loading of the underlying Pacific Plate by the growing volcanic edifices causes subsidence, forming deep basins at the base of the volcanoes. Once volcanism wanes and lava flows no longer reach the ocean, the volcano continues to submerge, while erosion incises deep river valleys, such as those on the Island of Kaua‘i. The edges of the submarine terraces that ring the islands, mark paleocoastlines that are now as much as 2,000 m underwater, many of which are capped by drowned coral reefs.

Direction Of Plate Movement

This diagram shows the relative topography along the Hawaiian chain. Note that with increasing age to the NE, the volcanoes are eroded and sink below sea level.

The volcano grows by adding layer upon layer of new pillow lava. Calderas are possible. Lava is alkalic in composition, with highcontent of sodium and potassium relative to the tholeiitic basalts of the shield-building stage. It may or may not emerge aboveCalderasalkalic

Giant Landslides Seamounts

The large size of seafloor features, high activity of volcanism, and rapid rates of subsidence make the Hawaiian islands an ideal place to study geologic processes.

The Hawaiian islands sink into the ocean at a measurable rate as the cessation of volcanism that provides the magma to build the islands.

Once extinct, the Hawaiian Islands continue to erode until they slip below sea level. The Emperor Seamount chain was once over the magma plumeand probably looked much like the Hawaiian Islands, but the volcanoes have since submerged. The Pacific Plate is carrying the entire chain of islands and seamounts to the north-north-west as it drifts slowly to the Aleutian Trench and its ultimate subduction.

The Hawaiian Isles volcanoes undergo a progression of eruption styles and chemistries as they age, from pre-shield stage (e.g., Lo'ihi), through the major shield-building stage (e.g., Kilauea), to post-shield (e.g., Haleakala) and rejuvenated stages (such as Diamond Head on Oahu when it erupted). As the enormous mountains build on top of the ocean crust, the crust flexes downward and the islands gradually sink, sometimes rapidly (Mahukona). Erosion takes its toll on the islands: giant landslides have occurred off all the islands, and some of the debris has traveled hundreds of kilometers offshore. Once extinct, the islands continue to erode until they slip below sea level. The Pacific Plate is carrying the entire chain of islands and seamounts to the north-north-west as it drifts slowly to the Japan Trench and its ultimate subduction.

Collapsed lava pillow on Puna Ridge, the submarine extension of Kilauea's East Rift Zone, April 2001, MBARI. Equivalent lava flow on land, on Kilauea's East Rift Zone, April 2001 Observations of submarine lava flows indicate volcanic processes that contradict conventional wisdom. Discoveries of lava ponds and flood basalts imply that lava may stay molten for a long time, despite the almost infinite heat sink of the ocean. Fragmental rocks-ash, hyalo- clastites, and volcaniclastic rocks, limu o Pele, and spatter imply that explosive eruptions take place even in the deep sea, where hydrostatic pressure is so great that steam bubbles should not be able to expand

This SPOT satellite image of the island of Oahu reveals the extensive erosion that has removed much of the material on the flanks of the Waianae and Koolau volcanoes. Note the stream valleys that reach the crest of the mountains. The resistant rocks mark the location of the rift zones of the volcanoes