As a "new" portion of lithosphere (it is actually ~90 million years old in the vicinity of Hawai'i) moves over the hotspot, the effects are not instantaneous; the degree and amount of partial melting and magma production are small. Small percentages of partial melting produce magma that is rich in alkali elements relative to silica, (called alkalic basalt). This low-efficiency melting produces only small amounts of magma meaning that the eruption rate of a young hotspot volcano is not high.

Lo'ihi seamount off the southeast coast of Hawai'i was known from bathymetric surveys, and thought to be a large slump. However, dredging in the 1970's recovered fresh lava samples, and the growing HVO seismic network began to record earthquake swarms centered on Lo'ihi. Submersible investigations have confirmed that Lo'ihi is actually the youngest Hawaiian volcano, with its summit some 975 m below sea level (~4000 m above the adjacent sea floor). Geochemical analyses of Lo'ihi samples show many of them to be alkalic basalt. Lo'ihi has a flat top that may be an almost-filled caldera, and as we'll see later this means it probably also has a magma chamber. Many of the pillow lavas observed on Lo'ihi have little or no sediment on them, a good indication of their recent formation.


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