Maui, the second largest island in the Hawaiian group, is 48 miles long, 26 miles wide, and covers 728 square miles. The principal town is Wailuku. Sugar cane and pineapples are the principal crops. Water is used chiefly for irrigating cane. The purpose of the investigation was to study the geology and the ground-water resources of the island.
Maui was built by two volcanoes. East Maui or Haleakala Volcano is 10,025 feet high and famous for its so-called crater, which is a section of Hawaii National Park. Evidence is given to show that it is the head of two amphitheater-headed valleys in which numerous secondary eruptions have occurred and that it is not a crater, caldera, or eroded caldera. West Maui is a deeply dissected volcano 5,788 feet high. The flat Isthmus connecting the two volcanoes was made by lavas from East Maui banking against the West Maui Mountains. Plate 1 shows the geology, wells, springs, and water-development tunnels. Plate 2 is a map and description of points of geologic interest along the main highways. Volcanic terms used in the report are briefly defined. A synopsis of the climate is included and a record of the annual rainfall at all stations is given also. Puu Kukui, on West Maui, has an average annual rainfall of 389 inches and it lies just six miles from Olowalu where only 2 inches of rain fell in 1928, the lowest ever recorded in the Hawaiian Islands. The second rainiest place in the Territory is Kuhiwa Gulch on East Maui where 523 inches fell during 1937. Rainfall averages 2,360 million gallons daily on East Maui and 580 on West Maui. Ground water at the point of use in months of low rainfall is worth about $120 per million gallons, which makes most undeveloped supplies valuable.
The oldest rocks on East Maui are the very permeable primitive Honomanu basalts, which were extruded probably in Pliocene and early Pleistocene time from three rift zones. These rocks form a dome about 8,000 feet high and extend an unknown distance below sea level. Covering this dome are the Kula volcanics, extruded probably in early and middle Pleistocene time, and characterized by andesites, andesitic basalts, and picritic basalts. They are 2.000 feet thick on the summit and 50 to 200 feet thick at the periphery. They contain a sufficient number of interbedded soils, thin vitric tuff beds, and lava-filled valleys in their upper part to give rise to valuable perched springs in wet areas. The Kula lavas accumulated during a waning volcanic phase which was followed by a quiescence long enough for the erosion of deep amphitheater-headed valleys in the east or wet half of the mountain. Volcanic activity was renewed in middle (?) to late Pleistocene time and continued until Recent time, during which the Hana volcanic series was laid down. The last lava flow was erupted about 1750. The Hana lavas comprise andesitic, picritic, and olivine basalts. They veneered large areas of the east and south slopes, partly filled the deep amphitheater-headed valleys, and deeply buried the smaller valleys in the eastern half of the mountain. The Hana rocks are exceedingly permeable and much rain sinks into them.
The oldest rocks on West Maui are the very permeable primitive Wailuku basalts, which were extruded probably in Pliocene and early Pleistocene time from two rifts and from many radial fissures. The basalts form a dome about 5,600 feet high and extend an unknown distance below sea level. Iao Valley is the eroded caldera of this dome. Forming an incomplete veneer over the dome are the Honolua soda trachytes and oligoclase andesites. They were extruded in late Pliocene (?) or early Pleistocene time, chiefly from bulbous domes. The clinker beds carry some water but the rocks are generally too dense to be good aquifers. During early (?) Pleistocene the West Maui volcano was cut by deep amphitheater-headed valleys and then all of Maui was deeply submerged.
Four scattered eruptions occurred on West Maui in middle (?) and late Pleistocene time. The cones and lavas cover only small areas and are called the Lahaina volcanic series.
The sedimentary rocks of both East and West Maui are chiefly late Quaternary and comprise fans, landslide debris, delta deposits, and valley fills, mostly of poorly permeable and poorly assorted bouldery alluvium. They are overlain on the Isthmus by extensive calcareous dunes of three ages. A mud flow more than 300 feet thick is exposed in Kaupo Valley. During the fluctuations of the ocean in the Pleistocene, the island was emerged and submerged several times. Calcareous fossiliferous marine conglomerates deposited during this period are found up to an altitude of 250 feet on West Maui.
The Homomanu, Wailuku, and Kula lavas are the chief aquifers. They supply 28 irrigation wells which yield an average of 170 million gallons a day of basal water. These wells are mine-like shafts with infiltration tunnels and are called Maui-type wells. Well 16 yields 40,000,000 gallons daily with a 22-foot drawdown, which is the largest amount yielded by any well in the Hawaiian Islands. The largest spring (no. 26) on the island is artesian. It yields 10,400,000 gallons daily and issues from Kula lavas near Nahiku. West Maui has numerous perennial streams supplied by springs from a dike complex. Twenty-three tunnels in West Maui recover 20.5 million gallons a day of high-level water, mostly from this dike complex. East Maui has few perennial streams in proportion to its size, and they are chiefly small due to the water sheds being underlain with permeable lavas. Forty tunnels recover 6 million gallons a day of high-level water in East Maui and all from structures other than dikes.
It is estimated that about 100 million gallons a day of basal water wastes into the sea from West Maui and about 700 million gallons a day from East Maui. A number of sites are described where wells could be sunk to recover this water. Sites are also described where tunnels could be driven to recover high-level supplies. The hydrology of East and West Maui is conspicuously different in many respects, mainly because of the difference in the stage of dissection, the extensive veneer of very permeable Hann lavas on East Maui, and the comparatively small area of the Lahaina lavas of similar age on West Maui. The only thermal water known in the Hawaiian Islands, except on the active volcano of Kilauea, is in a well in West Maui.
The Nahiku area has been mapped and studied in detail. The upper part of the Honomanu volcanic series, exposed in the sea cliffs, in petrographic character is transitional into the overlying Kula lavas, Kula and Hana time were characterized by a long succession of valley-cutting episodes, each valley being filled by lava erupted from the east rift zone. The lavas include olivine basalts, picritic basalts, and basaltic andesites,
In the Nahiku area basal ground water occurs largely in the Honomanu basalts. Perched water occurs in many of the later lavas, generally following the axes of buried valleys. The members which perch the water are mostly ashy soil beds, although an unusually extensive, thick layer of much decomposed clinker also appears to be a supporting member. Most of the water travels through the basal clinker members of aa lavas. Artesian water is encountered in the upper, transitional part of the Honomanu volcanic series. The aquifer is permeable porphyritic pahoehoe; the confining members are relatively impermeable nonporphyritic aa.
The lavas of East Maui are described according to stratigraphic groups. The oldest or Honomanu lavas are olivine basalts like the primitive lavas in other Hawaiian volcanoes. The later or Kula and Hana lavas include basalts, basaltic andesites, andesites, and picritic basalts. The normative nepheline of analyzed East Maui lavas has not been identified in the mode. The degree of differentiation is inversely proportional to the frequency of eruptions.
The lavas of West Maui volcano are divided into the Wailuku volcanic series, consisting largely of olivine basalts with less abundant olivine-poor basalts, hypersthene basalts, and picritic basalts; the Honolua volcanic series, consisting of oligoclase andesites and soda trachytes; and the Lahaina volcanic series, consisting of nepheline basanite and picritic basalts. Coarse-grained gabbros intrude the Wailuku lavas. Differentiation was undoubtedly partly by crystal settling, but the alkali curves of the variation diagram suggest that volatile transfer was of some importance.
