The geology of Nusa is very different from the great part of volcanic Bali but similar to its southern peninsula, the area of Bali commonly know as Bukit. All three islands have a limestone base and typical karst topography. The karst effect is particularly pronounced in Nusa Penida where a significant number of caves are found.
Previous geological studies of the southern part of Bali and Nusa Penida point to a formative period between the Miocene and Pliocene. According to these works, Nusa Penida was built up by both limestone units of the southern formation and local marine alluvial sediments.
The small archipelago lies between S0 3S' and S0 47' latitude south and between 115° 26' 27,9" and 115° 35' 27,9" longitude east. The three island's respective areas are: Nusa Penida (191.4625 km2), Lembongan (8.6875 km2) and Ceningan (2.69 km2). The total surface of the small archipelago is 202.84 km2 while the islands' altitudes range between 0 and 529 m. a.s.l.
Image left: Limestone pit near Limo, north Nusa Penida (Godi Dijkman, 2009)
The topography of Nusa Penida is characterised by a sequence of steep limestone knolls rising from the north to the south coast, while the northern area has a small coastal plain at sea level, the south coast is marked by vertical cliffs rising between 100 and 200m. a.s.l. The orographic structure of the island can be roughly divided along three plateau, the first being at sea level on the north coast; the second which runs between 400 and 450 m. a.s.l. at the centre; and the third on the south of the island which lies at about 200m. a.s.l. The island is marked by a central hilly ridge that runs across it from south-east to north-west with the highest hill being 529 m. Situated on this hill is one of the main temples of Nusa Penida: the Pura Mundi.
The high permeability of the limestone rock, which constitutes the island's main geological foundation, facilitates quick infiltration of the rainfall that rapidly vanishes into the porous rocks forming underground caves and rivers thereby leaving a dry surface. Throughout the island there are no permanent rivers and the few that do occur are merely short brooks closed to the coastal line where alluvial terrain prevails thereby allowing them to run over ground. These brooks generally follow the valley's floor until they reach the sea; they are generally dry through most of the year and become alive only during the rainy season. During that period the hills' slopes carry all the rain into the valley and replenish the dead rivers.
The rivers do not have any protective banks or flow regulating dams, and in the case of protracted rain they tend to overflow causing flash floods. Some areas of the island, notably: Suana, Penida, Pundukaha Kelod and Sentalkawan, appear to be particularly affected by this danger during the rainy season. In 1989 two flash floods literally cut the inhabited area of Suana from its main road. North of the village, the force of the first of these flash floods swept away a concrete road bridge and a large chunk of a hill. To the south of the village a second flood, coming from a parallel valley, eradicated a number of coconuts trees and filled with debris a large area of sea used for seaweed farming. The lower part of the village was covered with water for a day and the local people lost a significant number of domestic animals. Fortunately no human deaths occurred as a result of these events in Suana.
- Giambelli, Rodolfo A. - Reciprocating with Ibu Pretiwi. Social organisations and the importance of plants, land and the ancestors in Nusa Penida, Department of Anthropology Research School of Pacific and Asian Studies The Australian National University. Canberra 1995, p.1-2
Physiographic Regions (Whitten 1996:105-109)
The physical structure of Java and Bali is complex, and 128 land systems were recognized in the appropriate volume of the detailed Regional Physical Planning Project for Transmigration study (RePPProT 1989). Java and Bali can be divided into four major physiographic regions among which certain land systems may be shared. Apart from the Northern Alluvial Plains (Java), the Northern Foothills and Plains (all Java, including the Madura karstic hills and terraces), the Central Volcanic Mountains (al Java, and the Bali volcanic complex), the fourth is the Southern Dissected Plateaux and Plains, amongst which the (D3) Sewu-Lengkong karstic plateaux and (D6) Blambangan - Nusa Dua - Nusa Penida karstic terraces. This region (p.109) in total measures 30,620 km2.
The basic lithology of the region is one of young, mixed volcanic and calcareous marine sediments lying on the southern flank of the series of young volcanic piles aligned along the centre of the islands. There are common intrusions of older volcanics, and one subregion is dominated by such material. Most areas have been uplifted and tilted to the south: folding is not an important feature.
Image right: Cone karst landscape on Nusa Penida (A.J.Whitten, 1996)
The landforms are varied but all the non-alluvial plains and hilly areas are moderately to strongly dissected by a fine pattern of streams. There are several broad plateaux of tuffaceous sediments comprising undulating to rolling summit areas; in other places they have been tilted south and dissected to leave flat-topped ridges, widely separated by deep valleys. Karst scenery is very distinctive where well developed over pure limestone, but impure tuffaceous limestones and marls are more common, and karstic landscapes with some karst features mixed with non-karst topography are more usual. The limestones of the small eastern subregions are impure and exhibit the usual features of raised reefs and atolls. Large alluvial plains of riverine and estuarine origin are a characteristic of one subregion only. They are flat except for lightly incised floodplains and coastal beaches.
Six sub-regions are distinguished: 1. Tampang-Pangandaran Dissected Plateaux; 2. Citandauy-Serayu-Progo Ridges and Plains; 3. Sewu-Lengkong Karstic Plateaux; 4. Betiri Hills; 5. Negara Plains, Terraces and Hills; 6. Blambangan-Nusa Dua-Nusa Penida Karstic Terraces.
- Whitten, Tony (et al.) - The Ecology of Java & Bali, The Ecology of Indonesia Series, Volume II, Periplus Editions, 1996