- Discover Indonesia Online

You are currently in > Indonesia > Lesser Sunda Islands > Geology

Islands with vulcanos and corals

Nusa Tenggara forms the cental chain in the vast Indonesian chain of islands: it connects the Bigger Sunda Islands in the west with the Maluku islands and Papua in the east. The Lesser Sunda Islands form two separate bows. The long northern bow - Lombok, Sumbawa, Komodo, Flores, Lembata - has an vulcanic origin. The islands of the shorther, southern bow - Sumba, Savu, Roti, Timor - consist of raised coral reefs and deposits.
The islands in the vulcanic bow are relatively fertilel in the eastern part it can be dry in periods. The southern islands are branded by limestone plateaus and poor savannah's, on which cattle can hardly find any food. The western islands Lombok and Sumbawa are covered in wealthy vegetation which is characteristic for the humid tropical areas. The other islands have a dry season which lasts longer when you go to the east; parts of Timor have the lowest percipation of Indonesia.
On the whole the islands are remarkable less populated than Jawa and Bali, and the villages and cultures are located far from eachother. The agriculture of 'wet' rice on irrigated sawah's, just like elsewhere in Indonesia, is the most productive form of agriculture. On places where farmers depend on rain, corn, maniok and other crops are grown. On the dry islands of Roti and Savu the population survives the dry season with help of the nutricious juice from the lontar-palm.

A difficult start

The islands of Nusa Tenggara just appear from a clear, deep sea in the form of smoking vulcanoes or layered coral platforms. They have been created in the Late-Tertiairy period, about 70 milion years ago, and are fairly young for geologists. Pushed by magma flows, the Austratian-Indian Ocean Plate ( also called Sahul-plate ) steadily moved towards the west, until it collided with the Eurasian ( Sunda ) Plate. The less compact Sunda Plate was pushed up by the more heavy Sahul Plate, while the weight of the first forced the Sahul Plate to go down, where it was compressed and heated.

Picture: Colored lakes

When the plate had reached a depth of over 50 km, the rock formations melted, which found it's way up under big pressure through cracks and holes along the damaged border of the Sunda Plate. When reaching the surfave, the molten rock forms vulcanoes. Over the course of thousands of years the vulcanoes have increased in size and eventually they melted together in the form of the northern islands of Lombok, Sumbawa, Flores and the Solor- and Alor-archipelago. The bigger vulcanoes consist of layers of lava, ash, andother materials from eruptions.
The eruption of Gunung Tambora, on the island of Sumbawa, from 5-7 April 1815 was the biggest of the modern geological era. The force of the explosion was much bigger than the famous eruption of the Krakatau, of the western coast of Jawa in 1883. The eruption of the Tambora caused the release of 150 kubic km of ashes and rocks. The hight of the vulcano was brought down from 4200 to 3090 meter in just one day. The number of casualties is not exactly known, some say 12,000, others reported as much as 50,000.
Of the almost 40 identified vulcanoes in Nusa Tenggara, about 25 are in some form active. In 1988 a new vulcano emerged: Anak Ranaka.

The southern Chain

The islands of the shorter, southern chain - Sumba, Savu, Roti and Timor - contain old vulcanic material but are now totally not vulcanic. They mainly consist of geologically young, raised coral reefs from the Quarternary and somewhat older marine deposits from the Tertiary. The composition of the sediments varies from foulded and mixed coral limestone, marl and the strange 'flaking clay', named because this material flakes when it is exposed to the air. The clay soils contain small particles of all kinds of stone.
The deposit stones mainly originate from the zone where the two plates collided. While the Sahul-plate got under the Sunda Plate, the relatively soft marine deposit layer was scratched off and piled on top of eachother. On Timor you can find some remains of the Sunda Plate which broke off, and got stuck between the softer parts of the Sahul Plate.The findings of old, granite-like stone under younger sediments indicates that Sumba probably has another origin. Geologists thing that the island was torn from the Australian mainland, and was then transported to the north by a magma-flow.

Varied Landscapes

The northern chain is overseen bu vulcano's in all shapes and sizes, varying from the massive Tambota, a crates with a diameter of seven kilometer, to cones which don't even reach 100 meters in hight. Most, like the Rinjani, Tambora, Sangeang, Kimang ans Ujolewung have classical cone shapes. Several vulcano's raise above the 3000 meter level and some contain deep craterlakes. The most known of them are the mysteriously colored threesome on Gunung Keli Mutu on Flores. The colors are decided by minerals which are dissolved in the lake's water. Most of the times the lakes are black, lightgreen, turkois, but also sea-green, dark-blue and red are seen.
Other craterlakes, like the magnifique, five km long Danu Segara Anak on Gunung Rinjani in Northern Lombok, contains miniature vulcanic landscapes in the form of new daughter cones, complete with fresh lava flows.
A view on just a map of the area learns us that the northern and southern islands have completely different coastlines. The irregular coast of the northern islands consists of small and large protected sandy bays and open peninsulas. Off the coast are attractive islands surrounded by coral reefs.
The southern islands in contrary, have much more regular coastlines. They are branded by long stretches of rocky coastline, and sometimes small stretches of sandbeach, protected by ridges of coral reef.
Big parts of the coast of the southern islands consists of wide, hilly plateaus and terraces of reef limestone. These are especially remarkable north of Sumba, and along the eastern and western ends of Timor and Roti. The inlands of Sumba and Timor are rough and cut by deep valleys with steep sides. Around the summit of the highest mountain in Timor, the 2963 meter high Tata Mai Lau, clouds form. The mountains of Sumba are remarkably lower, the highest peak measures 1220 meter. These islands have only a few rivers which are permanently filled with water; often the grid-filled areas are dry, to be used after a heavy thundershower.
The islands of Nusa Tenggara know a big variety of soils. The vulcanic soils of the northern islands are on the whole young and fertile, however the youngest deposits contain too much ashes and are too porous to hold water. The reddish clay which covers several of the older vulcanic rocks, is heavily eroded and not fertile. The alluvial plains of the northern islands and the soils around the feet of the hills, built by mountain rivers, are best used for agriculture.
On the southern islands, the soil mainly consists of limestone and marl, with on top the usual thin layer of red or brown-like soil. On some places you can find dark soils, which have developped from a clay mineral which reacts with water and expands or declines. On some places this has lead to problems with construction; roads sometimes crack, as well as canals and sometimes even buildings.

The most dry Region of Indonesia

In total, the regioon forms - together with Malukku Tenggara - the most dry part of Indonesia. Nusa Tenggara is on the border of the ares where the northwest monsoon still has it's influence, and the wind which contains the rain - from the southeast - only reach the southern coasts. The inlands of the biggest islands are mountainous enough to catch some humid air, which causes some rain, which causes them to be the most wet parts of the islands.
In Nusa Tenggara the seasons are decided more by the changing patterns of wind and rain during both monsoons than by the temperature. The intensity of the two wet periods varies with the topographical situation, but rains never last long and most visitors which remind the islands as sunny and windy.
The places located in the south have most rain in May and June, when the southeastern winds from Australia pick up humid air from the Timor Sea. During this period the northern coasts, which are on the other sides of the mountains, are almost dry. During the northwest monsoon, from december until march, the winds over the Jawa Sea take humid air over the northern slopes of Lombok, Sumbawa and Flores. The most dry areas in this season are the protected southeastern slopes and the coastal areas.
During both seasons, the mountains from the biggest islands which catch the 'humid' air, get the most rain; furthermore little rain is left for the other areas. During the southeast monsoon, the areas of eastern Flores, Lembata and Alor are less wet than the southeastern coast of Timor, because they are in the shade of the high Timorese mountains. On similar way, the northern coasts of Sumba and Timor are protected by the mountains of Sumbawa and Flores during the northwestern monsoon.

Picture: Gunung Inerie

Due to these patterns the spread of percipation is very complex. It's mainly created by the local topography. In the higher parts of Lombok, Sumbawa, Flores and Timor sometimes gets more than 3000 mm a year, while the coastal plains or protected areas in the inlands often get less than 1500 mm. Some areas are even more dry: parts of Solor, Adonara, Lembata, Pantar, Alor, North Timor, East-Sumba and the northern coast of Flores get less than 1000 mm a year; the most dry places only get 500 to 750 mm a year.
Most rain falls in the form of short, heavy thundershowers in the afternoon. In the night and mornings it hardly rains. A continuous wet period is rare. Cyclones only reach the Timor- and Arafura Sea; the islands west of Timor and Alor are usually not touched.

Sun and Wind

The region is known for it's refreshing winds and high number of hours with sun, and therefore is a paradise for sun-lovers and wind-surfers. In most coastal areas the sun shines for seven or eight hours a day from June until October, and in December and January at least four to five hours a day. The inlands on the whole are more wet and less sunny. Timor is the island with most sunshine; even the inlands gets at least four to five hours, and sometimes ten hours a day.
On the northern islands, winds are refreshing the biggest part of the year, at least along the coast; the strongest wind blow from June until September, and quiet from December until March. Along the coast the average wind speed is 9 to 14 km an hour. On Timor however, an possibly also on Roti and Savu, the wind is more powerfull on the whole. On all islands, the wind decreased more inland. The most windy times are in the morning and late in the afternoon.
In all low areas the temperatures are high throughout the year. The months October and November are the warmest, with about 33C, but this doesn't say much, since the temperature is only 3 degrees lower in the coolest period, June and July. The temperatures at night reach 16 to 21 degrees, while humidity is between 70 and 80 per cent all year round; only on Timor, they are little more extreme..
The elevated areas are cooler and more bearable; the nights can even be cold. It stimulates to travel around at daylight. But don't forget the chance for sunburn is rather big.

Your website for tickets in Indonesia!
Looking for e-tickets for flights in Indonesia? Here's your solution! Order your e-tickets at
Add this page to your email, your own blog, MySpace, Facebook, or whatsoever via AddThis:
Bookmark and Share

Additional information, updates or feedback? Send them in!

Feedback Form


7 pictures in this gallery 

Created by · feedback & contact · © 2000-2020
Other websites by · · ·

137,037,223 pageviews Discover Indonesia Online at