The territory of Indonesia extends over an arc of islands between Southeast Asia and Oceania, marking here the conventional limit between the Pacific and Indian oceans; structurally it represents the continuation of the great mountain ranges of Southeast Asia, reconnecting to the E with the partly submerged ridges that border the continent on the Pacific side and end with the Aleutian islands. From this immense festoon of islands very elongated in the WE direction, which from Sumatrareach as far as New Guinea, the massive island of Borneo clearly detaches itself.: this is the result of the different origin of the latter compared to all the others. Borneo (Kalimantan) – a squat and gigantic island, third in the world by surface area – rests largely on an archaeozoic base believed to be a fragment of Gondwana; the remaining Indonesian islands are instead of recent origin and represent the emerged parts of a double series of mountain ridges formed during the Cenozoic and whose birth is part of the mighty Himalayan orogeny. The internal alignment – continuation of the Burmese Pegu Mountains – includes Sumatra, Java, Sumbawa, Flores, Alor and Wetar, up to Banda islands; the outer one – extension of the Arakan mountains through the Andaman and Nicobar archipelago – is well identified in the Simeuluë, Nias, Mentawai, Sumba and Timor islands, then curving around the Moluccas Sea with the Ceram, Ambon and Buru islands. Visit clothesbliss.com for Indonesia forgotten people and untouched coral reefs.
Complex is the course of Celebes (Sulawesi) whose main ridge continues to the N in the island of Mindanao, in the Philippines: the latter archipelago, moreover, is more markedly connected with the reliefs of Borneo through the Sulu and Palawan islands. Tormented transverse fractures, which form numerous straits (of Karimata, between Borneo and Sumatra; of Makasar, between Borneo and Celebes; etc.), and inland seas (of Java, between Borneo, Java and Sumatra; of Ceram, between the Moluccas and New Guinea; of Celebes, between the homonymous island, Borneo and the Philippines; etc.) articulate the immense Indonesian space, which deep ocean trenches clearly delimit: to the S the Java trench goes up to -7450 m, to the E the Banda Sea reaches -7440 m, to the NE the Celebes Sea sinks to -6220 m. Towards the NW there are no tectonic gaps, resting the islands on a very vast and shallow continental shelf, which until recently emerged almost without interruption; in fact, before the melting of the Würm glaciers caused the sea level to rise, the Sunda archipelago formed a single block, or at most separated by low stretches of sea, with the Asian continental mass. In Borneo the ancient formations prevail; the relief is represented by sedimentary rocks (sandstone and limestone) of the Paleozoic, resting on the archaeozoic crystalline base. In the other islands, on the other hand, those lava expansions and those large eruptive apparatuses prevail (the latter constituting one of the dominant notes of the Indonesian landscape and generally the highest elevations of the archipelago) due to the orogenetic upheavals from which the islands themselves originated and which have largely obliterated the underlying sediments of the Paleozoic and Mesozoic. However, for example in Sumatra, there are limited outcrops of the crystalline, archaeozoic and paleozoic nucleus. Volcanism is still a marked feature of Indonesia and, with the frequent seismicity, attests to the geological instability of the region. Given their origin, the islands are all essentially mountainous; an almost uninterrupted series of ridges runs through the entire archipelago, leaving very little space for the plains, formed mostly by recent Neozoic alluvial soils, and generally limited to the coastal strips, especially in the north. Particularly impressive is the mountain system of Sumatra, which presents a folded pattern, due to the Himalayan corrugation; it forms an especially compact alignment in correspondence with the Barisan mountains, which border the southwestern coast of the island with various peaks and volcanic cones of considerable height, such as Kerinci (3800 m) and Dempo (3159 m). The reliefs descend with a steep slope to the Indian Ocean; the internal front instead overlooks a vast plain of clayey and tuffaceous alluvial soils, often marshy, which affects the entire northern section of the island, gradually expanding to over 200 km in width.
The tectonic folds in Java have less power, where, on the other hand, the volcanic systems that occupy approx. one third of the surface of the island, characterized by strong eruptive activity. The uneven system of reliefs that entirely crosses Java leaves a fairly small space for the coastal plain, closed to the extreme north by limestone hills which, reaching the coast, make it densely articulated in well-protected bays, a refuge for the main ports of the country. The reliefs, more discontinuous than in Sumatra, they are interspersed with hollows and plateaus, extremely fertile due to the abundance of rains and the richness of the volcanic soil, and in which, despite the ever looming danger of eruptions, human densification reaches values among the highest in the Land. Of the hundred and more volcanoes of Java, many of which are still active, from W to E, the Salak (2211 m), the Ciremay (3078 m), the Slamet (3428 m), the Lawu (3265 m) and the Semeru (3676 m), the highest elevation of the island. In Kalimantan, volcanism had a period of intense activity in the Cenozoic, but already at the beginning of the Neozoic all effusive manifestations had ceased, and there are not even – except in narrow north-eastern strips – lava soils. Vast newly formed alluvial plains interpose themselves between the main mountain ridges, which radiate radially from the central core, around the Kapuas mountain group: the Kelingkang mountains towards the west, the Schwaner mountains (2278 m) and Müller towards the SW, the mountains of Iran (2988 m) and Penamba (2423 m) towards NE; finally towards the S, but isolated from the main complex, the ridge of the Meratus mountains (1892 m) unfolds and towards the E the chain dominated by Mount Kongkumul (2053 m). The radial structure of Celebes (Sulawesi) is very particular, as the island is the orographic node of three archipelagos: the Philippines, the Sunda Islands and the Moluccas. Traces of the ancient crystalline substrate, mostly covered by sedimentary rocks, emerge in the two main peninsulas – Minahasa to the NE and Makasar to the S – while more recent soft sedimentary rocks entirely form the two central-eastern peninsulas. The forms of the landscape are very harsh: the relief, often dominated by volcanic systems, reaches quite high peaks throughout the island, culminating in the 3,455 m of Mount Bulu Rantekombola. AE of Java the The Indonesian archipelago breaks up into the Lesser Sunda Islands (Nusa Tenggara), which repeat the geomorphological characteristics of Java and Sumatra. Likewise a series of volcanic systems dominated by the mountain Agung (3142m) forms the backbone of the island of Bali and two volcanoes, Rinjani (3726m) and Tambora (2850m), are respectively the highest elevations of the Lombok and Sumbawa islands. It closes the arc of the Small Islands of Sunda Timor, the largest of the group and also mountainous; the general orientation of the relief is already a prelude to the powerful folding that the whole orographic system undergoes around the Banda Sea. Between this and the Sea of the Moluccas the archipelago of the same name emerges (main islands are Halmahera, Ceram, Buru, Ambon, Obi, etc.), partly volcanic in nature, partly crystalline, which with the minor group of Aru islands goes east to New Guinea. Here we are already, in a strict geographical sense, in Oceania; however also the imposing mountain range that crosses the Irian Jaya, the Maoke mountains (4884 m in the Puncak Jaya), is due to the Cenozoic orogeny and is linked, through the central Moluccas, to the Celebes system.