Papua New Guinea's geography: land raised by colliding plates

How two plates built the Central Range

Britannica describes the country's relief as the product of a geologically recent history in which the northward-moving Australian Plate collided with the westward-moving Pacific Plate. The low southern plains are part of the Australian Plate and were joined to Australia by dry land until the Torres Strait flooded, some 8,000 years ago in Britannica's account. Today the strait is 152 kilometres wide and shallow, with the Arafura Sea to the west and the deeper Coral Sea to the east.

North of the plains the ground rises through a belt of limestone, widest around the Kikori River and Lake Kutubu, which Britannica calls an extraordinarily harsh country of karst, sinkholes, rock towers and jagged ridges under lowland rainforest. Beyond it stands the Highlands, a mountain zone running the length of the island from the west to Milne Bay in the far south-east. In Papua New Guinea the peaks pass 4,000 metres, and the highest of them is Mount Wilhelm in the Bismarck Range, part of the Central Range. Wikipedia's geography of the country gives two estimates of how much of it is mountain: at least 72% in one, 52% of mountains and hills in another.

The mainland has a simple outline for so complicated a surface. Its western edge, the border with Indonesia, is almost a straight line along the meridian of 141° east, and runs 820 kilometres from coast to coast. From there the land tapers south-east, past the deep bite of the Gulf of Papua, into a long finger that ends at the D'Entrecasteaux Islands and the Louisiade Archipelago. The country counts about 600 islands, and its land area is 452,860 square kilometres in the figure the Australian foreign ministry uses.

Where the Highlands basins hold the people

Between the ridges lie enclosed upland basins whose floors are usually at 1,370 metres or higher. Britannica explains why they are crowded: they hold old lake deposits, soil washed from the mountains around them and layers of volcanic ash from nearby volcanoes, which together make them very fertile. Wikipedia counts the Highlands among the most intensively used land in the country, together with the area around Blanche Bay on New Britain.

The settlement pattern follows the soil. Britannica describes the southern plains as only sparsely peopled by fairly mobile sago gatherers, the Highlands valleys as densely settled in villages or scattered hamlets, and the north coast and north-eastern islands as well populated despite eruptions and tremors, with Karkar Island and the Gazelle Peninsula of New Britain particularly dense. Only about 2% of the country is regularly cultivated, in Wikipedia's figure, and arable land was put at 0.49% in a 2005 estimate.

The soils are young everywhere because the ground is so active: Wikipedia's geography article puts Inceptisols on over half of the country and Entisols on a quarter. In 1975, 69.5% of the land was counted as entirely unused, mostly forest, and 25.6% as cultivated to some degree, half of it very lightly or as long fallow.

Highlands gardens are a landscape in their own right. Britannica describes some laid out in checkerboards of drainage ditches and others as circular mounds built over compost to warm and enrich the soil, the same wetland skills that UNESCO dates back thousands of years at the Kuk swamp near Mount Hagen, 1,500 metres up.

Why the rivers carry so much mud

Steep slopes, very heavy rain, unstable rock and fast clearing of land combine, in Britannica's words, to produce soil erosion rates among the highest anywhere, comparable with the Himalaya. The rivers are short for the volume they carry. Once out of the mountains, often through deep gorges, they slow across plains they have built from their own sediment and wander through swamp.

Wikipedia divides the mainland into nine drainage basins, of which the Sepik and Ramu system in the north and the Fly in the south are the two great lowland ones and the Purari and Markham the other major ones. The Sepik runs 1,126 kilometres and the Fly 1,050, in Wikipedia's figures, and both are navigable for long stretches. The Fly's gradient is the clearest sign of how flat the southern plains are: 820 kilometres from its mouth the river is 18 metres above sea level, an average fall of about 2.4 centimetres per kilometre. For about 80 kilometres near Kiunga the Indonesian border leaves the meridian to follow it.

The sediment that makes the plains also limits their use. Britannica notes that the deposition rates cause serious problems for any plan to run boats or hydroelectric schemes on these rivers. Lake Murray, in Wikipedia's list, covers 64,700 hectares.

How the volcanic arc shapes the north

The north coast of the mainland is the opposite of the south. Instead of swamps it drops steeply to the sea, and much of it is raised former seabed in a zone that remains tectonically active, with earthquakes and landslides common. Offshore runs what Britannica calls a complex, unstable volcanic arc in the Bismarck Sea, from the Schouten Islands past the Huon Peninsula and through New Britain, where it splits: one arm turns north-west through New Ireland and the Admiralty Islands, the other south-east through Buka and Bougainville towards Solomon Islands. Wikipedia counts 14 active and 22 dormant volcanoes in the country.

What eruptions and earthquakes have cost

The Smithsonian Institution's Global Volcanism Program records that an eruption of the Rabaul caldera in September 1994, its first in over 50 years, forced the evacuation of over 50,000 people and badly damaged Rabaul Town. On 29 August 2014 the Tavurvur cone erupted again and sent an ash plume to 18 kilometres. The same volcanic soils are why the area is so densely farmed.

Earthquakes and the waves they raise are the deadlier hazard. A magnitude 7.0 earthquake north of Aitape on 17 July 1998 raised a tsunami that Wikipedia's geography article says killed over 2,180 people, while Britannica gives some 2,000. A magnitude 7.5 earthquake struck the Highlands on 25 February 2018, and a landslide at Kaokalam in Enga Province on 24 May 2024 buried more than 2,000 people according to the national government's figure as Wikipedia reports it.

What lies offshore of the mainland

The islands make up a large part of the country and differ from the mainland and from each other. Wikipedia describes the islands north-east of the mainland as two arcs. The inner one holds small islands near the coast and New Britain, which is mostly not volcanic except along its north and on the Gazelle Peninsula around Rabaul. The outer one links Manus, New Hanover, New Ireland and Bougainville, which carries three large volcanoes. Together these islands cover around 68,000 square kilometres. Off the south-east tip lie the D'Entrecasteaux and Trobriand Islands, Woodlark and the Louisiade Archipelago, just over 7,000 square kilometres between them.

The Papuan Peninsula, the long tail of the mainland, has its own volcanoes, Mount Lamington on the northern side and Mount Victory and Mount Trafalgar near Cape Nelson, with the Owen Stanley Range running down its centre. The Admiralty Islands, with Manus among them, carry plants and animals distinct from the main island, and Wikipedia notes that Bougainville is related in its wildlife to the rest of the Solomon Islands chain more closely than to the rest of Papua New Guinea. Britannica adds that very small Micronesian and Polynesian communities live on some outlying islands and atolls, with chiefs at the head of their political structures, a form rarely found among the Melanesian majority.

What the rain does across the mountains

All of Papua New Guinea is tropical, and Britannica stresses that rainfall, not temperature, makes the seasons. In the lowlands the mean annual maximum is 30 to 32 °C and the minimum 23 to 24 °C, with little change through the year. In the Highlands night frosts are common above 2,100 metres, and Wikipedia records snow on the highest summits, Mount Wilhelm and Mount Giluwe among them.

Two wind systems set the pattern. The south-east trades blow for about seven months, May to November, over Milne Bay in the far south-east, and for shorter periods further north; the north-west monsoon dominates the north coast and the Bismarck Archipelago and reaches Port Moresby only from December to March. Because the trades run parallel to the coast there, the capital is dry: under 1,300 millimetres a year in Britannica's figure, under 1,000 in Wikipedia's. Where mountains face the trades, as on New Britain and the southern slopes of the Highlands, the total often passes 7,600 millimetres. The Highlands themselves get between 2,500 and 4,000 millimetres, with a dry spell in the middle of the year.

Wikipedia reports that average temperatures have risen by 0.2 °C per decade since the 1950s, and that rising seas are expected to force people off low islands of Bougainville such as the Carteret Islands.

What lives in forests cut off from Asia

Britannica describes a coast lined with mangroves, giving way inland to nipa palm in brackish water and stands of sago along the larger rivers, then primary lowland rainforest up to about 1,000 metres, oak, beech, cedar and pine above that, and moss forest of conifers, tree ferns, orchids and fungi above about 2,000 metres. Mangroves cover around 4,800 square kilometres, in Wikipedia's figure, and of all forest in 2018 over three-quarters was old growth and 11.9% heavily logged.

New Guinea and Australia were joined for much of the past and isolated together from other land for some 55 to 56 million years, which is why the island shares marsupials, tree kangaroos and echidnas with Australia and has almost no large mammals. The cassowary and the crocodile are its largest animals. Britannica puts the birds-of-paradise at some 40 species; the national tourism authority gives 38. Their export is banned, and hunters may use only traditional weapons. Wikipedia's country article says the land supports around 5% of all known species, with new ones still being described.

How the borders were fixed at sea

The land border is old. The meridian of 141° east was the eastern limit of the Dutch claim of 1828, and a Dutch and British exchange of 1895 gave British New Guinea the whole left bank of the Fly and moved the southern end east. A treaty concluded by Australia confirmed the line in 1974, before independence. At sea, a treaty of 1978 fixed the boundary with Australia so that it runs close to the New Guinea coast in the Torres Strait, leaving the islands of Dauan, Boigu and Saibai Australian, and a treaty of 1989 settled the line with Solomon Islands.

The sea is a larger part of the country than the land. The coastline is longer than 10,000 kilometres, the mainland's share of it 5,152 kilometres, and the National Seas Act of 1977 set out the territorial sea and the exclusive economic zone. The islands run from Mussau in the north to Sudest in the south, and the country spans two time zones, with Bougainville an hour ahead of the rest. How that terrain is crossed is in the travel article, and what it yields in the economy article.