Oceania is a region made up of thousands of islands throughout the Central and South Pacific. It includes Australia, the smallest continent in terms of total land area. Most of Oceania, including Australia, is under the Pacific, a vast body of water that is larger than all Earth’s continental landmasses and islands combined. The name “Oceania” justly establishes the Pacific Ocean as the defining characteristic of the region.

Oceania is dominated by the nation of Australia. The other two major landmasses of Oceania are the microcontinent of Zealandia, which includes the country of New Zealand, and the eastern half of the island of New Guinea, made up of the nation of Papua New Guinea. Oceania also includes three island regions: Melanesia, Micronesia, and Polynesia (including the U.S. state of Hawai‘i).

Oceania’s physical geography, environment and resources, and human geography can be considered separately.

Oceania can be divided into three island groups: continental islands, high islands, and low islands. The islands in each group are formed in different ways and are made up of different materials. Continental islands have a variety of physical features, while high and low islands are fairly uniform in their physical geography.

Continental Islands
Continental islands were once attached to continents before sea-level changes and tectonic activity isolated them. Tectonic activity refers to the movement and collision of different sections, or plates, of Earth’s crust.

Australia, Zealandia, and New Guinea are continental islands. These three regions share some physical features. All three have mountain ranges or highlands—the Great Dividing Range in Australia; the North Island Volcanic Plateau and Southern Alps in New Zealand; and the New Guinea Highlands in Papua New Guinea. These highlands are fold mountains, created as tectonic plates pressed together and pushed land upward. New Zealand and Papua New Guinea also have volcanic features as a result of tectonic activity.

Although they share some landscape features, each of these regions has distinct physical features that resulted from different environmental processes. Australia’s landscape is dominated by the Outback, a region of deserts and semi-arid land. The Outback is a result of the continent’s large inland plains, its location along the dry Tropic of Capricorn, and its proximity to cool, dry, southerly winds. New Zealand’s glaciers are a result of the islands’ high elevations and proximity to cool, moisture-bearing winds. Papua New Guinea’s highland rainforests are a result of the island’s high elevations, proximity to tropical, moisture-bearing winds, and location right below the warm Equator.

High Islands
High islands, also called volcanic islands, are created as volcanic eruptions build up land over time. These eruptions begin underwater, when hot magma is cooled and hardened by the ocean. Over time, this activity creates islands with a steep central peak—hence the name “high island.” Ridges and valleys radiate outward from the peak toward the coastline.

The island region of Melanesia contains many high islands because it is a major part of the “Ring of Fire,” a string of volcanoes around the boundary of the Pacific Ocean. This part of the Ring of Fire is on the boundary of the Pacific plate and the Australian plate. This is a convergent plate boundary, where the two plates move toward each other. Important volcanic mountains in Melanesia include Mount Tomanivi, Fiji; Mount Lamington, Papua New Guinea; and Mount Yasur, Vanuatu.

Low Islands
Low islands are also called coral islands. They are made of the skeletons and living bodies of small marine animals called corals. Sometimes, coral islands barely reach above sea level—hence the name “low island.” Low islands often take the shape of an irregular ring of very small islands, called an atoll, surrounding a lagoon. An atoll forms when a coral reef builds up around a volcanic island, then the volcanic island erodes away, leaving a lagoon. Atolls are defined as one island even though they are made up of multiple communities of coral.

The island regions of Micronesia and Polynesia are dominated by low islands. The Kwajalein Atoll in the Marshall Islands, for example, is composed of 97 islands and islets that surround one of the largest lagoons in the world, with an area of 2,173 square kilometers (839 square miles). The nation of Kiribati is composed of 32 atolls and one solitary island dispersed over 3.5 million square kilometers (1.35 million square miles) of the Pacific Ocean.

Island Flora and Fauna
The evolution of flora and fauna across Oceania is unique. Many plants and animals reached the islands from southern Asia during the last glacial period, when sea levels were low enough to allow for travel. After sea levels rose, species adapted to the environment of each island or community of islands, producing multiple species that evolved from a common ancestor. Due to its isolation from the rest of the world, Australia and Oceania has an incredibly high number of endemic species, or species that are found nowhere else on Earth.

Plants traveled between islands by riding wind or ocean currents. Birds carried the seeds of fruits and plants and spread them between islands with their droppings. Ferns, mosses, and some flowering plants rely on spores or seeds that can remain airborne for long distances. Coconut palms (Cocos nucifera) and mangroves, common throughout Australia and Oceania, produce seeds that can float on salty water for weeks at a time. Important flowering plants native to Australia and Oceania include the jacaranda, hibiscus, pohutukawa (Metrosideros excelsa), and kōwhai. Other indigenous trees include the breadfruit (Artocarpus altilis), eucalyptus, and banyan (Ficus benghalensis).

Birds are very common in Australia and Oceania because they are one of the few animals mobile enough to move from island to island. There are more than 110 endemic bird species in Australia and Oceania, including many seabirds. Many flightless birds, such as emus (Dromaius novaehollandiae), kiwis, cassowaries, wekas (Gallirallus australis), and takahēs (Porphyrio hochstetteri), are native to Australia, Papua New Guinea, and New Zealand. The Pacific Islands have more than 25 species of birds of paradise, which exhibit colorful plumage.

Lizards and bats make up the majority of Australia and Oceania’s native land animals. Lizard species include the goanna, skink, and bearded dragon. Australia and Oceania has more than a hundred different species of fruit bats.

The few native land animals in Australia and Oceania are unusual. Australia and Oceania is the only place in the world that is home to monotremes—mammals that lay eggs. All monotremes are native to Australia and Papua New Guinea. There are only five living species: the platypus (Ornithorhynchus anatinus) and four species of echidna.

Many of the most familiar animals native to Australia and Oceania are marsupials, including the koala (Phascolarctos cinereus), kangaroo, and wallaby (Macropus rufogriseus). Marsupials are mammals that carry their newborn young in a pouch. Almost 70 percent of the marsupials on Earth are native to Oceania. (The rest are native to the Americas.)

In Australia and Oceania, marsupials did not face threats or competition from large predators such as lions (Panthera leo), tigers (Panthera tigris), or bears. The red kangaroo (Macropus rufus), the world’s largest marsupial, can grow up to twp meters (six feet) tall, and weigh as much as 100 kilograms (220 pounds). In the Americas, marsupials such as possums are much smaller.

Marine Flora and Fauna
The marine environment is an important and influential physical region in Australia and Oceania. The region is composed of three marine realms: Temperate Australasia, Central Indo-Pacific, and Eastern Indo-Pacific. Marine realms are large ocean regions where animal and plant life are similar because of shared environmental and evolutionary factors.

The Temperate Australasia realm includes the seas surrounding the southern half of Australia and the islands of New Zealand. This realm is one of the world’s richest areas for seabirds. Its cold, nutrient-rich waters support a diversity of plants and fish that seabirds feed on. These seabirds include different species of albatross, petrel, and shearwater, as well as the Australasian gannet (Morus serrator) and rockhopper penguin (Eudyptes chrysocome).

The Central Indo-Pacific realm includes the seas surrounding the northern half of Australia, Papua New Guinea, Solomon Islands, Vanuatu, New Caledonia, Fiji, and Tonga. This marine realm has the greatest diversity of tropical coral in the world and includes the world’s two largest coral formations: Australia’s Great Barrier Reef and the New Caledonia Barrier Reef. The Great Barrier Reef, a UNESCO World Heritage Site off the coast of northeast Australia, is 344,400 square kilometers (133,000 square miles).

The Great Barrier Reef and the New Caledonia Barrier Reef are underwater hotspots for biodiversity. The Great Barrier Reef is home to 30 species of whales, dolphins, and porpoises; six species of sea turtles; 215 species of birds; and more than 1,500 species of fish. The New Caledonia Barrier Reef is home to 600 species of sponges, 5,500 species of mollusks, 5,000 species of crustaceans, and at least 1,000 species of fish.

The Eastern Indo-Pacific realm surrounds the tropical islands of the central Pacific Ocean, extending from the Marshall Islands through central and southeastern Polynesia. Like the Central Indo-Pacific realm, this realm is also known for its tropical coral formations. A variety of whale, tortoise, and fish species also inhabit this realm.