Miocene

The Miocene is the earliest epoch of the Neogene Period, spanning roughly from 23 to 5.3 million years ago. Introduced by the Scottish geologist Charles Lyell, its name derives from Greek elements meaning less recent, referring to the smaller proportion of modern marine invertebrate species compared with the succeeding Pliocene. The epoch followed the Oligocene and preceded the Pliocene, forming a critical interval during which Earth experienced major climatic, geological, and biological transitions.
The Miocene marks a phase of steady global cooling, as the planet shifted from the warmer conditions of the Oligocene toward the climatic fluctuations and glaciations that characterised the Pliocene and later Pleistocene. Although its boundaries are not defined by a single global event, they correspond to gradual continental, biological, and climatic changes observed across different regions.

Major Geological and Climatic Developments

During the Early Miocene, the Afro-Arabian landmass collided with Eurasia. This convergence severed the connection between the Mediterranean and Indian Oceans and enabled extensive faunal interchange between Africa and Eurasia. Proboscideans, apes, and other mammalian groups dispersed widely across the Old World during this time. The apes in particular underwent significant diversification during the Aquitanian and Burdigalian stages.
The Late Miocene witnessed one of the epoch’s most dramatic geological episodes: the closure of the Atlantic–Mediterranean connection. This led to the near-complete desiccation of the Mediterranean Sea in what is known as the Messinian salinity crisis. At the Miocene–Pliocene boundary, the Strait of Gibraltar reopened, producing the Zanclean flood and rapidly refilling the Mediterranean basin.
Global climate patterns were shaped significantly by intensified mountain building. The continuing uplift of the Himalaya and the Tibetan Plateau strengthened monsoonal systems and contributed to aridification in central Asia. In the Tian Shan, major uplift blocked westerly winds from entering the Tarim Basin, resulting in drier conditions. By the mid- and late Miocene, these large-scale tectonic changes were tightly associated with the accelerating global cooling trend.
In East Africa, uplift altered rainfall patterns and contributed to the retreat of tropical rainforests, a development with long-term implications for hominoid and hominin evolution. Forests elsewhere also diminished while extensive grasslands spread, reflecting a global shift toward more open habitats.
Although cooling dominated the epoch, evidence indicates the presence of warmer intervals, demonstrating that the decline toward Pleistocene glaciation was not linear.

Paleogeography and Global Continental Patterns

Continental positions during the Miocene broadly resembled those of today, though certain connections and landforms had not yet fully developed. The land bridge between North and South America was absent, yet South America was moving closer to the Pacific subduction zones, contributing to continued uplift of the Andes and the expansion of the Mesoamerican region.
Marine and continental Miocene deposits occur widely across the world. Coastal exposures are common in areas that coincide with present shorelines, while large continental deposits are found in the North American Great Plains and in regions of Argentina. Overall, aridity increased in many regions after about 7–8 million years ago as atmospheric moisture-holding capacity declined with global cooling.
In Eurasia, the collision between the Indian and Eurasian plates further reduced the remnants of the Tethys Ocean. By 20 million years ago, the closure of these marine corridors had already reduced water exchange by approximately 90 per cent. A subsequent closure at roughly 13.8 million years ago coincided with expansion of Antarctic glaciation and produced the final severing of the Indian Ocean–Mediterranean connection.
The Paratethys region experienced a complex history of transgressions, evaporite formation, and isolation. In the early Middle Miocene, the Brăila Strait closed, isolating the Eastern Paratethys and converting it into a saltwater lake. Later oscillations in sea levels and tectonic shifts led to alternating phases of open marine and brackish conditions and contributed to extinction events such as the Badenian–Sarmatian transition.
Around the same interval, the Fram Strait opened between Greenland and Svalbard, becoming a vital route for Atlantic water to enter the Arctic Ocean. Due to regional uplift, the Barents Seaway was closed to throughflow during the Miocene.
The present-day Mekong Delta began forming after approximately 8 million years ago. Sediment records from the Qiongdongnan Basin indicate that the Pearl River was already a major fluvial system during the Early Miocene, supplying large volumes of sediment to the northern South China Sea.
South American coastal regions also experienced Miocene marine transgressions, particularly in northern Brazil, Colombia, Chile, and Patagonia. In Patagonia, the Oligocene–Miocene transgression may have temporarily connected the Pacific and Atlantic Oceans through narrow inland seaways, suggested by fossil assemblages of mixed oceanic origins. Subsequent tectonic activity, including the northward migration of the Chile Triple Junction and the creation of slab windows, contributed to regional uplift and ultimately reversed the transgression. Mountain building during the Middle Miocene contributed to the formation of the Patagonian Desert through rain shadow effects.
In Australia, the far north experienced monsoonal conditions, although interpretations of wetter Miocene climates have been debated. Western Australia remained arid, consistent with modern patterns, particularly during the Middle Miocene.

Biological Evolution and Ecosystems

The Miocene is notable for the continued spread of grasslands and the reduction of dense forests, a trend already underway in the Oligocene. These environmental changes fostered the evolution and proliferation of grazing mammals. Many mammal and bird groups reached recognisably modern forms, while marine ecosystems saw the emergence of kelp forests, which developed into highly productive ecological systems along temperate coastlines.
Whales and pinnipeds flourished in Miocene oceans. The diversification of apes, especially in the Early and Middle Miocene, shaped primate evolution on a global scale. Near the end of the epoch, during the Messinian Stage, the evolutionary lineage leading to humans diverged from that of the Pan genus, marking an important milestone in hominin history.

Subdivisions of the Miocene

The Miocene is divided into internationally recognised stages as defined by the International Commission on Stratigraphy. These include, from oldest to youngest, the Aquitanian, Burdigalian, Langhian, Serravallian, Tortonian, and Messinian. Regional subdivisions based on mammalian fossils are also used in various parts of the world and overlap with both the earlier Oligocene and the later Pliocene.

Significance in Earth Sciences

The Miocene remains a focal point for palaeoclimatology, palaeobiology, and geology. It captures a dynamic interval marked by tectonic reorganisation, evolving climate systems, and major biogeographical transformations. Continuing uplift of the Himalaya and associated changes in atmospheric circulation patterns shaped much of the northern hemisphere climate, while the development of modern ecosystems and faunas provided a foundation for subsequent evolutionary pathways.
The epoch represents a bridge between ancient environments and the emergence of conditions more characteristic of the present day, offering insights into both deep-time Earth processes and the origins of modern biodiversity.