The Oku Volcanic Field or Oku Massif is group of volcanoes based on a swell in the Cameroon Volcanic Line, located in the Oku

Oku

Oku is Sub Division in the Northwest Cameroon. The term Oku also refers to the people who live in this region and the primary language that they speak . Oku is a rural area containing about thirty-six villages...

 region of the Western High Plateau

Western High Plateau

The Western High Plateau, Western Highlands, or Bamenda Grassfields is a region of Cameroon characterised by high relief, cool temperatures, heavy rainfall, and savanna vegetation. The region lies along the Cameroon line and consists of mountain ranges and volcanoes made of crystalline and igneous...

 of Cameroon

Cameroon

Cameroon, officially the Republic of Cameroon , is a country in west Central Africa. It is bordered by Nigeria to the west; Chad to the northeast; the Central African Republic to the east; and Equatorial Guinea, Gabon, and the Republic of the Congo to the south. Cameroon's coastline lies on the...

.
The Mount Oku

Mount Oku

Mount Oku, or Kilum Mountain, is the largest volcano in the Oku Massif, in the Cameroon Volcanic Line, located in the Oku region of the Western High Plateau of Cameroon.It is the second highest mountain in mainland West Africa....

 stratovolcano

Stratovolcano

A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions...

 rises to 3,011 m above sea level.

The massif has a diameter of almost 100 km and contains four major stratovolcanoes: Mount Oku, Mount Babanki 15 km SW of Oku, Nyos and Nkambe.
Rocks in the massif have ages from 24.9 to 22.1 million years ago, but more recent activity has occurred.
The massif is composed of rhyolitic and trachytic rock, and contains many maar

Maar

A maar is a broad, low-relief volcanic crater that is caused by a phreatomagmatic eruption, an explosion caused by groundwater coming into contact with hot lava or magma. A maar characteristically fills with water to form a relatively shallow crater lake. The name comes from the local Moselle...

s and basaltic cinder cones.

The Oku Volcanic Field includes two crater lakes, Lake Nyos

Lake Nyos

Lake Nyos is a crater lake in the Northwest Region of Cameroon, located about northwest of Yaoundé. Nyos is a deep lake high on the flank of an inactive volcano in the Oku volcanic plain along the Cameroon line of volcanic activity...

 to the north and Lake Monoun

Lake Monoun

Lake Monoun is a lake in West Province, Cameroon, that lies in the Oku Volcanic Field . On August 15, 1984, the lake exploded in a limnic eruption, which resulted in the release of a large amount of carbon dioxide that killed 37 people. At first, the cause of the deaths was a mystery, and...

 to the south.
On 15 August 1984, an earthquake and landslide triggered a major release of carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 from Lake Monoun, killing several people.
Lake Nyos is inside a maar formed by an explosion about 400 years ago, and is about 1,800 m wide and 208 m deep. There is a low-temperature reservoir of free carbon dioxide below the lake bottom, which seeps into the lake through a volcanic pipe.
On 21 August 1986, Lake Nyos released a massive amount of carbon dioxide which killed at least 1,700 people by suffocation, possibly triggered by a force 5 earthquake along the Foumban Shear Zone

Foumban Shear Zone

The Foumban Shear Zone, or Central Cameroon Shear Zone , is a fault zone in Cameroon that has been identified with the Pernambuco fault in northeastern Brazil, which splays from the Trans-Brazilian Lineament....

, which runs under the massif.