nephelinite

A normative feldspathoid classifies a rock as silica-undersaturated; an example is nephelinite.

If olivine is present, the rock may be classified as an olivine nephelinite.

The older effusive eruptions resulted in an occurrence of nepheline basalt and nephelinite.

A few igneous rock types with compositions unlike basalt, such as nephelinite, do occur on these islands but are extremely rare.

Olivine nephelinite lavas are thought to have come from much deeper in the mantle of the Earth than other lavas.

Genesis of the less common mafic rocks such as nephelinite is usually ascribed to more than one of the following three causes:

These lava flows are composed of olivine nephelinite, a type of lava not commonly found on Earth.

This lava, known as olivine nephelinite, is also unusual because it contains small, angular to rounded fragments of rock called nodules.

These include nephelinite, basanite and peralkaline phonolite, trachyte, and comendite lavas.

Melilite olivine nephelinite intrusives of Cretaceous age are found in the area around Uvalde, Texas.

Other igneous rocks such as nephelinite are present in small quantities; these occur often on the older volcanoes, most prominently Detroit Seamount.

The lava at Volcano Mountain is called olivine nephelinite, which is a very uncommon type of lava.

Nyiragongo, another African volcano known for its semipermanent lava lake activity, erupts lava made of melilite nephelinite.

An example is the Hamada nephelinite lava flow in southwest Japan which occurred in the late Miocene age.

Some of the lava flows in the field are similar to those that erupted at Volcano Mountain in the Yukon, where olivine nephelinite occurs.

Volcanic plugs in the Atlin and Maitland areas consist of olivine nephelinite and basanite magmas.

On the southern part of the Central Plateau, there are olivine melilite nephelinite, olivine nephelinite, quartz tholeiite lava flows.

Magmas of rock types such as nephelinite, carbonatite, and kimberlite are among those that may be generated following an influx of carbon dioxide into mantle at depths greater than about 70 km.

Basalt, alkali basalt, basanite, tephritic nephelinite, and nephelinite differ partly in the relative proportions of plagioclase and nepheline.

Olivine nephelinite flows also occur in the Wells Gray-Clearwater volcanic field in east-central British Columbia and at Volcano Mountain in central Yukon Territory.

Nyiragongo's lavas are made of melilite nephelinite, an alkali-rich type of volcanic rock whose unusual chemical composition may be a factor in the unusual fluidity of the lavas there.

In the late stages of eruption of individual volcanoes, more alkaline basalt also was erupted, and in the very late stages after a period of erosion, rocks of unusual composition such as nephelinite were produced in very small amounts.

Trinidade is a volcanic cone rising 5,500m above sea level and is dominated by rocks undersaturated in silica, in the form of pyroclastic deposits, domes, dykes and plugs, together with subordinate lava flows that include nephelinite, ankaratrite and phonolite, all of which are dated at 4Myr (ref. 21).