Gabbro is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term gabbro may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro is to basalt as granite is to rhyolite.
Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known to be the main host matrix for diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.70 g) diamond called the Star of South Africa in 1869 spawned a diamond rush and the digging of the open-pit mine called the Big Hole. Previously, the term kimberlite has been applied to olivine lamproites as Kimberlite II, however this has been in error.
Dunite, also known as olivinite, is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite, magnetite, and pyrope. Dunite is the olivine-rich endmember of the peridotite group of mantle-derived rocks.
Peridotite ( PERR-ih-doh-tyte, pə-RID-ə-) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.
Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.
Diopside is a monoclinic pyroxene mineral with composition MgCaSi
2O
6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the monoclinic prismatic class. It has two distinct prismatic cleavages at 87 and 93° typical of the pyroxene series. It has a Mohs hardness of six, a Vickers hardness of 7.7 GPa at a load of 0.98 N, and a specific gravity of 3.25 to 3.55. It is transparent to translucent with indices of refraction of nα=1.663–1.699, nβ=1.671–1.705, and nγ=1.693–1.728. The optic angle is 58° to 63°.
Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenites are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both types of pyroxenes. Closely allied to this group are the hornblendites, consisting essentially of hornblende and other amphiboles.
Carbonatite is a type of intrusive or extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geochemical verification.
The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.
Harzburgite, an ultramafic, igneous rock, is a variety of peridotite consisting mostly of the two minerals olivine and low-calcium (Ca) pyroxene (enstatite); it is named for occurrences in the Harz Mountains of Germany. It commonly contains a few percent chromium-rich spinel as an accessory mineral. Garnet-bearing harzburgite is much less common, found most commonly as xenoliths in kimberlite.
The Trans-Hudson orogeny or Trans-Hudsonian orogeny was the major mountain building event (orogeny) that formed the Precambrian Canadian Shield and the North American Craton, forging the initial North American continent. It gave rise to the Trans-Hudson orogen (THO), or Trans-Hudson Orogen Transect (THOT), which is the largest Paleoproterozoic orogenic belt in the world. It consists of a network of belts that were formed by Proterozoic crustal accretion and the collision of pre-existing Archean continents. The event occurred 2.0–1.8 billion years ago.
The Wyoming Craton is a craton in the west-central United States and western Canada – more specifically, in Montana, Wyoming, southern Alberta, southern Saskatchewan, and parts of northern Utah. Also called the Wyoming Province, it is the initial core of the continental crust of North America.
The Great Falls Tectonic Zone is a major intracontinental shear zone between the Hearne craton and Wyoming craton basement rock of the Archean Eon which form part of the North American continent. The zone is an area about 100 miles wide extending from the southwestern Idaho-Montana border across Montana to the northwestern Montana-Saskatchewan-North Dakota border. It is named for the Great Falls of the Missouri River, a major geologic feature of the area. The central and western portions of the zone are believed to be about 1.1 to 3.3 billion years old. The central part of the zone lacks Archean rock, however, leading at least one group of scientists to speculate that it was formed very late in the Paleoproterozoic Era.
The Adel Mountains Volcanic Field is an ancient volcanic field of heavily eroded 75-million-year-old igneous rocks about 40 miles long and 20 miles wide in west-central Montana about 30 miles southwest of the city of Great Falls. The area was named by geologist John Bartholomew Lyons, who first described the general geology of the region in 1944. The Adel Mountains Volcanic Field is a significant and abundant source of shonkinite, a very uncommon type of intrusive igneous rock found primarily in Montana, Ontario, and Timor. Because of its geologic structure, the Adel Mountains Volcanic Field has drawn the attention of geologists for more than 100 years.
Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava.
The Mackenzie Large Igneous Province (MLIP) is a major Mesoproterozoic large igneous province of the southwestern, western and northwestern Canadian Shield in Canada. It consists of a group of related igneous rocks that were formed during a massive igneous event starting about 1,270 million years ago. The large igneous province extends from the Arctic in Nunavut to near the Great Lakes in Northwestern Ontario where it meets with the smaller Matachewan dike swarm. Included in the Mackenzie Large Igneous Province are the large Muskox layered intrusion, the Coppermine River flood basalt sequence and the massive northwesterly trending Mackenzie dike swarm.
The Shonkin Sag is a prehistoric fluvioglacial landform located along the northern edge of the Highwood Mountains in the state of Montana in the United States. The Sag is a river channel formed by the Missouri River and glacial meltwater pouring from Glacial Lake Great Falls. It is one of the most famous prehistoric meltwater channels in the world.
The Okavango Dyke Swarm is a giant dyke swarm of the Karoo Large Igneous Province in northeast Botswana, southern Africa. It consists of a group of Proterozoic and Jurassic dykes, trending east-southeast across Botswana, spanning a region nearly 2,000 kilometres (1,200 mi) long and 110 kilometres (68 mi) wide. The Jurassic dykes were formed approximately 179 million years ago, composed of mainly tholeiitic mafic rocks. The formation is related to the magmatism at the Karoo triple junction, induced by the plate tectonic break up of the Gondwana supercontinent in the early Jurassic.
In Teria volcanic field is a volcanic field in Algeria. It consists of about 20 craters.
The Navajo volcanic field is a monogenetic volcanic field located in the Four Corners region of the United States, in the central part of the Colorado Plateau. The volcanic field consists of over 80 volcanoes and associated intrusions of unusual potassium-rich compositions, with an age range of 26.2 to 24.7 million years (Ma).
