Abstract
Large igneous provinces are conspicuous features of late Phanerozoic geology, and include continental flood basalts, rifted continental margin volcanic sequences and oceanic plateaus. Many large igneous provinces can be associated with mantle plumes. Where plumes ascend beneath spreading ridges, their energy is transformed into a large melt volume, producing over-thickened plateau crust. Comparing the timing of mass extinctions with the formation age of large igneous provinces reveals a close correspondence in several cases. The Columbia River Basalt Group is a large igneous province, which covers parts of the western United States. There are significant landforms associated with this formation that provide evidence as to the age and controlling mechanisms of its emplacement.

1. Introduction
Large igneous provinces (LIPs) are massive emplacements of extrusive and intrusive mafic rock. Processes other than seafloor spreading form these mass emplacements and they are the primary mechanism of terrestrial magmatism on earth. LIPs are distinguishable from mid-ocean ridge magmatism and arc magmatism on the basis of petrologic, geochemical, geochronological and physical volcanological data. Large igneous provinces occur in both continental and oceanic crust.(Coffin and Eldholm, 1991). The purpose of this review is to give an overview of the types of LIPs, their formation, and the possibility of their contribution to mass extinction events.

2. Types of Large Igneous Provinces
LIPs are characterized by the attributes of their primary mafic rocks. Research has primarily focused on continental flood basalts such as the Deccan Traps or the Columbia River Flood Basalt (Map 1). Most continental flood basalts are part of extensional sedimentary basins. It is not currently ascertainable as to whether magmatism or extension occurred first (Coffin and Eldholm, 1991). There are four types of oceanic LIPs. Oceanic plateaus are broad flat-topped features, which are generally 2000m or more above the surrounding seafloor. An example of an oceanic plateau is the Shatsky Rise located in the Pacific Ocean Southeast of Japan (Korenaga and Sager, 2012). Submarine ridges are steep sided, elongated landforms such as the Ninetyeast Ridge in the Indian Ocean (Smith, 1978). Seamounts are a more localized type of elevated seafloor, the most well known example being the Emperor Seamount Chain which includes the Hawaiian Islands. Ocean basin flood basalts, while being the least researched type of LIP, are known to be composed of sills and flows, which post-date the formation of normal oceanic crust.

3. Formation of Large Igneous Provinces
LIPs often have an areal extent of a few million km² and volumes on the order of 1 million km³. In most cases, the majority of a basaltic LIP's volume is emplaced in less than 1 million years. One of the problems pertaining to LIPs is understanding how enormous volumes of basaltic magma are formed and erupted in such short period of time. Although most volcanic activity on Earth is associated with subduction zones or mid-oceanic ridges, there are significant regions of long-lived, extensive volcanism, known as hot spots, which are only indirectly related to plate tectonics. The Emperor Sea Mount chain, located on the Pacific plate, is one example, recording millions of years of relative motion as the plate moves over the Hawaiian hot spot. Numerous hot spots of varying size and age have been identified across the world. These hot spots move slowly with respect to one another, but move an order of magnitude more quickly with respect to tectonic plates, providing evidence that they are not directly linked to tectonic plates but rather anchored at the core-mantle boundary. This hypothesis, also known as the mantle plume model attempts to provide a natural explanation for the time-progressive chains of older volcanoes seen extending out from some "hot spots".(Humphreys and Schmandt, 2011). A major alternative to the plume model is a model in which ruptures are caused by plate-related stresses that fractured the lithosphere, allowing melt to reach the surface from shallow heterogeneous sources. The high volume of molten material that forms an LIP is postulated to be caused by a process of convection in the upper mantle which is secondary to the large scale convection driving tectonic plate motion (Hagstrum, 2005).

4. LIPs and Mass Extinction Events
One hypothesis, which attempts to explain global mass extinctions, is that the formation of large igneous provinces by flood basalt events could have produced dust and particulate aerosols which inhibited photosynthesis, and thus caused food chains to collapse both on land and at sea. These events also emitted sulfur oxides, which were precipitated as acid rain and poisoned many organisms, contributing further to the collapse of food chains. Flood basalt events could have emitted carbon dioxide, and thus caused sustained global warming once the dust and particulate aerosols dissipated (Alroy, 2008). Episodes of global warming and marine anoxia show a good correlation with flood basalt volcanism (Wignall, 2001).

5. Coloumbia River Basalt Group
The Columbia River Basalt (CRB) Group is a large igneous province that lies across parts of the Western United States. It is found in the U.S. states of Washington, Oregon, Idaho, Nevada, and California. The Basalt group includes the Steens, Grande Ronde, Wanapum and Saddle Mountain basalt formations. During the middle to late Miocene epoch, the Columbia River flood basalts engulfed about 163,700 km2 (63,200 sq mi) of the Pacific Northwest, forming a large igneous province with an estimated volume of 174,300 km3 (41,800 cu mi). Eruptions were most vigorous from 17–14 million years ago, when over 99 percent of the basalt was released. Less extensive eruptions continued from 14–6 million years ago (Carson and Pogue, 1996). The oldest flow formation member of the CRB is the Steens Basalt. Geomagnetic dating(reversed-to-normal polarity transition) along with 40Ar/39Ar have been used to date this member to approximately 16.7 Ma (Jarboe et al., 2008). Landforms associated with the Steens member include the Oregon Plateau, Steens Mountain, Catlow Peak and Poker Jim Ridge. The next oldest flows, dated to 15.6 Ma comprise the Grande Ronde Basalt. Grande Ronde is significant in that it is estimated to make up 85 percent of the total flow volume. Large quantities of upwelled magma were erupted onto surface forming the Chief Joseph Dike Swarm, which is hypothesized to be comprised of up to 20.000 dikes. Much of the lava flowed north into Washington as well as down the Columbia River channel to the Pacific Ocean, these tremendous flows created the Columbia River Plateau. The Wanapum Basalt is made up of the Eckler Mountain Member (15.6 Ma), the Frenchman Springs Member (15.5 Ma), the Roza Member (14.9 Ma) and the Priest Rapids Member (14.5 Ma). They originated from vents between Pendleton, Oregon and Hanford, Washington. The Frenchman Springs Member flowed along similar paths as the Grande Ronde basalts, but can be identified by different chemical characteristics. It flowed west to the Pacific, and can be found in the Columbia Gorge, along the upper Clackamas River, the hills south of Oregon City. and as far west as Yaquina Head near Newport, Oregon—a distance of 750 km (Bishop, 2003). The Saddle Mountains Basalt, seen prominently at the Saddle Mountains, is made up of the Umatilla Member flows, the Wilbur Creek Member flows, the Asotin Member flows (13 Ma), the Weissenfels Ridge Member flows, the Esquatzel Member flows, the Elephant Mountain Member flows (10.5 Ma), the Bujford Member flows, the Ice Harbor Member flows (8.5 Ma) and the Lower Monumental Member flows (6 Ma) (Carson and Pogue, 1996).

Conclusions
Oceanic plateaus, submarine ridges, seamount chains, volcanic passive margins, ocean basin flood basalts, and continental flood basalts have geological characteristics which suggest an origin apart from igneous rocks formed at mid-ocean ridges and arcs. The landforms associated with LIPs provide an opportunity to ascertain their age and emplacement mechanisms. There is evidence to suggest that LIPs have contributed to catastrophic environmental change. As always, there is a need for further research into the origin and control mechanisms of both continental and oceanic LIPs.

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