The igneous material is of two types; one is extrusive, and the other is intrusive. The extrusive igneous bodies are formed from the magma poured out at the surface of the earth. The lava flows are examples of extrusive igneous material. While the igneous bodies are formed by the consolidation of magma at some depth below the earth’s surface. Such rock or igneous bodies show considerable variations in their size and shape. Examples of intrusive bodies are batholith, stock, dyke sill, etc.
1). Extrusive Igneous Bodies: Extrusive igneous rock bodies are formed as a result of the accumulation of magma above the earth’s surface. Examples of these rock bodies are lava flows.
2). Intrusive Igneous Rock Bodies: Intrusive Igneous bodies are formed by the consolidation of magma at some depth below the earth’s surface. Such rock bodies show considerable variations in their sizes and shapes. Best examples of the intrusive igneous bodies are batholith, stock, dyke, sill, diabase, diorite, gabro, pegmatite, etc.
The intrusive bodies are further classified into two groups; one group includes discordant bodies, and the other group includes concordant bodies.
- Discordant Bodies: The discordant igneous bodies are those, which cut through the overlying strata. Examples of these are batholiths, stocks, and dykes.
- Concordant Bodies: The concordant igneous bodies are those, which lie between rock beds. Such bodies do not show transgressive relation to the rocks they invade. Examples of concordant bodies are sills, lopoliths, and laccoliths.
Features of different Igneous Bodies:
1): Batholith: Batholiths are large intrusive igneous bodies, which have transgressive relations with the adjacent country rocks. Their diameter is usually 100 km or more and their outcrop at the surface is roughly circular or oval. In cross-section, batholiths possess steep outwardly dipping contacts and they are through to be bottomless. The composition of batholiths is usually granitic or granodioritic.
2): Stock and Boss: Irregular igneous masses of batholithic habit are called “stocks”. They are of smaller size and their diameter is usually between 10-20 kilometers. The term “boss” is applied to those stocks which have an approximately circular outcrop.
3): Lopolith: A “lopolith” is a saucer-shaped concordant igneous body, which is bent downward into a basin-like shape. Its diameter is usually 10-20 times its thickness. Thus “lopoliths” are much larger than the “laccolliths”. The Lopoliths are commonly composed of basic material.
4): Laccolith: A ”laccolith” is a lens-shaped intrusive igneous body, which causes the overlying beds to arch in the form of a dome. It has a flat base and a domed top. A laccolith maybe 2 to 3 kilometers in diameter and several hundred meters in thickness. It differs from a batholith in being much smaller and having a known floor.
5): Phacolith: Phacoliths are igneous rock bodies of crescent shape, which occupy crests and troughs of folded strata. Phacoliths are formed when igneous material invades the folded region The igneous material accumulates at the crests and troughs of folds because these are the zones of minimum stress.
6): Sill: A ”sill” is a sheet-like igneous body, which runs parallel to the bedding planes of the pre-existing strata. They may be horizontal, inclined, or vertical depending upon the attitude of strata in which they’re intruded. Sills vary in thickness from a few centimeters to several hundred meters, but they are always thin as compared to their length along beds. Sills are commonly made up of dolerites and basalts.
7): Dyke: A “dyke” is a wall-like igneous body that cuts across the strata of the pre-existing rocks. Dykes are often vertical or steeply inclined. Their thickness varies from a few centimeters to a hundred meters or even more. Dykes tend to occur in groups where they run parallel to one direction or are radial to a center. A dyke having a circular outcrop and a conical form and circular outcrops are described as “cone-sheet”. Dykes probably represent a crustal fracture into which the magma is injected.
8): Volcanic Plug: A volcanic plug is a vertical cylindrically shaped igneous body, which has a roughly oval or circular cross-section. It represents the vent of an extinct volcano. Volcanic plugs range in diameter from a few hundred meters to a kilometer or more.
9): Lava Flows: The volcanic igneous rocks occur as lavaflow. The lavaflows are tabular in shape and may range in thickness from a few meters to several hundred meters. They are formed when lava erupts on the earth’s surface from fissures. The lavas cover a very large area before solidifying and a considerable thickness of rock is formed from repeated eruptions. The scale of lava flows may be measured by distance traveled from the vent, or by area, volume or thickness. The Keweenaw has flows that are large, based on volume and thickness, but not on distance travelled. This is because the lavas accumulated within the rift zone, in the case of the Greenstone Flow, ponding to as much as 500 m. The Greenstone flow is one of the Earth’s largest lava flows; according to Longo (1984), it has an aggregate volume of 1650 km3 (396 mi3), comparable to the Roza flow of the Columbia River Flood basalts, which is estimated to be 1500 km3 (360 mi3) by Swanson et aI. (1975). The areal extent of the Roza, 40,000 km2 (15,450 mi2), is much larger than the Greenstone flow, 5000 km2 (1930 mi2), a comparison which results from the ponding of the Greenstone within the rift basin.