1774_Forms of Ig.Rocks (1).pptx forms of igneous rock

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1. FORMS OF IGNEOUS ROCKS Rajnikant P atidar Department of Geology M.L. Sukhadia University, Udaipur

2. The Rock Cycle Igneous Rock Solidification Magma Partial Melting Mantle Rock See Kehew, Fig. 1-6

3. Extrusive igneous rocks, or volcanics , form when magma makes its way to Earth's surface. The molten rock erupts or flows above the surface as lava, and then cools forming the rocks. These volcanic rocks are occur in two types of “Extrusive Forms (Shapes)”. Lava Flows Pyroclastic Rocks Extrusive Forms

4. When magma comes out on surface it called “Lava”. The out pouring and spread of lava gives rise the sheet like form (shape) called “Lava Flow”. Lava Flows

5. During the volcanic eruption the lava comes out along with fragments of counry rocks. This mixture of lava and rock fragment is called pyroclastic rocks. Pyroclastic Rocks

6. The forms of igneous rocks are assumed by the intrusive bodies depend upon the major geological structures like fold, fault, bedding planes etc. Accordingly there are two major categories of forms of intrusive igneous rocks: Discordant Forms: In this case the intrusive mass happens to cut the structures of pre-existing rocks. Concordant Forms: In this case intrusive bodies run parallel to the structures of country rocks. Intrusive Forms

7. Intrusion in Unfolded Region Intrusion in Folded Region Concordant Forms Sill Laccoliths Lopoliths Phacolith Concordant Batholiths Discordant Forms Dykes Cone Sheet Volcanic Neck Ring Dykes Discordant Batholiths Chonolith

8. Sills are relatively thin tabular sheet of magma that penetrates parallel to the bedding planes. The Sills are generally horizontal and the thickness vary from few inches to many meters. Similarly the length may extends for few meters to 100s of km. Since basic magma are more fluid then acidic magma- mostly sills are made up of gabbros, dolerites and basalts Sills

9. It is a concordant body, with flat bottom and convex upward. It is dome or mushroom shaped. When viscous magma is injected rapidly along the bedding, as it cannot spreads it pushes up the overlying layers and keep on piling up. It causes folding of the overlying rock layers. Laccoliths

10. These are basin or saucer-shaped concordant bodies with top nearly flat and convex bottom They may be very huge body with diameter upto 150 miles (app. 240 km) Lopoliths

11. Laccolith Lopolith

12. Dykes and sills are the most common forms of the intrusive igneous bodies. They are discordant as such cut across the bedding or structures of the rocks in which they intrude. Dykes are formed by the intrusion of magma in the country rocks. Dykes may be vertical or inclined. Thickness vary widely from an inch upto hundred of feet. Dykes may be simple dyke, multiple dyke or dyke-swarm (number of dykes). Dykes

13. Mafic dyke Vertical Dyke Inclined Dyke Simple Dyke Multipal Dyke Dyke - swarm

14. Cone – sheets are intrusion of dykes which are generally inclined towards common centre. Their outcrops are arcuate in outline. Ring-dykes are having circular ring-shaped outcrop. These are intrusions of dykes which are generally outwardly dipping. Cone-sheets & Ring Dykes Cone-sheets Ring-dykes

15. A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when magma solidify within a vent on an active volcano. Volcanic neck Volcanic Neck, Devils Tower National Monument. USA

16. These are concordant bodies of igneous rocks that occurs along the crests and troughs of the folded sedimentary strata. Phacolith

17. Batholiths are the largest kind of plutons , irregular in shape and occupies large area. These may show both concordant and discordant relationship. Their occurrence is commonly associated with the mountain-building process These are generally of granites or granodiorites in composition. The Batholith of British Columbia is probably the largest plutonic body ( 2000 kms long and 140 kms wide). Batholiths

18. Batholith Stock Boss

19. Stocks : These are smaller irregular bodies with 10 km in maximum dimension, and are associated with batholiths. Boss : When such stock has roughly circular outline on surface, called as Boss.

20. This term is applied to all other intrusive igneous bodies with irregular shape, i.e. the body with no specific shape. Chonoliths

21. Thanks