Understanding Igneous Rocks: Intrusive Activity and Formation

1. Chapter 3 Igneous Rocks, Intrusive Activity, and the Origin of Igneous Rocks Index

2. Picture on pg. 53 

3. Igneous Rocks • Igneous Rocks – A rock formed or apparently formed from solidification of magma. • Igneous rocks may be either extrusive if they form at the earth’s surface (e.g., basalt) or intrusive if magma solidifies underground. 

4. How do we know? • Unlike the volcanic rock in Hawaii, nobody has ever seen magma solidify into intrusive rock. So what evidence suggests that bodies of granite (and other intrusive rocks) solidified underground from magma? 

5. Proof I • Mineralogically and chemically, intrusive rocks are essentially identical to volcanic rocks. • Volcanic rocks are fine-grained. • Experiments have confirmed that most of the minerals in these rocks can form only at high temperatures. More evidence comes from examining intrusive contacts, such as shown in Fig. 3.1and Fig. 3.2. (A contact is a surface separating different rock types. • Preexisting solid rock, country rock, appears to have been forcibly broken by an intruding liquid, with the magma flowing into the fractures that developed. Country rock is an accepted term for any older rock into which an igneous body intruded. 

6. Proof II • Close examination of the country rock immediately adjacent to the intrusive rock usually indicates that it appears “baked” close to the contact with the intrusive rock. • Rock types of the country rock often match xenoliths, fragments of rock that are distinct from the body of igneous rocks in which they are enclosed. • In the intrusive rock adjacent to contacts with country rock are chill zones, finer-grained rocks that indicate magma solidified more quickly here because of the rapid loss of heat to cooler heat. 

7. Different Types of Igneous Rocks • Fine-grained rocks – A rock in which most of the mineral grains are less than one millimeter across (igneous) or less than 1/16 mm (sedimentary). • Plutonic rocks – Igneous rock formed at great depth. • Coarse-grained rocks –Rock in which most of the grains are larger than 1 millimeter (igneous) or 2 millimeters (sedimentary). (Fig. 3.3) 

8. Identification of Igneous Rocks • Igneous rock names are based on texture (notably grain size) and mineralogical composition (which reflects chemical composition). Mineralogically (and chemically) equivalent rocks are granite-rhyolite, diorite-andesite, and gabbro-basalt. The relationships between igneous rocks are shown in Fig. 3.4. Table 3.1Fig. 3.5 

9. Intrusive Bodies • Intrusions, or intrusive structures, are bodies of intrusive rock whose names are based on their size and shape, as well as their relationship to surrounding rocks. They are important aspects of the architecture, or structure, of the earth’s crust. The various intrusions are named and classified on the basis of the following considerations: (1) Is the body large or small? (2) Does it have a particular geometric shape? (3) Did the rock form at a considerable depth or was it a shallow intrusion? (4) Does it follow layering in the country rock or not? 

10. Volcanic neck • A volcanic neck is an intrusive structure apparently formed from magma that solidified within the throat of a volcano. One of the best examples is Ship Rock in New Mexico. (Fig. 3.6) 

11. Dikes and Sills • Dike – A tabular, discordant intrusive structure. Fig. 3.7Fig. 3.8 • Discordant – Not parallel to any layering or parallel planes. • Sill – A tabular intrusive concordant with the country rock. Fig. 3.9 • Concordant – Parallel to layering or earlier developed planar structures. 

12. Intrusives That Crystallize at Depth • Pluton – An igneous body that crystallize deep underground. • Stock – A small discordant pluton with an outcropping area of less than 100 square kilometers. • Batholith – A large discordant pluton with an outcropping area greater than 100 square kilometers. (Fig. 3.10) • Diapir – Bodies of rock (e.g., rock salt) or magma that ascend within the earth’s interior because they are less dense than the surrounding rock. (Fig. 3.11) 

13. Geothermal Gradient • Geothermal Gradient – The rate at which temperature increases with increasing depth beneath the surface It is, on the average, to be about 3oC for each 100 meters (30oC/km) of depth in the upper part of the crust. • Fig. 3.13 

14. Factors That Control Melting Temperatures Pressure The melting point of a mineral generally increases with increasing pressure. Pressure increases with depth in the earth’s crust, just as temperature does. Water Under Pressure If enough gas, especially water vapor, is present and under high pressure, a dramatic change occurs in the melting process. Water vapor sealed in under high pressure by overlying rocks helps break down crystal structures. High water pressure can significantly lower the melting points of minerals. (Fig. 3.14) Effect of Mixed Minerals Two metals – as in solder – can be mixed in a ratio that lowers their melting temperature far below that of the melting points of the pure metals. (Fig. 3.15) 

15. Differentiation and Bowen’s Reaction Theory • Differentiation is the process by which different ingredients separate from an originally homogenous mixture. In the early part of the twentieth century, N. L. Bowen conducted a series of laboratory experiments demonstrating that differentiation is a plausible way for silicic and mafic rocks to form from a single parent magma. • Bowen’s reaction seriesis the sequence in which minerals crystallize from a cooling magma, as demonstrated by Bowen’s laboratory experiments. In simplest terms, Bowen’s reaction series shows that those minerals with the highest melting temperatures crystallize from the cooling magma before those with lower melting points. However, the concept is a bit more complicated than that. 

16. Crystallization • Crystallization begins along two branches, the discontinuous branch and the continuous branch. In the discontinuous branch, one mineral changes to another at discrete temperatures during cooling and solidification of the magma. Changes in the continuous branch occur gradationally through a range in temperatures and affect only the one mineral, plagioclase. Crystallization takes lace simultaneously along both branches. 

17. Assimilation • A very hot magma may melt some of the country rock and assimilate the newly molten material into the magma (Fig. 3.18) This is like putting a few ice cubes into a cup of hot coffee. The ice melts and the coffee cools as it becomes diluted. 

18. Mixing of Magmas • If two magmas meet and merge within the crust, the combined magma will be compositionally intermediate. (Fig. 3.19) 

19. Fig. 3.22 • Pg. 71 

20. Fig. 3.23 • Pg. 72 Back to the Beginning

21. Fig. 3.1 • Pg. 54 Back

22. Fig. 3.2 • Pg. 54 Back

23. Fig. 3.3 • Pg. 55 Back

24. Fig. 3.4 • Pg. 56 Back

25. Table 3.1 • Pg. 56 Back

26. Fig. 3.5 • Pg. 57 Back

27. Fig. 3.6 • Pg. 60 Back

28. Fig. 3.7 • Pg. 61 Back

29. Fig. 3.8 • Pg. 61 Back

30. Fig. 3.9 • Pg. 62 Back

31. Fig. 3.10 • Pg. 62 Back

32. Fig. 3.11 • Pg. 63 Back

33. Fig. 3.13 • Pg. 64 Back

34. Fig. 3.14 • Pg. 65 Back

35. Fig. 3.15 • Pg. 66 Back

36. Bowen’s Reaction Series • Pg. 3.16 Back

37. Fig. 3.18 • Pg. 69 Back

38. Fig. 3.19 • Pg. 69 Back