How the Earth’s Rocks Were Formed

1. Chapter 5 How the Earth’s Rocks Were Formed

2. I will be able to … • Explain the principle of uniformitarianism and relate it to the formation of igneous, sedimentary, and metamorphic rocks. • Discuss the differences between plutonic and volcanic igneous rock. • Define rock texture and list some factors that control the texture of igneous rock. • Name and describe members of the granite, diorite, and gabbro igneous rock families

3. Igneous Rock

4. Uniformity of Process • Modern geology began in 1795 • This is when Scottish geologist James Hutton described a new concept called uniformitarianism

5. Uniformity of Process • Before Hutton goelogists thought • Earth was formed by spectacular events and catastrophes • They believed these catastrophies formed mountains, canyons, waterfalls and almost all land forms

6. Uniformity of Process • James Hutton’s ideas were different. • He came to the conclusion, “the present is the key to the past” • This statement has 2 key concepts • The geological process now at work were also active in the past • The present physical features of Earth were formed by these same processes, at work over long periods of time

7. Uniformity of Process • For example: a river valley • Otheres would describe this as a sudden split of the Earth’s crust • Hutton explained it as the slow steady wearing away of the land by the river over thousands of years

8. Three Groups of Rock • A rock is defined as a group of minerals bound together in some way. • Using Hutton’s principle, and studying different rocks around the Earth goelogists have concluded all rock from the earths crust is formed in one of 3 ways

9. Three Groups of Rock • Igneous Rock – formed by the cooling and hardening of magma from inside the earth. • Magma is hot molten rock (under surface).

10. Three Groups of Rock • Sedimentary rock –formed by hardening and cementing of layers of sediments. • Sediments may consist of rock fragments, plant and animal remains, or chemicals that form on lake and ocean bottoms.

11. Three Groups of Rock • Metamorphic Rocks – formed when rocks that already exist are changed by heat and pressure into new rocks. • Garnet is an example

12. Recognizing Igneous Rock Plutonic, or intrusive, igneous rock • Rocks that form underground from cooled magma • These rocks are seen on the surface only after the rocks that cover them are cleared away. • Example: Granite

13. Recognizing Igneous Rock Volcanic, or extrusive, igneous rock • Magma that pours onto the earths crust during a volcanic eruption is called lava • These rocks are formed when the lava cools • They can also be formed out of the dust and ash of a volcanic eruption

14. Recognizing Igneous Rock • Volcanic rock example: • Scoria – formed from lava slowly cooling

15. Recognizing Igneous Rock • Plutonic and volcanic rocks are simular in mineral composition, but volcanic rocks lack mineral grains • Example: obsidian – volcanic rock formed from lava cooling rapidly

16. Kinds of Magma • There are 2 general kinds that are most common • Felsic • Mafic

17. Kinds of Magma Felsic • when hardens forms rocks with mainly light coloured minerals • high-silica • Does not have much clacium, iron or magnesium • Thick and slow flowing magma

18. Kinds of Magma Felsic • Granite is a common felsic rock • It contains light coloured minerals like quartz and orthoclase • Most plutonic rocks are felsic

19. Kinds of Magma Mafic • Low percentage of silica • Higher percentage of calcuim, iron, and magnesium • Magma is hotter, thinner, and more fluid than felsic • When solid forms rocks with dark ferromagnesian minerals

20. Kinds of Magma Mafic • An example is basalt • It contains a number of dark minerals like hornblende, augite, and biotite • Most volcanic rock are mafic.

21. Textures of an Igneous Rock • Texture is another way to group rocks. • A rock’s texture depends on the size, shape, and arrangement of it’s mineral crystals. • Igneous rock textures range from glassy-smooth (obsidian), to course grained (granite).

22. Textures of an Igneous Rock • Crystal size is most important for texture. • This depends on • how fast the magma hardens. • The amount of gas dissolved in the magma

23. Textures of an Igneous Rock Coarse-grained texture Fine-grained and smooth texture Magma that reaches the Earth’s crust as lava hardens quickly This rock has tiny crystals The crystals are too small to see without a microscope Volcanic rock like basalt is fine-grained; obsidian is glassy • Magma deep within the crust cools slowly • Creates large mineral grains of fairly uniform size • Plutonic rock has this texture • Granite is an example

24. Porphyritic Texture • Porphyry (POR fur ee) rock has 2 distinctly different textures.

25. Porphyritic Texture • How are these rocks formed? • There are 2 stages of cooling • Magma is at a large depth, and here is cools slowly so large crystals can form of one mineral, the rest remains liquid • As the magma gets closer to the surface the rest of the magma cools into fine grained rock around the larger crystals

26. Families of Igneous Rock • Igneous rock is grouped into families according to mineral composition • Granite Family • Gabbro Family • Diorite Family • And some igneous rocks don’t fit into any of these families

27. Families of Igneous Rock • Granite Family • Forms from felsic magmas • Consist mainly of orthoclase feldspar and quartz • These are light in colour, so the rocks in this family are usually light-coloured • other minerals likely present are plagioclase feldspar, mica, and hornblende • Rocks in this family will have different textures, but simular chemical compositions

28. Families of Igneous Rock Granite Family Granite Family Fine-grained rhyolite • Course grained granite

29. Families of Igneous Rock Granite Family Granite Family • Glassy obsidian

30. Families of Igneous Rock • Gabbro Family • Forms from mafic magma • Made mainly of dark plagioclase feldspar and augite • Other likely minerals are olivine, hormblend, and biotite. • These are generally darker and more dense than those found in the granite family

31. Families of Igneous Rock Gabbro Family Gabbro Family Fine-grained basalt • Course-grained gabbro

32. Families of Igneous Rock • Gabbro Family • Glassy example: basalt glass

33. Families of Igneous Rock • Diorite Family • Composition and colour between the granite and gabbro families

34. Families of Igneous Rock Diorite Family Diorite Family Andesite: fine-grained • Diorite: Course-grained • Has less quartz than granite • Has less dark plagioclase than gabbro

35. Families of Igneous Rock • Some igneous rock do not fit into any of these 3 families.

36. Families of Igneous Rock Don’t Fit Don’t Fit Pyroxenite – nearly all pyroxene Coarse, dark and heavy May be like rock in Earth’s mantle • Granodiorite • Course grained rock • Composition between granite and diorite

37. Families of Igneous Rock Don’t fit Don’t Fit Peridotite – a mix of pyroxene and olivine Coarse, dark and heavy May be like rock in Earth’s mantle • Dunite – almost all olivine • Coarse, dark and heavy • May be like rock in Earth’s mantle

38. Description of Common Igneous Rock • Granite • Made of: • quartz – looks like tiny chips of cloudy or grayish glass • orthoclase feldspar – often has smooth cleavage surfaces and is white, gray, pink or orange • and one other mineral like mica (shiny black flakes) or hornblende (tiny dull black chunks or sticks) • Ranges in colour from light to medium gray and pinks • Have course-grained texture

39. Description of Common Igneous Rock • Granite • The most common continental igneous rock • Occurs in thr Rockies, the Aairondacks of New York State, the Black Hills in South Dakota, the White Mountains of New Hampshire, and in many other mountain areas • It is plutonic, and when it appears on the surface it shows errosion removed thousands of meters or rock since the magma cooled

40. Description of Common Igneous Rock • Granite Felsite is the general name for light-coloured, fine grained granite Rhyolite is an example of a fine-grained, light gray to pink rock

41. Description of Common Igneous Rock • Obsidian • Volcanic glass of the granite family • Contains many of the same minerals and other light coloured rocks like granite • Usually dark brown or black • This is due totiny amounts of dark coloured minerals scattered throughout the rock • Is hard and brittle • It has a conchoidal, or shell-like, fracture

42. Obsidian

43. Descriptions of common Igneous Rocks • Pumice • Formed from felsic lava that hardened while steam and other gases were still bubbling out of it • It looks like a sponge with many small holes in it • Because of all the air holes, it is sometimes light enough to float on water

44. Pumice

45. Descriptions of common Igneous Rocks • Basalt • The most common rock in the gabbro family • A fine-grained rock • Ranges in colour from dark green to black • Basalt is the igneous rock on the ocean floor • On land it is the most common rock formed from flows of lava • Large areas occur in the lava flows of: Iceland, Hawaiian Islands, and Columbia and Snake River in western United States

46. Basalt

47. Descriptions of common Igneous Rocks • Gabbro • Has about the same composition as basalt • It is cooled slowly deep underground, so it is course-grained • Very dark in colour

48. Gabbro

49. Descriptions of common Igneous Rocks • Diabase • Has a composition simular to gabbro • It has a finer texturethangabbro, but courser than basalt

50. Diabase