F_pg46

1. f_pg46

2. Minerals Naturally occurring (cannot be man-made) Solid (not a gas or liquid) Inorganic (not living or once-living) Definite chemical composition (not a mixture) Specific internal crystal structure (molecules aren’t randomly arranged)

3. Building blocks of rocks Many form under narrow ranges of physical conditions Can indicate pressure & temperature during formation Some form from ocean water Evidence of former marine conditions Some form under arid conditions Evidence of former arid/tropical areas Magnetic minerals record direction of Earth’s magnetic field at time of their formation Radioactive minerals allow age dating of rocks

4. Over 4000 mineral species have been identified Only about 400 may be considered “common” If you become a geologist, you need to be familiar with about 200 Only 25 or so form the majority of rocks

5. The common minerals are usually identified on basis of easily seen/measured properties Color Streak Luster Cleavage Hardness Density Crystal form Other special properties These properties not covered here; those in lab will see them up close & personal

6. The common rock-forming minerals are broken into two groups Silicates (contain silicon & oxygen) Non-silicates (do not contain silicon)

7. Silicates Make up the bulk of the crust Only 8 elements make up the majority of these Note that silicon & oxygen constitute about 75% (by weight) of the crust; oxygen is most abundant Basis of silicates: a silicon atom is surrounded by four oxygen atoms, forming a silicate tetrahedron These can combine somewhat like polymers, by sharing one or more oxygens

8. t04_02_pg50

9. Common rock-forming silicates These can form from molten rock Quartz – silicon dioxide (silica) Potassium feldspar group Orthoclase & microcline Potassium aluminosilicate Plagioclase feldspar group Several species Sodium-calcium aluminosilicates

10. Muscovite (mica) – potassium aluminosilicate w/water Biotite (mica) – magnesium-potassium-iron aluminosilicate w/water Pyroxene group – aluminum-calcium-magnesium-iron silicates Amphibole group - aluminum-calcium-magnesium-iron silicates w/water Olivine group – magnesium-iron silicates Biotite, pyroxene, amphibole, & olivine are collectively called ferromagnesian minerals

11. Clay minerals Silicates of hydrogen, aluminum, magnesium, iron, & potassium Structure is similar to mica Formed by the weathering of aluminum-bearing silicates Cover about 75% of continental surfaces, & are the most abundant materials deposited in oceans “clay” also refers to a grain size (more on this later)

12. Nonsilicate minerals Do not contain silicon Make up about 8% of crust Includes carbonate, sulfide, sulfate, chloride, & oxide minerals, among others As far as rocks, we have two main groups: Carbonates Evaporites

13. Carbonates Contain carbon & oxygen (carbonate radical) Calcite Calcium carbonate Main constituent of limestone & marble Shells of some organisms; can also precipitate directly from seawater or groundwater Identified by reaction with hydrochloric acid, and by its cleavage (if in reasonably crystallized masses) Aragonite Also calcium carbonate, but different crystal structure Forms skeletons of corals & molluscs (clams, snails) Makes up “mother-of-pearl” Metastable – changes to calcite over time

14. Dolomite Calcium magnesium carbonate Has cleavage like calcite Reacts with hydrochloric acid only if powdered Forms from limestone by reaction with magnesium-bearing waters (the exact process isn’t really known)

15. Evaporites Form by evaporation of water containing dissolved salts Indicates arid climate Several, the most common are halite and gypsum

16. Halite (rock salt) Sodium chloride – common table salt Cubic cleavage Salty taste (!) Gypsum Calcium sulfate dihydrate Used to make Plaster of Paris & drywall Soft – can be scratched by fingernail Has two cleavages, but is often fine-grained or fibrous

17. Rocks Areally extensive aggregates of one or more minerals Three groups Igneous – crystallized from lava or magma Sedimentary – from sediments that were compressed and/or cemented Metamorphic – rocks that were changed by the action of heat, pressure, and/or chemical activity

18. Once formed, a rock is rarely left alone On the Earth, anyway Environmental conditions change It can be weathered (mechanically broken down, chemically changed) It can be heated/compressed It can be remelted The rock cycle Not a Flintstone-era exercise machine made from rock

19. f04_17_pg56

21. Rock groups classified according to their origin Specific rock types within each group identified based on two characteristics: Texture – the size, shape, & arrangement of the grains/particles Mineral composition We now go through the rock groups….

22. Igneous rocks “Fire-formed” Crystallized from hot, molten lava or magma as it cooled Magma – molten rock beneath the surface of the Earth Lava – molten rock which has flowed out onto the surface of the Earth

23. Cooling history & texture Magma cools & crystallizes before it reaches the surface Termed intrusive (intruded other rocks) or plutonic (from Pluto, Roman god of the underworld) Magma reaches surface, erupts as lava Called extrusive (wonder why?) or volcanic (from Vulcan, Roman god of fire; not from Mr. Spock)

24. Texture of igneous rocks Mainly, the grain size – how big are the individual crystals Related to the cooling history of the (igneous) rock Coarse grains (basically, you can see the grains without need of magnification) indicate slow cooling Fine grains indicate fast cooling Large crystals in fine groundmass Porphyritic Some crystals grew deep in the Earth, then were incorporated in the lava as it erupted at the surface Glass – very rapid cooling

25. f04_18_pg57

26. f04_21a_pg58

27. f04_21c_pg58

28. f04_21b_pg58

29. f04_24_pg60

30. Why the different compositions? Studied by geologist Norman Bowen in early to mid 1900’s Melted rock samples, let them cool over time, quenched samples to see what minerals had formed Started with basaltic rock Found a sequence of mineral crystallization Certain minerals crystallized at higher temperatures, others at lower temperatures Some minerals seemed to react with early-formed minerals as the temperature lowered Evolved to Bowen’s Reaction Series

31. f04_26_pg62

32. Distribution of the major igneous rocks Igneous rocks comprise more than 90% of the crust, by volume Continental crust is dominated by granodiorite (similar to granite, but with more dark minerals) Oceanic crust is dominated by basalt (think Hawaiian Islands)

33. Characteristics of volcanic activity Type of volcanic activity depends on the lava’s viscosity (how thick) and its water content Viscosity also related to the silica (Si & O) content & temperature “runny”, low-viscosity lavas (low silica, high temperature) give gentle eruptions (Hawaii) Thick, high-viscosity (high silica, high water content, lower temperature) lavas give violent explosive eruptions (Mt. St. Helens)

34. f04_27_pg63

36. Sedimentary rocks Provide more info on Earth’s past than igneous or metamorphic rocks Form by compressing and/or cementing loose sediments This process is called lithification (“making rock”) Common cements – Calcium carbonate (calcite) Silica (quartz) Iron oxides (hematite)

37. 3 most abundant sed. rocks (there are others) Sandstones (sand-sized grains that are compressed/cemented together) Shales (similar, but much smaller grain sizes, also often contain clay minerals) Carbonate rocks (limestones, mainly) grain sizes……we’ll get there, hang on

38. Origin of sediments (the grains) From weathered, disintegrated, decomposed older rocks (of any type) In the beginning, they were igneous (most likely) Next, the evolution of an igneous rock of granite/granodiorite composition (a continental-type rock)

39. f04_31_pg66

40. Classifying sedimentary rocks Again, texture & composition Texture – the size, shape, & arrangement of grains Composition – the minerals Two big groups Clastic - made up of clasts (grains, broken fragments of minerals, rocks, fossils) Chemical/biochemical – carbonates & evaporites And a minor one – organic coals

41. Clastics Derived from weathering of pre-existing rocks; particles transported to a depositional basin Composed of clasts (the bigger pieces), matrix (finer-grained sediment surrounding the clasts), and cement The texture is related to the grain sizes, which have specific meanings geologically

42. Four grain sizes Gravel – grains are larger than 2 mm Rounded clasts – conglomerate Angular clasts – breccia Sand – grains are 1/16 to 2 mm Sandstone – various types, more on them later Silt – grains 1/256 to 1/16 mm Siltstone – gritty (tooth test) Clay – less than 1/256 mm Shale or claystone “mud” – technically, a mixture of silt & clay

43. Chemical/biochemical rocks Carbonates – both chemical & biochemical processes Limestones Contain calcite, aragonite, dolomite Many names – some important ones Micrite – very fine-grained; basically, carbonate mud Oolitic limestone – made of sand-sized, rounded grains Coquina – a fossil hash; made of pieces of shells Chalk – made of microscopic shell pieces of planktonic organisms Evaporites Halite (rock salt) Gypsum Travertine (calcium carbonate) Technically a carbonate, but deposited in caves & around hot springs from water solutions

44. Siliceous rocks (generally, biochemical) Dominated by silica Silica-secreting organisms such as diatoms, radiolarians, some sponges Chert, from reactions of silica in solution on limestones (e.g., by leaching from volcanic ash) Diatomite looks like chalk, but very low density Diatomaceous earth – pool filters Can buy it powdered; very fine powder, but feels very gritty Chert a massive, hard, microcrystalline form of quartz

45. Organic sedimentary rocks (coals) Formed from organic matter (mainly plant matter) Rocks CAN be organic in origin Four types – based on depth of burial (temperature & pressure) Peat – brownish plant fragments resembling peat moss Lignite – crumbly & black Bituminous – dull to shiny & black; burns sooty; layers may be visible Anthracite – extremely shiny and black; low density; no soot when burns Often considered a metamorphic rock due to higher temperatures & pressures it has undergone; surrounding rocks are still sedimentary

46. Metamorphic rocks “changed form” – texture and mineralogy of other rocks changed Caused by: High temperatures (but not to the point of melting) High pressures Chemical reactions with solutions & hot gasses Here, new elements may be added to the mix, often not

47. Two major types of metamorphism Contact metamorphism Rock altered by heat from adjacent lava or magma Regional metamorphism Rock altered over a large area by heat & pressure Deep burial or tectonic pressure Creates the majority of metamorphic rocks

48. Metamorphic index minerals Minerals formed under specific temperature & pressure conditions Allows us to decipher the growth history of old mountain regions “grade” – the relative combination of temperature & pressure

49. f04_38_pg70

50. Textures of metamorphic rocks Foliated Parallel alignment of mineral grains Nonfoliated Granular; equidimensional grains, no preferred orientation

51. Parent rock The rock type from which a given metamorphic rock was formed

52. Foliated rocks Slate very fine grained, parent rock = shale Phyllite Slighter coarser grained; often wrinkled surface Ultimate parent rock = shale Schist Platy or needle-like minerals visible to unaided eye Named according to dominant mineral (i.e. mica schist) Ultimate parent rock = shale, sometimes fine-grained volcanic rocks Gneiss Coarse grained, minerals segregated into bands Parent rocks often high-silica rocks or “dirty” sandstones

53. Nonfoliated rocks Marble Metamorphosed limestone Quartzite Metamorphosed quartz sandstone Greenstone A fine-grained, dark green rock Formed by low-grade metamorphism of basalt Hornfels Hard, fine-grained rock often composed of mica & garnet Usually formed from contact metamorphism of shale or other fine-grained rocks

54. What metamorphic rocks indicate about Earth’s history Presence of these rocks indicate periods of past deformation, uplift, & erosion Minerals & textures indicate direction of compressional forces, amount of pressure, how much heat involved, type of pre-existing rock

55. Metamorphic rock may also indicate conditions prior to metamorphism Marble had to have limestone, thus conditions similar to those in some present seas Quartzite Some have relict structures, similar to those seen in sandstones (more on such structures later) Indicate depositional processes similar to those today occurred in the past