Lecture 6: Earth Materials

Transcription

1 Introduction to Landforms GEOG 1113K Dr. Thieme Lecture 6: Earth Materials The 8 most abundant elements in Earth s crust are: Oxygen (O): 46.6% Silicon (Si): 27.7% Aluminum (Al): 8.1% Iron (Fe): 5.0% Calcium (Ca): 3.6% Sodium (Na): 2.8% Potassium (K): 2.6% Magnesium (Mg): 2.1% All other 95 elements: 1.7% These 8 elements most available to form rocks and minerals mineral - a naturally occurring, inorganic substance that has a definite chemical composition and characteristic crystal structure rock - an assemblage of one or more minerals three major categories of rocks: 1) igneous 2) sedimentary 3) metamorphic 1

2 Mineral Groups 1) silicates Si and O base usually combined with one or more metal cations 2) oxides O base combined with one or more metal cations 3) sulfides S base combined with one or more metal cations 4) carbonates metal cations combined with the carbonate anion (CO 3-2 ) "Rock-forming" Minerals quartz, SiO 2 Feldspars potassium (Kspar) plagioclase (sodium and calcium) mica muscovite (white or tan) biotite (black) Ferromagnesian olivine (Fe- and Mg SiO 4 ) pyroxene (Augite) amphibole (Hornblende) 2

3 Metallic Luster Native copper Galena (PbS 2 ) - both shiny and dull Pyrite (FeS 2 ) Color often due to minor impurities rather than the mineral's crystalline structure Fluorite (CaF 2 ) comes in many colors with the same octahedral form. Colored varieties of quartz (SiO 2 ) include amethyst, citrine, rose quartz, smoky quartz, etc... Streak the color obtained by rubbing the mineral across a piece of unglazed porcelain Both of these samples of hematite (Fe 2 O 3 ) have a reddish brown streak. 3

4 Hardness Tampa Girls Can Flirt And Other Queer Things Can Do Cleavage tendency of a mineral to break along planes of weak bonding 3 at 90 o 4 at < 90 o 3 at < 90 o Fracture Quartz has such strong bonds between Si and O in all directions that it has no cleavage. Instead, it breaks apart by conchoidal fracture. Obsidian (volcanic glass) also has conchoidal fracture. 4

5 Specific Gravity ratio of the weight of a mineral to the weight of an equal volume of water Quartz (SiO 2 ) = 2.65 Galena (PbS 2 ) = 7.5 Gold (Au) = 20 Special Properties Taste some minerals can be identified by taste, example, halite. Effervescence a drop of dilute HCl will cause calcite to effervesce Double Refraction - clear samples of some minerals will bend light, giving a doubleimage if placed over an object or writing. Double refraction in a sample of calcite. Figure

6 Igneous Rocks Figure 13-5(a) Igneous Rocks form when melted rock cools and solidifies magma molten rock beneath earth s surface lava molten rock above earth s surface intrusive rocks form when magma cools below the surface extrusive rocks form when lava cools above the surface Igneous Rock Textures Aphanitic igneous rocks with very fine-grained crystals, individual crystals are too small to be seen with the unaided eye. Phaneritic igneous rocks with individual mineral crystals large enough to be seen with the naked eye. Glassy igneous rocks that cooled so quickly there was no time for any alignment of minerals 6

7 Igneous Rock Textures Phaneritic Aphanitic Glassy Aphanitic cooled quickly above ground Phaneritic cooled slowly below ground Glassy cooled very rapidly (quenched) above ground Igneous Rock Composition Felsic igneous rocks that are composed mostly of quartz and feldspar, with some minor additions of biotite mica and amphibole. Mafic igneous rocks composed mostly of ferromagnesian minerals with some minor additions of feldspar Intermediate igneous rocks that fall between rocks with felsic or mafic compositions Intermediate Felsic Mafic 7

8 Viscosity resistance to flow, depends upon composition of magma or lava 1. Felsic magmas have high viscosity (are thick) because their high silica content makes them sticky 2. Mafic magmas have low viscosity (are thin) because of their low silica content 3. Intermediate magmas have intermediate viscosity, and ultramafic magmas have very low viscosity Naming Igneous Rocks based upon texture and mineralogy Granite a phaneritic, felsic rock. Most common intrusive Rhyolite an aphanitic, felsic rock Andesite an aphanitic, intermediate rock Diorite a phaneritic, intermediate rock Basalt an aphanitic, mafic rock. Most common extrusive Gabbro a phaneritic, mafic rock Obsidian a glassy, felsic rock Pumice a glassy, felsic rock with numerous vesicles 8

9 gives the order that minerals crystallize in as magma cools Forming Sedimentary Rocks diagenesis describes any chemical, physical, and/or biological changes that take place after sediments are deposited lithification the process by which unconsolidated sediments are transformed into solid sedimentary rocks compaction compressing of sediment by the weight of overlying sediment cementation precipitation of minerals in pore spaces leading to a linkage of the sedimentary particles 9

10 COMPACTION CEMENTATION Figure Sedimentary Rock Classes detrital (clastic) sedimentary rocks composed of material (sand, silt, clay) that was deposited chemical sedimentary rocks formed when material is precipitated out of solution organic sedimentary rocks formed from organic material 10

11 Top left: detrital sedimentary rock, Top right: chemical sedimentary rock, Bottom left: organic sedimentary rock. Detrital Sedimentary Rocks Most are composed of clays and quartz Particle size is the primary basis for distinguishing detrital rocks: shale consists of silt and clay-sized particles where the particles are aligned parallel to one another sandstone consists primarily of sand-sized grains conglomerate consists primarily of rounded gravel Parallel alignment of clay particles in a sample of shale. Detrital Sedimentary Rocks sorting the degree of similarity in particle size in a sedimentary rock well sorted most of the particles are of the same size poorly sorted the particles are of highly variable size Well Sorted Poorly Sorted 11

12 Detrital Particles rounded - particles have smooth, curved edges angular - particles have sharp, sudden direction changes along edges Rounded particles Angular particles Chemical Sedimentary Rocks Classification based on composition limestone composed mainly of calcite (fizzes in acid) chert hard rock made of silica (SiO 2 ) Limestone Chert arrowhead Biochemical Sedimentary Rocks limestone composed of shell material coquina limestone made of shells and shell fragments chalk made of the tests (shells) of microscopic marine organisms coral coral reefs are made up of limestone secreted by the coral polyps and algae coal formed when plant materials are preserved in a swamp, buried under sediments, and then compressed by weight at depth lignite a soft brown coal bituminous a harder black coal formed by further compression of lignite 12

13 Sedimentary Structures beds horizontal layering of sediment shown in the rock bedding planes the boundary between beds, flat surfaces along which rock tends to break Beds and bedding planes shown in a Utah rock outcrop. Sedimentary Structures cross-bedding beds deposited on an incline relative to horizontal graded beds particles within a single bedding layer gradually change from coarse at the bottom to fine at the top Cross-beds Graded beds Sedimentary Structures ripple marks wave-like features that develop on a sand surface asymmetrical formed by a current symmetrical formed from a back and forth motion mud cracks indicate a sediment that was alternatively wet and dry Ripple marks Mud cracks 13

14 Fossils Fossil - evidence of past life remains preserved bones, skin, etc trace footprints, coprolites, etc By combining rock type (detrital, chemical, organic) and name, sediment size, shape, and sorting (if detrital), structures, and fossils, we can determine sedimentary environments, important in landform genesis. Remains Trace fossils Sedimentary Environment A geographic setting where sediment is accumulating Determines the nature of the sediments that accumulate (grain size, grain shape, etc.) Sedimentary Environments Continental Transitional Marine Floodplain Delta Continental Shelf Alluvial Fan Barrier Island Continental Slope Playa Lake Lagoon Continental Rise Desert Tidal Flat Deep-sea Fan Glacier Margin Abyssal Plain n = 14 14

15 Continental Environments fluvial or alluvial stream dominated environment glacial environment dominated by moving ice lacustrine lake environment eolian wind dominated environment Fluvial Glacial Lacustrine Eolian Transitional Environments beaches sandy deposits along the shoreline tidal flats mud-covered areas that are alternately flooded with shallow seawater and the exposed to air as the tide rises and falls barrier islands elongated islands of sand that run parallel to the coast Beach Tidal Flat (salt marsh) Barrier Island Transitional Environments lagoons area of sheltered water between a barrier island or reef and the mainland deltas complex accumulation of sediment built out into the ocean or a lake at the mouth of a river Lagoon Delta 15

16 Marine Environments shallow marine water up to about 700 feet deep deep marine water over 700 feet deep Metamorphic Rocks Figure 13-5(c) Metamorphism The transition of one rock into another by temperatures and/or pressures unlike those in which it formed Metamorphic rocks are produced from Igneous rocks Sedimentary rocks Other metamorphic rocks 16

17 Metamorphism Type Contact (thermal) Hydrothermal Regional Impact Grade (from Low to High) Contact Metamorphism Hydrothermal Metamorphism Chemical alteration caused when hot, ion-rich fluids, called hydrothermal solutions, circulate through fissures and cracks that develop in rock Most widespread along the axis of the mid-ocean ridge system 17

18 Regional Metamorphism Produces the greatest quantity of metamorphic rock Associated with mountain building Ocean-Continental Plate Convergence 18

19 Subduction Zones Mountainous terrain adjacent to subduction zones has distinct linear belts of metamorphic rocks High-pressure, low-temperature zones nearest the trench High-temperature, low-pressure zones further inland in the region of igneous activity Continent-Continent Plate Collisions Compressional stresses deform the edges of the plate Formation of the Earth s major mountain belts including the Alps, Himalayas, and Appalachians Foliation Figure 13-13(a) 19

20 LOW GRADE HIGH GRADE Naming Metamorphic Rocks Metamorphic rocks are named based on their foliation, or if lacking foliation, based on their composition (chemistry) Chemistry is inherited from the - Parent rock the rock from which a metamorphic rock forms 20

21 foliation any planar arrangement of mineral grains or structural features within a rock slaty cleavage closely spaced planar surfaces along which rocks split schistosity layered appearance or orientation of mica and chlorite grains gneissic banding segregation of minerals in the rock produces a banded look Slaty Cleavage Schistosity Gneissic Banding Foliated fine-grained rocks Phyllite Slate Foliated coarsegrained rocks Garnet-mica schist Amphibolite gneiss 21

22 Foliated Rocks slate a fine-grained rock composed of mica flakes too small to be seen with the unaided eye. Parent rock: shale phyllite a rock composed of slightly larger mica flakes, large enough to cause a sheen when rotated in light. Parent rock: slate schist medium-grained rock, platy minerals large enough to be seen with the unaided eye dominate. Parent rock: phyllite gneiss a coarse-grained rock with banding, mineralogy is usually similar to granite. Parent rock: schist or granite Non-foliated Rocks rocks that do not display any orientation of minerals Examples of two nonfoliated metamorphic rocks Marble, a nonfoliated metamorphic rock formed from limestone. 22

23 Quartzite, a nonfoliated metamorphic rock formed from quartz sandstone. Nonfoliated Rocks marble a coarse-grained rock composed of calcium carbonate. Parent rock: limestone or dolostone quartzite a medium-grained very hard metamorphic rock composed of quartz. Parent rock: quartz-rich sandstone anthracite a shiny black organic rock. Parent rock: bituminous coal Impact Metamorphism Occurs when high speed projectiles called meteorites strike Earth s surface Products are called impactites or tektites 23

24 Impact Metamorphism Tektites from Nullarbor Plain, Australia 24