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04_weathr_sed_09_post - 4: Weathering & Sedimentary...

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Unformatted text preview: 4: Weathering & Sedimentary Rocks Weathering of granite produces sediment. Photo: S. Marshak. Sedimentary layers in Grand Canyon. Photo: P.L. Kresan Canyon. Why this is important • Weathering is the first step in the process of wearing down what tectonics builds up. Sedimentary rock: rock that formed at or near the Earth’s surface by precipitation of minerals from water solutions, by growth of skeletal material in organisms, or by cementing together of shell fragments or loose small pieces (grains) derived from preexisting rocks. Sediment: loose (unconsolidated) fragments. L e c tu re 4 : 1 Photo: P.L. Kresan L e c tu re 4 : 2 Why this is important Why this is important • W e a th e rin g p ro d u ce s so ils a n d co n ce n tra te s ce rta in m in e ra l re so u rce s. • S e d im e n ta ry ro ck s a re th e p rim e re co rd e rs o f g e o lo g ic h isto ry . P h o t o : P .L . K r e s a n P h o to : C h e v ro n C o rp . P h o to : L . L e fe v e r • S e d im e n ta ry ro ck s co n ta in fo ssil fu e ls & m a n y n o n -m e ta llic m in e ra l reso u rces. L e c tu re 4 : 3 P h o t o : P .L . K r e s a n L e c tu re 4 : 4 • O n ly ro ck ty p e th a t co n ta in s fo ssils, th e re co rd o f th e h isto ry o f life . Origin of sediment Fig. 7.16 Two types of weathering • M e ch a n ica l (p h y sica l) weathering: reduces th e siz e o f r o c k a n d m in e r a l fr a g m e n ts w ith o u t c h a n g in g th e ir c o m p o sitio n • Weathering: mechanical disintegration and chemical decomposition of rock to produce sediment. • Erosion: removal of (generally weathered) rock or rock by-products. • Transportation: movement of weathered material by moving air, water, ice. • Deposition: accumulation of previously transported sediment. L e c tu re 4 : 5 Mechanical weathering • C h e m ica l w e a th e rin g : r e su lts in a c h a n g e in c h e m ic a l c o m p o sitio n . L e c tu re 4 : 6 Mechanical weathering • Root wedging & burrowing: mechanical weathering produced by plants and animals T a y lo r V a lle y , V icto ria L a n d , A n ta rctica . P h o to : M . H a m b re y • Frost wedging: water in c r a c k s e x p a n d s b y 9 % w h e n it fre e z e s Analogy: freezing water bursts pipes and breaks water bottles placed in freezer. L e c tu re 4 : 7 See Fig. 7.6b Fig. 7.6a Photo: P.L. Kresan Photo: L e c tu re 4 : 8 Chemical weathering D isso lu tio n : s o m e m in e ra ls (h a lite , g y p su m , ca lcite ) d isso lv e in w a te r, e sp e cia lly a cid ic w a te r. Chemical weathering • Oxidation: elements combine with oxygen (iron rusts) Arches National Park, Utah. Photo: P.L. Kresan L e c tu re 4 : 9 Chemical weathering L e c tu re 4 : 1 0 Weathering Fig. 7.9 • Hydrolysis: hydrogen ions in water replace ions of silicate minerals and convert them to insoluble residue (commonly clay minerals like kaolinite) and dissolved ions. C h e m ic a l w e a t h e r in g a id s p h y s ic a l w e a t h e r in g b y w e a k e n in g t h e attachments between mineral grains so that rock “falls apart”; c h e m ic a l w e a t h e r in g t h e n a t t a c k s t h e c r u m p le d p ie c e s . A n a lo g y : y o u c a n ( c a r e f u lly ) h o ld u p a n in t a c t jig s a w p u z z le , b u t r e m o v e s o m e o f t h e p ie c e s ( o r m a k e t h e m s o f t ) , a n d it w ill f a ll a p a r t . L e c tu re 4 : 1 1 L e c tu re 4 : 1 2 Review Questions / iClicker 4-1. What type of rock is most likely to contain fossils, oil, natural gas, and coal? A. igneous B. sedimentary C. metamorphic 4-2. Which of the following is an example of chemical weathering? A. burrowing B. frost wedging C. hydrolysis D. root wedging Types of sedimentary rocks Clastic: made from weathered rock fragments (clasts). Biochemical: cemented shells of organisms. Organic: the carbon-rich remains of plants (e.g., coal) C h e m ic a l: m in e r a ls th a t c r y sta lliz e (p r e c ip ita te ) d ir e c tly fr o m w a te r . 4-3. Which of the following is an example of mechanical weathering? A. dissolution B. frost wedging C. hydrolysis D. oxidation 4-4. Water expands by ___ when it freezes. A. ~0.1% B. ~1% C. ~10% D. ~100% L e c tu re 4 : 1 3 Clastic sediments / rocks Clastic Biochemical Organic Chemical L e c tu re 4 : 1 4 Clastic sediments / rocks Conglomerate • Classification based on grain size (Table 7.3) Sandstone Shale Photo: B.P. Kent C o n g lo m e ra te S h a le S iltsto n e S a n d sto n e L e c tu re 4 : 1 5 L e c tu re 4 : 1 6 See Fig. 7.19 Chemical sediments / rocks C h e m ic a l s e d im e n t s / r o c k s E v a p o rite s : ro ck s m a d e o f m in e ra ls th a t fo rm w h e n w a te r e v a p o ra te s. Wet playa, Salar de Uyuni in the Altiplano of western Boliva; Photo: G. Zandt Fig. 7.23 Salar de Uyuni with evaporite (salt) deposits covering dry lake bed; Photo: D. Richards L e c tu re 4 : 1 7 L e c tu re 4 : 1 8 Chemical sediments / rocks Photo: P.L. Kresan Chemical sediments / rocks • Classification based on chemical composition Sediment Evaporites generally indicate hot, dry conditions. L e c tu re 4 : 1 9 L e c tu re 4 : 2 0 Rock Composition Minerals Biochemical sediment / rocks Review Questions / iClicker 4-5. Which term does not belong with the others? A. conglomerate B. sandstone C. siltstone D. limestone 4-6. Which rock contains the largest grain size? A. conglomerate B. sandstone C. shale D. siltstone • Classification of chemical & biochemical sediments & rocks is based on chemical composition L e c tu re 4 : 2 1 Photo: P.L. Kresan 4-7. A. True / B. False: Chert is composed principally of calcium carbonate. 4-8. Which of the following is an example of a clastic sedimentary rock? A. chert B. limestone C. rock salt D. shale L e c tu re 4 : 2 2 Review Questions / iClicker Controls on weathering • P r o p e r t ie s o f p a r e n t r o c k : d if f e r e n t r o c k s ( m in e r a ls ) w e a t h e r a t d if f e r e n t r a t e s Questions 4-9 & 4-10 apply to the table below. Sediment Name Rock Name Particle Size Boulder Conglomerate >256 mm Cobble Conglomerate 256 - 64 mm Pebble Conglomerate 64 - 2 mm Sand Sandstone 2 - 0.062 mm Silt Siltstone 0.062 – 0.0039 mm Clay Shale or claystone <0.0039 mm 4-9. The name of the sediment for a particle size of 20 cm is: A. boulder B. cobble C. pebble D. sand 4-10. The name of the sedimentary rock for a particle size of 0.00004 inches is: A. conglomerate B. sandstone C. siltstone D. shale Both gravestones are about the same age, but the one made of marble (calcite) has weathered more than the one made of granite (quartz, feldspar, Na-plagioclase) L e c tu re 4 : 2 3 L e c tu re 4 : 2 4 Controls on weathering: climate Controls on weathering • P r o p e r t ie s o f p a r e n t r o c k : d if f e r e n t r o c k s ( m in e r a ls ) w e a t h e r a t d if f e r e n t r a t e s Dissolve in water High Crystallization temperature Low • Numerous freezethaw cycles favor physical weathering. • High moisture & high temperatures favor chemical weathering. Weathering minerals L e c tu re 4 : 2 5 Controls on weathering • Presence and abundance of cracks (joints) L e c tu re 4 : 2 6 Controls on weathering: iClicker Volume=L3 Surface area=L2 1m 1m • M e c h a n ic a l w e a t h e r in g in c r e a s e s n u m b e r a n d w id t h o f c r a c k s . L e c tu re 4 : 2 7 1m Q 1 . T h e v o lu m e a n d su rfa ce a re a o f th e cu b e o n th e le ft is: A. 1 m3; 1 m2 B. 1 m3; 3 m2 C. 1 m3; 6 m2 D. 3 m3; 3 m2 E. 6 m3; 6 m2 L e c tu re 4 : 2 8 Fig. 7.8 Controls on weathering: iClicker Fig. 7.8 Controls on weathering Fewer cracks, Fewer blocks, lower surface area More cracks, more blocks, higher surface area Fig. 7.8 0.1 m 1m 1m 1m 0.5 m 1m 1m Q 2 . W h a t is th e su rfa ce a re a fo r a ll o f th e cu b e s in th e m id d le ? A . 0 .2 5 m 2 B . 0 .5 m 2 C . 1 .5 m 2 D. 12 m2 L e c tu re 4 : 2 9 0.5 m 1m Q 3 . C a lcu la te th e su rfa ce a re a fo r a ll th e cu b e s o n th e rig h t. Chemical weathering rates increase as surface area increases, a result of mechanical weathering. L e c tu re 4 : 3 0 Features of clastic sediments Fig. 7.16 Features of clastic sediments Fig. 7.18 Increasing weathering & transport Increasing weathering & transport As the amount of weathering and transport distance increase, the grain size decreases. Sorting: a measure of the uniformity of grain (clast) size. A s t h e a m o u n t o f w e a t h e r in g a n d t r a n s p o r t d is t a n c e in c r e a s e , r o u n d n e s s a n d s o r t in g in c r e a s e . L e c tu re 4 : 3 1 L e c tu re 4 : 3 2 Origin of sedimentary rocks Origin of sedimentary rocks • Lithification and diagenesis: formation of rock from sediment • Compaction: w e ig h t o f o v e rly in g m a te ria l (o v e rb u rd e n ) sq u e e z e s g ra in s to g e th e r a n d sq u e e z e s w a te r out Fig. 7.17 • C e m e n ta tio n : p ro ce ss o f “ g lu in g” g ra in s to g e th e r w ith m in e ra ls p re cip ita te d fro m w a te r. Cement Grain Grain Grain Photo: P.L. Kresan L e c tu re 4 : 3 3 L e c tu re 4 : 3 4 Sedimentary structures • Bedding or stratification: subdivision of a succession of sedimentary rocks into layers bounded by generally parallel surfaces called bedding planes, which were originally horizontal. Recognized by changes in color, composition, grain size, etc. Sedimentary structures • Bedding is produced by changes in materials or conditions (e.g., flow rate of moving water) during deposition. Slowmoving stream L e c tu re 4 : 3 5 http://www.joshushund.com/terragen_01-06/wall_of_strata.jpg L e c tu re 4 : 3 6 Fastmoving stream Slowmoving stream Fig. 7.26 Sedimentary structures Sedimentary structures • Graded bedding: upward decrease in grain (particle) size reflecting decreasing energy (turbulence) during deposition. • Mudcracks: polygonal cracks produced by drying out of sediments. Fig. 7.30 Largest grains settle out See Fig. 7.31 Smallest grains settled out Photos: P.L. Kresan Modern sediment se L e c tu re 4 : 3 7 L e c tu re 4 : 3 8 Sedimentary structures Sedimentary structures • Cross bedding: individual layers are inclined at an angle to the main b e d , fo r m e d b y m o v in g w in d a n d w a te r c u r r e n ts. Fig. 7.29 Fig. 7.29 C ro s s b e d s a re in c lin e d f r o m u p p e r le f t t o lo w e r r ig h t , in d ic a t in g t h a t t h e c u r r e n t f lo w e d f r o m l e f t t o r i g h t. Photo: P.L. Kresan L e c tu re 4 : 3 9 Ancient rock L e c tu re 4 : 4 0 Sedimentary structures Sedimentary structures • Ripple marks: undulations on bedding surface produced by flowing a ir o r w a te r. Photo: R. Siever • S y m m e trica l rip p le s a re p ro d u ce d b y b a ck -a n d -fo rth (b id ire ctio n a l) cu rre n ts, su ch a s p ro d u ce d b y w a v e s o n a b each . Fig. 7.28 Photo: P.L. Kresan L e c tu re 4 : 4 1 L e c tu re 4 : 4 2 Sed. structures / iClicker Sedimentary structures • A sy m m e trica l rip p le s a re p ro d u ce d b y u n id ire ctio n a l cu rre n ts, su ch a s in a riv e r. S te e p sid e o f rip p le slo p e s fro m u p p e r le ft to lo w e r rig h t, in d ica tin g cu rre n t fl o w e d fro m le ft to rig h t, p e rp e n d icu la r to cre st lin e o f rip p le . These questions refer to the sketches below that show cross sections (side views) of a dune (a) and ripple marks (b and c). Q1. The flow direction that produced the dune in (a) was A. left to right B. right to left C. both Q2. The flow direction that produced the ripple marks in (b) was A. right to left B. left to right C. both Fig. 7.28 L e c tu re 4 : 4 3 Q3. The flow direction that produced the ripple marks in (c) was A. right to left B. left to right C. both L e c tu re 4 : 4 4 Review Questions / iClicker 4-11. A. True / B. False: Chemical weathering is most effective in regions with that have a hot, humid climate. 4-12. A. True / B. False: Mechanical weathering is most intense in regions that have numerous freeze-thaw cycles. 4-13. Which term applies to a process that turns sediment into rock? A. Weathering B. cementation C. deposition D. transportation 4-14. A. True / B. False: Compaction results from the weight of overlying sediment. 4-15. A. True / B. False: Grain size increases with increasing weathering and transportation. 4-16. In what environment do symmetrical ripples most likely form? A. beach (waves) B. desert (wind) C. alluvial (stream) D. glacial 4-17. A. True / B. False: Graded bedding shows an upward increase in grain size. L e c tu re 4 : 4 5 ...
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This note was uploaded on 09/25/2011 for the course GEOLOGY 100 taught by Professor Lepre during the Fall '11 term at Rutgers.

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