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CHAPTER 6 Weathering and Soil
2. Distinguish between mechanical weathering and chemical weathering?
At Earth’s surface, bedrock is subject to both mechanical weathering that produces rock and minerals
fragments, and chemical weathering that dissolves or oxidizes soluble materials and changes silicate
minerals except for quartz to clay, salts, and colloids.
9. Explain how sheet joints and exfoliation domes originate.
Rock is elastically compressible. Removal of overburden by erosion results in tensional stress in the rock
above rock below that expands due to lessening confining pressure. In near-surface rock the result is brittle
fracture that produces rock joints perpendicular and sheet joints conformable to the outward expansion of
the rock below. Granite stocks are massive dome-like plutonic emplacements so as they are exposed, the
rock jointing conforming to their form exfoliates curved sheets of rock.
17. What role do hydrogen and hydroxyl ions play in chemical weathering?
In chemical weathering, hydrolysis is when H+ and OH- ions displace atoms in a mineral and in doing so
bring into being a different mineral (i.e. one with a different crystal structure). Example: Orthoclase, a
feldspar with a three-dimensional framework of silicate tetrahedra, is replaced by Kaolinite, a clay mineral
with parallel sheets of silicate tetrahedra. Clays have a greater volume than common rock forming minerals
that as a product of hydrolysis they replace. Hydrolyzing crystals in a rock swell and the thereby strained
rock crumbles.
4AlSi3O8
orthoclase
6H+ + 2(OH)–
—>
acidic water
hydrogen and hydroxyl ions
+
Al4Si4O10(OH)8
kaolinite
+
8SiO2
colloidal quartz
+
4K+
potassium ions
36. Compare and contrast pedalfer, pedocal, and laterite.
Essay 1
Three soil types that weathering produces in climates different in the vegetation they naturally support are:
1) pedalfer (ped means soil, al refers to Al in clay, fe refers to iron oxide) is acidic soil of deciduous and
pine forest.
2) pedocal, (cal refers to calcium carbonate) is alkaline soil of grassland.
3) laterite, is highly leached acid soil of tropical rain forest.
Essay 2
In pedalfer soils, clays and iron are eluviated from the A soil horizon and illuviated in the B soil horizon.
Pedalfer soil includes soil types Spodolols (acidic, coniferous forest soils) and Alfisols (acidic, deciduous
forest soils)
In pedocal soils, poor flushing of salts allows calcite to accumulates in the B soil horizon. Pedocal soil is
soil type Molisols (alkaline, grassland soils).
In laterite soils, leaching is ultimately to the point that only hydroxides of aluminum and iron remain as the
soil. Laterite soil includes soil types Ultisols (extensively weathered acid soils) and Oxisols (extremely
weathered tropical acid soils).
CHAPTER 7 Sedimentary Rocks
10. Explain why the sediments in windblown sand dunes are better sorted than that in glacial
deposits.
Wind that normally blows cannot move clasts larger than sand size. It can cause sand-sized clasts to roll
and hop (saltate) and deposits these as mounds of just sand called dunes. Wind cannot pickup clasts
smaller than sand but when dust (silt and clay-sized clasts) is kicked up into the air by say saltating sand or
the hooves of animals, the dust can be carried in suspension as far as the wind blows to settle out as
blanketing deposits of just dust called loess.
Glacial ice can transport clasts of all sizes equally easily and where the ice ablates (melts or evaporates)
these are deposited in the order that they arrive without regard to size. The glacial deposit that accumulates
is called till. Till is composed of clasts with the size of clay, silt, sand, pebbles, and boulders all mixed
together without layering or sorting according to clast size.
16. Why is quartz the predominant mineral in most sandstones? What is a sandstone called that
contains at least 25% feldspar?
Continental crust is composed predominantly of granitic igneous rocks, and metamorphic gneiss and
schists. Sand-sized crystals of quartz occurs abundantly in these and at Earth’s surface, weathering
releases the grains of quartz while altering the other silicate minerals present to clay. Wind and running
water that transport these and deposit the sand as quartz sandstone and clay as shale different places. The
silicate minerals that weather to clay do so at different rates with feldspar lasting longer than
ferromagnesian minerals. Although weathering does not stop during transportation, sand-sized clasts of
feldspar can survive to accumulate along with quartz sand if the distance or duration of transportation to
the a place of final deposition is not far. Sandstone with noticeable amounts of feldspar (usually pink
colored K-feldspar) is called arkose.
22. In what fundamental way do chemical sedimentary rocks differ from detrital sedimentary rocks?
Chemical sedimentary rocks make possible the chemical industry of our civilization. The minerals of
chemical sedimentary rocks are released by weathering in solution in water. The minerals of detrital
sedimentary rocks are released by weathering as solid particles and being silicates are not amenable to
chemical manipulation and so are at best the containers(glassware, pottery, and substrates) for chemical
experiments and production.