Volcanic rocks and Sedimentary rocks by lm0N43u


									Volcanic rocks and Sedimentary rocks

   •   The Long Valley Caldera, near the Sierra Nevada Mountains, exploded about
       700,000 years ago and produced an immense ash fall called the Bishop Tuff
   •   About 30 km to the northwest lies Mono Lake, with an island in the middle and a
       string of craters extending south from its south shore

   •   Hot springs and tufa deposits can be found along the lake
   •   You can see the lake on Google Earth™ at Lat 37° 59' 56.58'' N Long 119° 2'
       18.2'' W

   •   The depression known as Mono Lake formed as a volcanic caldera with its floor
       beneath the local water table. Its islands and nearby domes imply resurgent
       volcanic activity; the area is potentially subject to future volcanic hazards

Earth not unique in having volcanoes

   •   The largest known is an extinct shield volcano, Olympus Mons, found on Mars

   •   The most volcanically active body in the solar system is Io, a moon of Jupiter

                                    Sedimentary rocks

   •   Although rare in the crust as a whole, sediments and sedimentary rocks dominate
       solid Earth materials at the surface

   •   Clastic sedimentary rocks are primarily classified on the basis of grain size.

Clastic sedimentary rocks

   •   First, physical and chemical weathering break up and alter the parent rock to form
       detrital fragments of parent material, dissolved ions, and clay
   •   This material is then eroded from the parent surface and transported away from
       the source by water, wind, or glacial ice (or directly by gravity, in the case of
       large clasts on a slope)
   •   Ultimately, the detritus will settle out of the transport medium and the dissolved
       ions may react to form chemical precipitates (deposition includes both physical
       settling and chemical precipitation).
   •   Eventually, loose grains of deposited sediment may become buried under
       additional sediment, compacted, and cemented to form sedimentary rock.

Change in grain characteristics

   •   Grain size and shape, sorting, sphericity, and angularity change as sediments
       move downstream.
   •   Mechanical forces such as tumbling and abrasion wear on sediments as they are
       transported downstream

   •   Angular protuberances are especially likely to be broken off

   •   As a result, grain size decreases, with grains becoming more spherical and more
       rounded (less angular)

Velocity and deposition

   •   The speed at which the water in a stream is traveling decreases along its course

   •   The capacity of the stream to carry sediment is directly related to its rate of flow

   •   So larger grains are deposited upstream from finer grains (grains become sorted as
       they travel downstream)

Layer structure

   •   The layering of rock is termed stratification (or bedding)
          – cross beds signify deposition along a slope and can be used to indicate
              paleocurrent direction

   •   Other features of interest within sedimentary rocks include fossils, ripple marks,
       and mud cracks

Cross beds

   •   Cross beds form from sediment deposited on the leeward side of dunes and

   •   These angled beds dip downward in the downstream direction (parallel to the lee
       side of the dune or ripple)

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