Volcanic rocks and Sedimentary rocks
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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
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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
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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
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The largest known is an extinct shield volcano, Olympus Mons, found on Mars
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The most volcanically active body in the solar system is Io, a moon of Jupiter
Sedimentary rocks
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Although rare in the crust as a whole, sediments and sedimentary rocks dominate
solid Earth materials at the surface
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Clastic sedimentary rocks are primarily classified on the basis of grain size.
Clastic sedimentary rocks
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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
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Grain size and shape, sorting, sphericity, and angularity change as sediments
move downstream.
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Mechanical forces such as tumbling and abrasion wear on sediments as they are
transported downstream
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Angular protuberances are especially likely to be broken off
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As a result, grain size decreases, with grains becoming more spherical and more
rounded (less angular)
Velocity and deposition
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The speed at which the water in a stream is traveling decreases along its course
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The capacity of the stream to carry sediment is directly related to its rate of flow
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So larger grains are deposited upstream from finer grains (grains become sorted as
they travel downstream)
Layer structure
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The layering of rock is termed stratification (or bedding)
– cross beds signify deposition along a slope and can be used to indicate
paleocurrent direction
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Other features of interest within sedimentary rocks include fossils, ripple marks,
and mud cracks
Cross beds
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Cross beds form from sediment deposited on the leeward side of dunes and
ripples
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These angled beds dip downward in the downstream direction (parallel to the lee
side of the dune or ripple)