Earth: Portait of a Planet - Chapter 8 - Metamorphism: A Process of Change
Earth: Portrait of a planet - Chapter 9 - The Wrath of Vulcan: Volcanic Eruptions
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Oktober 2022
CHAPTER SEVEN
Pages of the Earth’s Past: Sedimentary Rocks
Introduction
Sedimentary rock; forms either by the cementing together of fragments broken o
preexisting rock or by the precipitation of mineral crystals out of water solutions at or near
the Earth’s surface.
7.2 Classes of Sedimentary Rocks
Clastic sedimentary rock consists of cemented together clasts, solid fragments and grain
broken o of pre-existing rocks.
Biochemical sedimentary rock consists of shells grown by organisms.
Organic sedimentary rock consists of carbon-rich relicts of plants or other organisms.
Chemical sedimentary rock consist of minerals that precipitated directly from surface-water
solutions.
CLASTIC ROCKS
Sandstone consists of loose clasts which have become a solid mass. These clasts consist
of individual mineral fragments. The production of clastic rocks involves ve steps:
- Weathering: the clasts come from the disintegration of pre-existing bedrock into separate
grains due to physical and chemical weathering.
- Erosion: Clasts produced by weathering may be removed from the outcrop by erosion;
moving water, ice or air separates clasts from their substrate and carries them away.
- Transportation: The clasts and dissolved ions are carried away by a transporting medium
(water, wind, ice).
- Deposition: The sediment eventually undergoes deposition.
- Lithi cation: The loose sediments then transform into solid rock.
- The sediment had been buried -> pressure generated by the weight of overlying
material squeezes ou the water and air that had been trapped between clasts and
presses them together tightly (compaction)
- After that the sediment binds to make coherent sedimentary rock -> minerals
precipitate from groundwater and to ll spaces between clasts (cementation).
CLASSIFICATION
- Clast size: diameter of the grains. (Boulder, cobble, pebble, sand, silt and mud)
- Gravel for an accumulation of pebbles and cobbles
- Mud for an accumulation of wet clay and very ne silt.
- Clast composition: Refers to the build up of clasts in sedimentary rock.
- Larger clasts include rock fragments, meaning that clasts aggregate of many mineral
grains.
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, - Smaller clasts consists of individual minerals.
- Angularity and sphericity:
- Angularity indicates the degree to which clasts have smooth or angular corners and
edges.
- Sphericity refers to the degree to which a clast has the same dimensions in all
directions. (Resembles the shape of a sphere)
- Sorting: to the degree the clasts all have the same size.
- If a sedimentary rock contains larger clasts surrounded by much smaller clasts then
the mass of smaller grains
constitutes the matrix of the rock.
- Sedimentary maturity: refers to the
degree to which a sediment has
evolved from being just a crushed-up
version of its source rock into well-
sorted and well rounded collection of
clasts consisting only of the minerals
that are most resistant to weathering.
- Character of cement: Not all clastic
sedimentary rocks contain of the
same kind of cement.
- The way the rock breaks (determines
di erence between mudstone and
shale, the last splits into thin sheets).
ORIGINS
Large blocks of rock tumble o a cli and slam into other blocks already at the bottom.
Producing angular clasts with sharp edges. If these are cemented together breccia is
formed.
If these clasts land in a riverbed they are rounded and produce a conglomerate.
If the gravel stays put for a long time, it undergoes chemical weathering, the cobbles and
pebbles break apart into individual mineral grains. A mixture of quartz and some feldspar
grains turn into arkose.
The sediment washes downstream and the sand loses feldspar clasts and ends up being
composed almost entirely of durable quartz sand grains. Meanwhile silt and clay may
accumulate in the at areas bordering streams, regions called oodplains, that become
submerged only during oods. And some silt and mud settles in a wedge, called a delta, at
the mouth of the river, in lagoons (protected bodies of quiet water), or in mud ats (broad,
quiet water areas exposed at low tide) along the shore.
- The silt when lithi ed becomes siltstone, and the mud, when lithi ed, becomes shale or
mudstone.
Diamicton is a very poorly sorted sediment that contains cobbles or boulders surrounded
by a matrix of sand, silt and clay: when lithi ed becomes diamictite.
- Diamictities can form from the lithi cation of debris ows both on land and underwater or
of glacial till, the debris left behind as glacial ice melts.
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