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Assigned: October 26
Due: November 2
Get the fill in sheets here!
The Excel file has two worksheets in the workbook,
one with the met ID sheet and the other with the Sed ID sheet.
Fill in the rock identification sheets for the sedimentary rocks 53 - 72. Do not do #
59 (orthoquartzite).
Hints
- Note that the sedimentary rocks in the trays
are organized with clastic rocks first (increasing in grain size with increasing number)
then chemical and biochemical rocks.
- Use the text as a guide to composition and
texture (see fig. 4.3).
- Note any sedimentary structures/unusual
features that may aid in identification in the "other" column e.g. reaction with
HCL in limestones.
Sedimentary
Rocks
Sediment refers to any loose or fragmented material produced by mechanical or chemical
weathering. Various processes operating at the earth’s surface (e.g. running water,
wind, glaciation, biological action) produce, transport and deposit sediment.
Lithification occurs via processes such as compaction, cementation and direct
precipitation. Sediment may be sorted and rounded during transport and deposition. The
great bulk of most sedimentary rocks is comprised of only four constituents: (1) quartz,
(2) calcite, (3) clay minerals, and (4) rock. Common environments for sedimentation
include fluvial, esturine, deltaic, deep and shallow marine, intertidal, reefs, deserts,
glacial, lagoonal e.t.c.
Classification
Sedimentary rocks are classified on the basis of texture, composition and process of
formation. Two main groups are recognized based on how the rock was formed:
- Clastic
- formed from mechanically weathered/transported fragments of other rocks,
minerals, etc.
- Nonclastic - formed by chemical processes (e.g. precipitation of dissolved
minerals from water) or biochemical (organic) processes (e.g. precipitated from organisms
or comprised of organic fragments).
Within these groups further classification is based upon texture and composition.
Clastic Rocks
- Consist predominantly of fragments and debris
of other rocks.
- Size of the clastic particles ranges e.g.
coarse grained > 2 mm, medium grained 1/16 - 2 mm, fine grained 1/256 - 1/16 mm, very
fine grained < 1/256 mm. Particle size provides information on transport and
depositional environment.
- Sediments may vary in degree of rounding.
Rounding provides information on distance of transport.
- Sediments may vary in degree of sorting.
Sorting provides information on transport and depositional environment.
- May contain cement altering the texture of
the rock e.g. calcite, quartz, iron, chert.
- May exhibit sedimentary structures e.g.
stratification, graded beds, trace fossils, tool marks e.t.c. indicating environment of
deposition.
Nonclastic Rocks
May be further split into chemical and organic (biochemical) rocks.
Chemical Rocks
- Precipitated directly from fluids as the
result of evaporation or changes in the fluid chemistry.
- Limestone and dolomite most abundant
chemically formed rocks.
- Crystalline textures may be described as
coarse > 2 mm, medium 1/16 - 2 mm, and fine or microcrystalline < 1/16 mm.
- Common materials in chemical rocks include
calcite, opal/chert, halite, gypsum.
- May exhibit oolitic texture, banded
appearance.
Organic Rocks
- Most composed of fragments of calcite shells
of invertebrate animals e.g. coquina, fossiliferous limestone, chalk, diatomite.
- Texture may be similar to a clastic texture
but the material will be organic and originate in the environment of deposition.
- Some organic rocks formed from alteration of
plant debris e.g. peat, coal.
Examine the metamorphic rocks 73, 75, 77, 78, 79, 80, 81, 82, and 84 (skip 74, 76, 83).
Fill in the metamorphic rock ID sheets provided. You may use this ID sheet to identify
metamorphic rocks during the final exam.
Hints
- Use figures 5.2 and 5.3 in the text plus
handouts as a guide to precursor rocks and metamorphic grade
- To decide what minerals may be present think
about what minerals you may expect to find in the precursor rock
- List metamorphic grade as high, medium, low.
Non-foliated metamorphic rocks are usually low to medium.
Although the text states that slate
is the precursor to phyllite, phyllite is the precursor to schist, and schist the
precursor to gneiss, in a continuing series of increasing metamorphism, this may not
always be the case. Schists and gneisses may also form from coarser, crystalline
sedimentary rocks (e.g. sandstones) and also possibly from igneous rocks e.g. granites,
basalts etc. The mineral content may give you a clue to the precursor rock.
Metamorphic Rocks
A metamorphic rock is a pre-existing rock that has been altered physically and/or
chemically by heat (> 200 °C), pressure (> 0.1 GPa), and interaction
with hydrothermal fluids whilst in the solid state.
Typical Metamorphic Scenarios
- Subduction zones - pressure/friction between
lithospheric plates, increasing temperature with depth (see p. 62 in lab text).
- A collision boundary between continental
lithospheric plates - generates high pressures and temps due to collision and burial
during mountain building (orogenisis) e.g. Himalayas.
- Magmatic emplacement - may vary in size from
regional e.g. subduction zones, to local e.g. small dike (Contact metamorphism).
- Fault and fold zones - friction/pressure due
to tectonic movement
- Areas of crustal weakness/fracturing - many
of the scenarios described above fracture the earth’s crust allowing hydrothermal
fluids to migrate through the rocks, leading to metamorphism.
Two scales of metamorphism are
usually distinguished, regional metamorphism (large scale, covers whole regions,
>100 mi3) and contact metamorphism (small scale, localized
metamorphism only, may be on the order of mm to m).
Mineral changes during metamorphism (Metamorphic Processes)
- Recrystallization - increase in
crystal grain size, no change in composition
- Neomorphism - change in crystal grain
size and composition using only the elements present in the system (closed system)
- Metasomatism - change in mineral
composition as elements are lost or added to a system via hydrothermal fluids (open
system)
Classification of metamorphic rocks
is based on texture and composition. Main textural categories are foliated and non-foliated.
Within foliated rocks, texture is further described as
- Slaty cleavage – implies nearly
perfect, planar, parallel foliation of very fine grained platy minerals (think of what
slate looks like) Source: Siltstone, fine grained sedimentary rocks
- Phyllitic texture – a wavy
foliation of fine grained platy minerals (e.g. mica, chlorite), exhibiting a shiny or
glossy luster. Phyllite (as the name suggests) exhibits a phyllitic texture. Basalt,
Slate
- Schistose texture (schistosity) –
parallel to sub-parallel foliation of medium to coarse grained platy minerals. Medium
grained igneous rocks, phyllite
- Gneissic texture - parallel to
sub-parallel foliation of medium to coarse grained platy minerals in alternating layers of
different composition (dominated by different minerals). Granite, Schist
Non-foliated metamorphic rocks lack
obvious layers of platy minerals. They may rather exhibit a granular crystalline texture
with randomly oriented, equally sized, visible crystals that have recrystallized from
their precursor rock.
Some metamorphic rocks contain large crystals called porphyroblasts, set in a
finer grained groundmass (analogous to phenocrysts in igneous rocks). This porphyroblastic
texture may be present in both foliated and non-foliated metamorphic rocks.
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