Geology Test Two
Terms
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- Metamorphic Rocks
- Rocks altered by heat, pressure and/or chemical activity. The amount of alteration determines the metamorphic grade.
- Low Grade
- very little change
- High Grade
- massive alteration to structure and/or mineralogy. Dolostone and coal are metamorphic.
- Changes to Rock Texture (3)
- Growth of mineral crystals Alignment or minerals (foliation) Formation of new minerals
- Mineralogical Changes
- As temperature and pressures increase, some minerals transform into others. One possible sequence: Clay-Chlorite-Muscovite Muscovite-Biotite-Garnet Garnet-Staurolite Note: early structural changes are minor
- Types of Alterations
- Heat Pressure Chemical
- Heat
- Caused by proximity to magma (or less commonly lava) or deep burial. Amount of alteration depends upon the temperature, time, and mineralogy. Result- minerals tend to grow larger or may alter into new minerals.
- Pressure
- Caused by deep burial or tectonic activity. Usually accompanied by an increase in temperatures, but now always. Result- rocks may become foliated and minerals may grow in size, new minerals may form, or minerals may be pulverized.
- Chemical
- Usually due to water Often caused by reactions with chemically active fluids migrating out of nearby magma or forced out of pre-existing rocks by high pressures. Result- recrystallization of existing minerals or formation of new minerals.
- Types of Metamorphism
- Contact Regional Dynamic
- Contact
- Alteration due to nearby magma Types of alteration: heat/chemical Degree of alteration depends on: -Composition of magma -Types of preexisting rock -Distance from magma body -Amount of fluids -Temperature
- Regional
- Alteration usually due to mountain building process Types of alteration - Pressure/heat/chemical Degree of alteration depends on: -Amount of pressure -Type of preexisting rock -Amount of fluids
- Dynamic
- Alteration usually due to impacts, fault motion. Type of alteration: PRESSURE Rocks may be shattered and fused back together - Fault Breccia (cataclastic rock).
- Migmatites
- part igneous and part metamorphic, lighter minerals have melted.
- Foliated(4)
- Slate Phyllite Schist Gneiss
- Non-Foliated(2)
- Marble Quartzite
- Slate Protolith and Grade
- Shale Low grade
- Phyllite Protolith and Grade
- Slate Low to intermediate grade
- Schist Protolith and Grade
- Shale Intermediate grade
- Gneiss Protolith and Grade
- Granitic rocks High grade
- Marble Protolith and Grade
- limestone or dolostone moderate
- Quartzite Protolith and grade
- quartz-rich sandstones or chert moderate
- Relative age
- The sequence of past events from oldest to youngest NO NUMBERS
- Absolute age
- The age of events or objects, expressed as a NUMBER. Doesn't work well with sedimentary, mainly igneous.
- Unconformity
- a surface that represents a gap in the geological record
- Disconformity
- an unconformity in which the contact representing missing rock layers separates beds that are parallel to each other
- Angular unconformity
- an unconformity in which the contact separates overlying younger layers from tilted or folded layers that have been eroded
- Nonconformity
- an unconformity in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary rock
- Contacts
- Surfaces separating successive rock layers(beds)
- Formation
- body of rock of considerable thickness with recognizable characteristics allowing it to be distinguished from adjacent rock layers.
- Original horizontality
- beds of sediment deposited in water formed as horizontal or nearly horizontal layers.
- Superposition
- within an undisturbed sequence of sedimentary or volcanic rocks, layers get younger from bottom to top.
- Lateral continuity
- original horizontal layer extends laterally nil it tapers or things at its edges.
- Cross-cutting relationships
- igneous intrusions and faults are younger than the rocks they cut through
- Inclusions
- fragments included in a host rock are older than the host rock. The included rocks are older than the rock that contains them. The rock containing the smaller rock is younger.
- Baked contacts
- contacts between igneous intrusions and surrounding rocks, where surrounding rocks have been contact metamorphosed.
- Correlation
- determining the time-equivalency of rock units within a region, a continent or between continents. Filling in the time gaps.
- Similarity of rock types
- assumes similar sequences of rocks formed at the same time. This can be inaccurate if very common rock types are involved.
- Correlation by fossils
- similar fossil assemblages can be used to back-up rock-based correlation
- Geological Time Scale
- a worldwide relative time scale. It is divide into Eons, Eras, Periods, and Epochs.
- Precambrian Eon
- the vast amount of time prior to the Paleozoic era; relatively few fossils preserved.
- Hadean Eon
- the oldest (has nothing to represent it)
- Proterozoic Eon
- the youngest
- Phanerozoic Era
- the most recent (and current) eon, subdivided into 3 eras: Paleozoic- "old life" many complex fossils Mesozoic- "middle life" dinosaurs, began and ended with major mass extinctions. Cenozoic- "recent life"
- Weathering
- group of processes that cause rocks to disintegrate and decompose at or near the Earth's surface.
- Erosion
- requires the transportation of material by some agent (water, wind, ice, etc.).
- Mechanical Weathering
- the disintegration of rocks and minerals into smaller pieces, without any substantial chemical changes. Done with force and sometimes called physical weathering. Maintains identity, breaks into pieces. increases surface area
- Regolith
- what mechanically weathered rocks become (ground up rocks).
- types of mechanical weathering
- frost wedging growth of mineral crystals root wedging thermal expansion unloading AFFECT SEDIMENTARY ROCKS MOST
- Frost wedging
- water in fractures expands as it freezes, causing rocks to break apart. Common process in mid-latitude climates(works well on shale).
- Growth of mineral crystals
- similar to frost wedging but not involving halite, gypsum or other minerals precipitated from water. This is common in drier climates.
- Root Wedging
- the growth of plan life on most rocks will slowly widen pre-existing fractures, allowing water and roots to extend even deeper.
- Thermal expansion
- rocks exposed to extreme daily temperature ranges may expand and contract enough to weaken their internal structures. Possibly DESERT process.
- Unloading
- release of confining pressure can cause intrusive igneous and some metamorphic rocks to fracture and exfoliate. This is much more common in large masses of rock than smaller ones.
- Chemical Weathering
- very heavily reliant on mechanical weathering. The decomposition of rock by the large-scale alteration of its mineralogical components. Chemical weathering requires the presence of chemically active fluids, usually water. Chemical weathered rocks may become SOIL (turns regolith into soil).
- Types of Chemical Weathering
- Solution- water(calcite, halite, and gypsum) Oxidation- Minerals that have high iron content will oxidize, leading to the formation of hematite and/or goethite. (olivine pyroxene gabbro) Hydrolysis- combine with water to form new minerals (feldspars +waters = clays)