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Geology I - Tufts University


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Medford Gabbro - Radiometric Date
180 million years
Quincy Granite - Radiometric Date
420 million years
Salem Gabbro - Radiometric Date
485 million years
Dedham Granite - Radiometric Date
585 million years
Beginning of Earth
4,500 million years
End of Precambrian
585 million years
End of Palezoic
245 million years
End of Mesozoic
66 million years
Order of Geologic Time Scale from most recent to earliest
Quarternary, Tertiary, Cretaceous, Jurassic, Triassic, Permian, Pennsylvanian, Mississippian, Devonian, Silurian, Ordovician, Cambrian, Precambrian
Relationship between Tufts Sandstone, Brighton Volcanics, Cambridge Mudstone, Roxbury Conglomerate, and Dedham Granite
Unconformity in Dedham Granite with Roxbury on top (derived fragments as well); Roxbury interfingers Cambridge M.S.; Brighton and Tufts slightly younger
Ages of Wamsutta Formation and Pondville Conglomerate
Age of Lynn Volcanics
Salem Gabbro and Quincy Granite have to intrude Lynn so Lynn is older than 485 million years
Boston area folding
Wamsutta and Pondville are folded but Medford Gabbro is not - folding occured between Penn. and Jurassic
Fault types: Strike-Slip
Occurs along strike: right lateral or left lateral
Fault types: Dip Slip: Normal
Hanging wall moves down (a fault due to gravity where rocks are pulling apart)
Fault Types: Dip-Slip: Reverse
Hanging wall moves up...>45 = reverse; <45 = thrust; <10 = overthrust
Fault Types: Oblique
Combination of strike-slip and dip-slip faults
Fold Types
Synclines - youngest in the middle, arrows facing inward; Anticlines - oldest in the middle, arrows facing outward
Columnar - top breaks during cooling down into columns; Rectangular - if everything cools at same time, shrinks into subrectangular plane
Earthquake waves (in order of arrival)
P-waves (solids and liquids), S-waves (solids only), L-waves (rolling or elliptical, etc). Difference in arrival used to determine epicenter
Earthquakes Scales and Magnitudes
Richter scale based on numbers and calculations; Mercalli intensity scale based on extent of damage seen
Sediments on top of igneous
Gap in geologic time
Angular unconformity
Horizontal on top of tilted or folded rocks
Do sequence diagrams on p. 24 - 27 in lab manual
Minerals in Basalt/Gabbro
Olivine, Pyroxene, Plagioclase (Ca and Na)
Minerals in Andesite/Diorite
Plagioclase, Amphibole, minor Pyroxene
Minerals in Rhyolite/Granite
Abundant Quartz and Potassium Feldspar
Bowen's Reaction Series
Series that demonstrates the full range of igneous rocks which could be produced from the same mafic magma
What element becomes more and more present as the composition goes from Ultramafic to Felsic
SiO, Silicia
Minerals produced by Ultramafic
Mostly Olivine, Ca-Plagioclase
Minerals produced by Mafic
Ca-Plagioclase, Pyroxene, Olivine
Minerals produced by Intermediate
Ca-Na-Feldspar, Mica, Amphibole
Minerals produced by Felsic
K-Feldspar, Quartz
Moving magma incorporates preexisting rock as it rises; unmelted "foreign" rocks are carried
Dome shaped and concordant
Sag downward
Massive disconcordant pluton with huge surface areas
Mineral cements in sedimentary rocks; composition
Quartz;SiO2 Carbonite;CaCO3 Hematite;FeO
T/F: The half life of a radioactive element becomes shorter as time passes
T/F: Radioactive radon gas forms as one short step in the slow decay of uranium into lead
What do pyroclastic deposits contain?
Pumice, glass, etc
Angular mix of rocks
Breccia - forms when one magma invades another then pulls out
Deer, bear, primate fossils
Fish fossils
Dinosaur fossils
Three steps in divergent plate margins
Rift valley, young narrow sea, then a mature ocean basin
Types of convergent plate boundaries
Ocean-ocean, ocean-continent, continent-continent
Temp at which basalt curies and iron lines up with magnetic field
Three pieces of evidence for midocean ridge spreading
Ages of igneous rocks, ages of sedimentary rocks, magnetic reversals
Name for crust
Name for mantle
At what depth do shear waves slow down?
70 - 200 km (Low Velocity Zone)
Bowen's Diagram: Sialic rocks
felsite, GRANITE
Bowen's Diagram: Intermediate rocks
andesite, DIORITE
Bowen's Diagram: Mafic
basalt, GABBRO
Bowen's Diagram: Ultramafic rocks
Contact metamorphism and replacement deposits
Acid wears away limestone and valuable ore sits in cracks and caves
Disseminated deposit
Water level lowers and supergene rests at new water table
Concentration of valuable elements in crust
Cu --> <1/100%; Silver --> <1/100,000%
Source rock
Often shale, with large accumulation of organisms and RAPID DEPOSITION and burial
Pressure, temperature, and time makes heavy oil into light oil and natural gas
Reservoir Rock
Porous and permiable, like limestone and sandstone where fluids can flow through or rest in rock
Cap Rock/Traps
Impermiable rock like shale which traps the oil
To find oil
Exploration, seismic surverys, drilling (randomly = wildcat well vs. production wells)
Plateau basalts
Fissure eruptions covering thousands of square miles
Caldera forming event

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