Geology Exam 3
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Title & author of text book.
- How Does the Earth Work?
Age of the Earth
4.5 Billion years
Study: time scale, orders of magnitude
3 ways to estimate rates of earth processes...
- 1. spreading rate of an ocean basin - width/age of oldest rocks found in basin
- 2. vertical uplift associet with orogeny- altitude of mtn/age of fossils
- 3. viscosity of mantle- norway's ice melted and rebounded the depressed lithosphere 500m b/c beach rocks are @ that elevation.
-when rocks form at earth's surface in the layers, the lowest layer formed first and each successively higher layer is younger than the one below
NEW on top of OLD
principle of superposition
sedimentary layers are horizontal, or nearly so, when they were deposited. Nonhorizontal layer indicates disruption of the beds at some time after depostion.
principle of original horizontality
...geologic features, such as dikes and faults, that cut across otherwise continuous rocks formed after the rocks that they cut across.
principles of cross-cutting relationships
...objects enclosed by rock formed prior to inclusion within the rock
principle of inclusions
...former land or seafloor surfaces between rock layers that represent time intervals not recoreded in the local rock record
...separate sedimentary or volcanic rocks that are inclined at different angles. the time required for tilting the lower rocks and eroding them to produce a new surface of deposition is not represented in the geologic record
...separte sedimentary or volcanic rock that are inclined at the same angle but are separated by an irregular erosion surface that indicates a pause in deposition or erosion of the older rocks
...separate sedimentary or volcanic rocks from underlying plutonic-igneous or metamorphic rocks. the time required to erode overlying materials until more recent sedimentation is not represented in the geologic record.
william smith (late 1700s to early 1800s) noted....
the principle of faunal succession - that coal seams in different underground mines were found in predictibable positions between other fossil bearing sediementary layers
principle of faunal succession...
- 1. fossils of different organisms appear at distinct times
- 2. fossils of related organisms appear in teh smae order every place they occur
- 3. fossil species disappear from the rock record everywhere when they become extinct and do not reappear in younger rocks
Geologic time scale.
570 . Paleozoic: come out some day & maybe play poker
245 . Mesozoic: TJC
66 . Cenozoic: pigeon eggs often make people puke
- free oxygen/life at 570 Ma
- first major coals at 245 Ma
- pangea? at 66 Ma ????
absolute dating of earth: 2 ways
- radioactive decay (250 million - 1.3 billion)
- uranium-lead dating/meteorites (4.5 billion)
isotopic applications can be used to....
to determine characteristics about a rock/process similar in half-life years as isotopic system.
continental crust characteristics
- 25-50 km thick
- plutonic 7 metamorphic rocks
- rock comp is felsic (quartz) in upper crust & mafic (amphilbole/olivine) in lower crust
- deepest drillhole is 12km
oceanic crust characteristics
- 7 km thick
- mafic igneous composition
- gabbro sills. basalitic dike. basalt lava flows. sea floor.
- deepest drillhole is 2.1 km
mantle composition is...
how do geologists learn about earth's interior?
- earth's overall density has been calculated to be 5.5 g/cm3knowing the density of crustal rocks and peridotitie tell us teh deeper material must be much denser
How do earthquakes make images of earth's interior?
- surface waves - move in many directions w/ plastic behavior
- body waves - move in 2 directions (p & s) below the earth's surface w/ elastic behavior
A ... wave moves fastest through rock.
A ... does not travel through a gas or liquid, because it twists material at right angles.
distance from epicenter to a seismometer can be expressed 3 ways
- straightline distance
- circumference distance along surface
- angle between the two with the point at the core center of the earth
...areas wehre seismometers do not record p or s waves.
- Shadow zones
- p waves are refreacted through core and mantle boundaries
- s waves are blocked entirely by the liquid outer core
...crust- mantle boundary, which is 5-20 km beneath ocean basins and 25-85 km beneath continents
The core is .... times as dense as the mantle.
much of the hypothetical inner earth gradient is derived from ....
radioactive decay of what elements
geothermal gradients measured near teh surface are higher than the interior.
temp .... depth of 100-150 km
temp .... at moho
temp .... at inner core
- 1000-2000 *C
- 3000-4000 *C
- >5000 *C
heat sources includes
- radioactive decay
- cystallization of iron in the core
- residual heat of accretion
- residual heat of core formation
...transfer of heat and material
how does a lava lamp use all three types of energy transfer?
- conduction at the base
- convection up through the column
- radiation at the top to lose heat
....a property of matter that resiste convective flow, demostrates the principle of driving force versus resisting force, i.e. metal marble on top of a stick of butter
heat eneryg is more than .... times greater than annual human energy consumption
explain mantle convection.
- conductive COOLING through the lithosphere (at top )
- conductive HEATING form the core causes convection to occur b/c hotter/less dense material rises
- pressure helps in the whole process as well
explain how chemical convection works in the core.
- compositional differences (rather than temp diff's) produce density variations that cause fluid motion in the outer core
- aka crystallization of iron core
current angular distance between earth's magnetic and geographic poles is about....degrees
Two measurement define the oerientation of the field at any location on earth's surface:
- declination - horizontal angle btwn the mag & geo poles
- inclination - vertical angle btwn the filed force line and earth's surface
why does the magnetic field reverse?
the interactions between teh magnetic fields in the inner and outer cores cause polarity reversals
Alfred Wgener discovered....
plate tectonics/the idea of continental drift- by matching continental boundaries according to shape and geologic features.
the zone of BRITTLE rock btwn the earth's surface and teh asthnosphere.
consists of the entire crust and smidgen of mantle
ultramafic igneous composition
forms the plates of plate tectonics
plastic layer of earth
70 - 660 km deep
part of the mantle
composed of rock peridotite
can flow very slow allowing rigid layer above to move
what is the evidence that plates are rigid?
- earthquakes outline plate boundaries
- volcanoes along plate boundaries
- mountain belts
....explains the magnetic record
evidence that plates move apart at divergent plate boundaries?
- visualizing age of seafloor
- visualizing higher heat flow at ridges
- east africa forming the next ocean basin
how do continental rift valleys become oceans?
- tension w/ a continent creates rift
- gap widen
- new lithosphere forms
- gap widens until continnent splits
- new ocean forms with mid-ocean ridge down the center
evidence that subduction occurs at convergent plate boundaries?
earthquakes under the mountainous continental side of subduction zone Benioff zone)
three types of convergent plate boundaries:
- oceanic - continental ex. south american appalachians
- oceanic - oceanic ex. japan
- continental - continental ex. hymalayas
- hot spots form where narrow columns of unusually hot mantle convectively rise from the moho, occuring on the surface of Hawaii/yellowstone with young volcanic rock or @ iceland haveing divergent zones with unusually prolific volcanism
- plume locations are stationary in the mantle, therefore we can calculate plate velocity
...% of earth's surface is land
...% is ocean
Bimodal distribution: tow elevation are the most common
- .5 km above sea level
- 4.5 km below sea level
principle of isostasy determines....
- determines elevation
- principle explains variation in surface elevations due to type/thickness/density
mountains (orogeny) result from both...
- tectonic deformation
- isostatic response to erosion
erosion and depostion effects are a result of....
- mass is redistributed (deposition) and thickness is changed (erosion), causing the area to rise and the affected area to lower
tectonic shortening and stretching effects are a result of...
- flexural isostasy
- crust is either compressed or stretched and elevation is changed by forming a thick root or rift valley/continental shelf, respectively
why does the sea level change?
- change in amount of water (glacier melt)
- change in depth of basins
- plate tectonics
when the global average lithosphere decreases, the average seafloor elevation...
rank major mnts youngest to oldest:
- canadian shield
...one exotic way that continents grow
accretion, non-subductable continental crust, unusually thick oceanic crust, or volcanic-arc crust or (theory II) new subduction zone is formed
one mountain building way that continents grow:
volcanic arcs converge with oceanic lithosphere
layers are continuous until obstructed
principle of lateral continuity
- 1. sequence of events
- 2. time required for each step
fossils occur in 3 types of rocks
the study of ancient life
...are atoms of teh same element with the smae number of protons, but different number of neutrons
...a change in the number or protons, neutrons, or both that transforms an unstable isotope (parent) towards a stable one (daughter)
...permits a compass to work for navigation
...protects earth from harmful cosmic radiation
...allows migrating animals ot find their way
geodynamo result from ... in the outer core
- 1. molten iron conduct electricity and can be magnetized
- 2. electrical current generates the mag field
- 3. earth's rotation influences the convection pattern and orientation of the mag field
oceanic crust is ..... dense than continental crust?
- that's why it subducts & there is no ocean floor older than 180 million yrs
the primary for plate motion is...
dense, sinking subducting slabs (slab pull/slab suction)
what happened at...
- north america was near equator. gondwana
- caledonian mountains, sea level high
- appalachians formed & pagnea assembled
- pangea began splitting (mesozoic)
- africa seperated from south america
- himalayas formed when india hit asia
...buildup of free oxygen
...early multicelled organisms
...extinction of dinosaurs/early primates
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