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![Page 1: Shelter Rock and the Geologic History of Long Island Dr. J Bret Bennington Hofstra University What is Shelter Rock?](https://reader035.fdocuments.in/reader035/viewer/2022062322/56649cb85503460f9497e052/html5/thumbnails/1.jpg)
Shelter Rock and the Geologic History of Long
Island
Shelter Rock and the Geologic History of Long
Island
Dr. J Bret BenningtonHofstra University
What is Shelter Rock?
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It’s a really big rock!
• App. 40’ long by 20’ wide by 17’ high• App. 1800 tons• Largest boulder on Long Island• Also known as “Manhasset Rock” and “Milestone Rock”• Archeological evidence for use as a temporary shelter by Matinecock Indians• Not originally part of Long Island
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How did Shelter Rock form?Where did it come from?
How did it get here?
Geologists study rocks because every
rock has a story to tell!
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Shelter Rock looks like a kitchen countertop.
• Large randomly distributed crystals
• Quartz, feldspar, biotite mica
• GRANITE - an igneous rock
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So, what is the origin of Granite?
Igneousrock
Crystallized from
molten magma
Magma
But where does magma come
from?
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Magma originates in the hot, upper mantle of rocky planets like the Earth, Venus, and Mars
Mars has only one kind of igneous rock• basalt• same as the ocean floor and volcanic islands on Earth• Granite is very different from basalt.• Mars doesn’t make granite.• How does the Earth make granite?
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Dry mantle magma(basalt)
Continental crust magma(granite)
Plate Tectonic Setting
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In the distant geologic past, Shelter Rock was part of a deep magma
chamber above an ocean-continent collision zone.
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Garnet
Layered Mica
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What explains these mineral changes in the
Granite?
Igneousrock
MetamorphicRock
Changed by heat and pressure.
Shelter Rockis actually aMetagranite
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• Mountain building events (orogenies) are usually caused by collisions between the Earth’s tectonic plates.
Mountain building buries rock under tremendous pressure and
heat, causing metamorphic changes.
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Taconic Mountain Building Event(450 million years ago)
Eastern North America Volcanic Island Arc
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Taconic Mountain Building Event(450 million years ago)
Eastern North America Volcanic Island Arc
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Taconic Mountain Building Event(450 million years ago)
Deep igneous rock is metamorphosed
Eastern North AmericaVolcanic Island Arc
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After forming as granite, Shelter Rock was
metamorphosed under a rising mountain range.
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Modern Geologic Time Scale
Pa
leo
zoic
Ordovician
Cambrian
Silurian
Devonian
Mississippian
Pennsylvanian
Permian
Me
so
zoic
Triassic
Jurassic
Cretaceous
Paleocene
Eocene
Oligocene
Miocene
Pliocene
Pleistocene
Ce
no
zoic
Te
rtia
ry
Holocene
Qu
at.
Pal
eoge
neN
eoge
ne
540
510
439
408
360
323
290
245
208
146
57
35
23
5
1.6
.01
0
Ca
rb.
Quat. = QuaternaryCarb. = Carboniferous
Ma
65RIP
Hadean
Archean
Proterozoic
Phanerozoic
3800 Ma
4600 Ma
2500 Ma
540 Ma
0 Ma
M
C
P
Grenville
Taconic
Acadian
Alleghenian
Orogenies
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Radiometric Dating of Rocks
During formation of Igneous and Metamorphic Rocks...• Parent atoms are trapped during crystallization of
minerals.• Blocking temperature - mineral becomes a closed
system.• The clock starts as daughter atoms form and remain
trapped in the rock.
Zircon
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U
UUU
UU
U
U
UU
U
UUU
U U
U
U
UU
U
UU UU
U
U
U
UU
T = 0 half lives(rock crystallizes, blocking temperature reached)
U
UU
UU
U
UU
U
U
U U
U
U
U
PbPb
PbPb
Pb
Pb
Pb
Pb
Pb
PbPb
PbPb
Pb
Pb
T = 1 half life(1 billion years old)
T = 2 half lives(2 billion years old)
U
U
UU
U U
U
U
PbPb
PbPb
Pb
Pb
Pb
Pb
Pb
PbPb
PbPb
Pb
Pb
Pb
Pb
PbPb
Pb
Pb
Pb
U U
U
U
PbPb
PbPb
Pb
Pb
Pb
Pb
Pb
PbPb
PbPb
Pb
Pb
Pb
Pb
PbPb
Pb
Pb
Pb
Pb
Pb
Pb
Pb
T = 3 half lives(3 billion years old)
Zircon grain
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Mass spectrometer - counts number of atoms of each isotope in a mineral sample.
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Shelter Rock is also a large sedimentary particle,
eroded, transported, and deposited on Long Island.
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Igneousrock
Cooled frommolten rock
The Rock Cycle
Magma / Lava
Uplift and Erosion
SedimentaryRock
Deposited inlayers on thesurface.
Uplift and
Erosion
Redeposition
burial and subsidence
MetamorphicRock
Changed by heat and pressure.
melting
uplift and erosion
Burial and subsidence
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Igneousrock
Cooled frommolten rock
The “Shelter Rock” Cycle
Magma / Lava
Uplift and Erosion
SedimentaryRock
Deposited inlayers on thesurface.
MetamorphicRock
Changed by heat and pressure.
Burial and subsidence
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What force could possibly move a boulder as large as
Shelter Rock?
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What force could possibly move a boulder as large as
Shelter Rock?
Ice!
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Formation of Long Island
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Kettle
Kame
Outwash
Moraine
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Major Glacial Features of Long IslandMoraines
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Major Glacial Features of Long IslandMoraines
Roanoke Point Moraine
Ronkonkoma Moraine
Harbor Hill Moraine
?
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Glacial till
Loess
erratics
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Erratic boulder, Lloyd Neck, New York
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Glacial polish, erratic boulder
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Tunnel Valleys - narrow valleys carved by water flowing beneath the glacier
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outwashplain
Harbor Hillmoraine
tunnelvalleys
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kam
e and kettl
e topogra
phy
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Glacial flutes - Central Park
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Glacial grooves - Central Park
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Orchard Beach, Bronx
N
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Long Island
Glacial striations
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Glacial striations
N
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Questions?Questions?