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?

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

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!

Shelter Rock looks like a kitchen countertop.

• Large randomly distributed crystals

• Quartz, feldspar, biotite mica

• GRANITE - an igneous rock

So, what is the origin of Granite?

Igneousrock

Crystallized from

molten magma

Magma

But where does magma come

from?

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?

Dry mantle magma(basalt)

Continental crust magma(granite)

Plate Tectonic Setting

In the distant geologic past, Shelter Rock was part of a deep magma

chamber above an ocean-continent collision zone.

Garnet

Layered Mica

What explains these mineral changes in the

Granite?

Igneousrock

MetamorphicRock

Changed by heat and pressure.

Shelter Rockis actually aMetagranite

• 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.

Taconic Mountain Building Event(450 million years ago)

Eastern North America Volcanic Island Arc

Taconic Mountain Building Event(450 million years ago)

Eastern North America Volcanic Island Arc

Taconic Mountain Building Event(450 million years ago)

Deep igneous rock is metamorphosed

Eastern North AmericaVolcanic Island Arc

After forming as granite, Shelter Rock was

metamorphosed under a rising mountain range.

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

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

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

Mass spectrometer - counts number of atoms of each isotope in a mineral sample.

Shelter Rock is also a large sedimentary particle,

eroded, transported, and deposited on Long Island.

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

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

What force could possibly move a boulder as large as

Shelter Rock?

What force could possibly move a boulder as large as

Shelter Rock?

Ice!

Formation of Long Island

Kettle

Kame

Outwash

Moraine

Major Glacial Features of Long IslandMoraines

Major Glacial Features of Long IslandMoraines

Roanoke Point Moraine

Ronkonkoma Moraine

Harbor Hill Moraine

?

Glacial till

Loess

erratics

Erratic boulder, Lloyd Neck, New York

Glacial polish, erratic boulder

Tunnel Valleys - narrow valleys carved by water flowing beneath the glacier

outwashplain

Harbor Hillmoraine

tunnelvalleys

kam

e and kettl

e topogra

phy

Glacial flutes - Central Park

Glacial grooves - Central Park

Orchard Beach, Bronx

N

Long Island

Glacial striations

Glacial striations

N

Questions?Questions?