Shelter Rock and the Geologic History of Long Island Dr. J Bret Bennington Hofstra University What is Shelter Rock?

It’s a really big rock! App. 40’ long by 20’ wide by 17’ high App 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? Igneous rock 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? Igneous rock Metamorphic Rock Changed by heat and pressure. Shelter Rock is actually a Metagranite

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 AmericaVolcanic Island Arc

Taconic Mountain Building Event (450 million years ago) Eastern North AmericaVolcanic 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 Paleozoic Ordovician Cambrian Silurian Devonian Mississippian Pennsylvanian Permian Mesozoic Triassic Jurassic Cretaceous Paleocene Eocene Oligocene Miocene Pliocene Pleistocene Cenozoic Tertiary Holocene Quat. Paleogene Neogene Carb. Quat. = Quaternary Carb. = Carboniferous Ma 65 RIP 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 UU U UU UUU U U U U U U U U U U U U UUUUU U UU U T = 0 half lives (rock crystallizes, blocking temperature reached) U U U U U U U U U U UU U U U Pb T = 1 half life (1 billion years old) T = 2 half lives (2 billion years old) U U U U UU U U Pb UU U U 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.

Igneous rock Cooled from molten rock The Rock Cycle Magma / Lava Uplift and Erosion Sedimentary Rock Deposited in layers on the surface. Uplift and Erosion Redeposition burial and subsidence Metamorphic Rock Changed by heat and pressure. melting uplift and erosion Burial and subsidence

Igneous rock Cooled from molten rock The “Shelter Rock” Cycle Magma / Lava Uplift and Erosion Sedimentary Rock Deposited in layers on the surface. Metamorphic Rock Changed by heat and pressure. Burial and subsidence

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 Island Moraines

Major Glacial Features of Long Island Moraines 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

outwash plain Harbor Hill moraine tunnel valleys

kame and kettle topography

Glacial flutes - Central Park

Glacial grooves - Central Park

Orchard Beach, Bronx N

Long Island Glacial striations

N

Questions?