Why Are the Appalachian
Mountains So High?
John Cipar, PhD
Weston Observatory
Boston College
April 16, 2014
1
Adirondacks from Mt. Marcy, NY
Steep Topography
(30 m at Lake Champlain)
High elevation
(1630 m at summit)
Photo by Jef Renaud
Old Rocks from deep crust
(1200 Ma Anorthosite, crystallized at 15-20 depth)
2
Eastern North America Topography
Canadian
Shield
Adirondacks
Northern
Appalachians
Catskills
Alleghany
Plateau
Central
Appalachians
Blue Ridge
Southern
Appalachians
Coastal
Plain
From Geological History of Jamestown, Rhode Island web site
3
Earth’s Interior
• Pressure at center is 350 Gpa
(3,500,00 atmospheres)
UC Berkeley Seismo Lab
• Temperature at center: 5430 ⁰C
(9800 ⁰F)
4
Continents v. Oceans
Continental crust
• Light granite: “floats” on surface
• Formed by continental collision
• Old (4000 My) to very young
30-50 km
Oceanic crust
• Heavy basalt: subducts into mantle
• Formed at mid-ocean ridges
• Young <185 My to right now!
Rigid & elastic
PhysicalGeography.net
5
Plate Tectonics
• Continental Drift was first proposed
by Alfred Wegner in 1912
• Strong geological evidence for drift,
but no physical mechanism for moving
the continents through the oceans
• Modern theory of sea floor spreading
/Plate Tectonics developed in 1950s 1970s
US Geological Survey
6
Plate Map
2011 Tohoku, Japan
earthquake
Boston
2004 Sumatra
earthquake
Earth’s Dynamic Systems /W. Kenneth Hamblin and Eric H. Christiansen
7
Plate Boundaries
8
Hawaiian Hot Spot
May 30, 2011
9
Subduction zones
Earth’s Dynamic Systems /W. Kenneth Hamblin and Eric H. Christiansen
10
Wilson cycle
Lynn S. Fichter/James Madison University
11
Building the Appalachians:
Precambrian to Paleozoic
12
Geology of North America
13
Geologic Map of Eastern North America
USGS
14
“Suspect” Terranes
Hibbard et al, 2006
15
Rodinia - 700 My
Laurentia
Paul Hoffman, Science, v. 252, p. 1409
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Passive Margin
Early Cambrian - 550 My
Laurentia
(North America)
Iapetus
Ocean
Amazonia
(South America)
• Maps from Ron Blakely
• Cross-sections from Virginia
Geology web site and USGS
17
Taconic Orogeny
Ordovician - 450 My
Ron Blakely and USGS
Boston
18
Acadian Orogeny
Devonian - 385 My
Ron Blakely and USGS
19
Alleghany Orogeny
Pennsylvanian & Permian - 290 My
North America
The Paleozoic Appalachians
were probably similar to
today’s Himalaya Mountains
Boston
Africa
Ron Blakely and USGS
20
Rifting of Pangaea
Triassic - 195 My
North America
Africa
Ron Blakely and JMU
21
Triassic rifting:
Opening of the Atlantic
~200 My
~185My
Earth’s Dynamic Systems /W. Kenneth Hamblin and Eric H. Christiansen
22
“How many years can a mountain exist
Before it's washed to the sea?”
- Bob Dylan, Blowin’ in the Wind
23
The Last 200 My
• Eastern North America is a “passive” margin
– Sedimentation along the Atlantic coast
– Far from active plate boundaries
• Passage over the Great Meteor hotspot 12590 My
Unsolved question is the persistence of high
Appalachian topography to the present
24
Great Meteor Hotspot Track
125 My
White Mountains
125-100 My
105 My
85 My
McHone, 1996
Hot spot in New England 125-100 My
25
North America 115 My ago
Greenland
North
America
Great Meteor
hotspot
Farallon
plate
Atlantic
Ocean
Map by Ron Blakely
26
Evidence for Hotspot Uplift
Devonian sediments are truncated
Crough, 1981
27
Thermochronology
Apatite grain w/tracks
Apatite fission track (AFT) ages tell when a rock cooled
through approximately 100 °C, or about 4-5 km depth.
Assumed geothermal
gradient converts
temperature to depth
28
New England AFT ages
125-178 My
~70-125 My
125-178 My
Roden-Tice and Tice, 2005
29
Offshore sediments
30
Sediment Supply to Atlantic Basins
Pulse in sedimentation (& uplift?) ~15 My
31
Seismic structure of the Earth
Stephen A. Nelson, Tulane University
32
S-wave velocities in New England
Slow S-wave ≡ high T
Li et al (2002)
Remnant of Great Meteor hot spot ?
33
Structure of North American mantle constrained by simultaneous inversion of multiple‐frequency SH,
SS, and Love waves
Tian, Yue, Ying Zhou, Karin Sigloch, Guust Nolet, and Gabi Laske (2011)
Map view
Cross-section
Slow S –wave velocity in NH & VT
Journal of Geophysical Research: Solid Earth
Volume 116, Issue B2, B02307, 15 FEB 2011 DOI: 10.1029/2010JB007704
http://onlinelibrary.wiley.com/doi/10.1029/2010JB007704/full#jgrb16621-fig-0013
Farallon plate beneath North America
eastern US
High S velocity =
• low temp
• high density
35
IRIS Earthscope TA Stations
Incorporated Research Institutions in Seismology
36
The “Great” Unconformity
near Alexandria Bay, NY
540 My
1100 My
~560 My
Chris Murray, Thousand Islands Life web site
37
Saga of the Potsdam sandstone
38
Questions?
39
Further Reading
• Mountain Press Roadside Geology Series
– Fr. James Skehan, S.J.: MA and CT/RI
• USGS Geology of the New York City Region
– http://3dparks.wr.usgs.gov/nyc/index.html
• Geological Evolution of Virginia and the Mid-Atlantic Region – Lynn
S. Fichter/James Madison University
– http://csmres.jmu.edu/geollab/vageol/vahist/
• Geological History of Jamestown, Rhode Island
– http://www.jamestown-ri.info/northern_appalachians.htm
• PhysicalGeography.net
– http://www.physicalgeography.net/
• Ron Blakely Paleogeographic Maps
– http://cpgeosystems.com/paleomaps.html
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Bibliography
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Crough, S. Thomas (1981). Mesozoic hotspot epeirogeny in eastern North America, Geology, v. 9, pp. 2-6.
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