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Rob
ert J
. Li l l
i eR
ober
t J. L
i l li e
Himalayas, I ndia/ Tibet
Very High Mountains During Collision
Parks and Plates©2005 Robert J. Lillie
Ali Onceloncel@kfupm.edu.sa
Department of Earth SciencesKFUPM
Gravity and Isostasy
Introduction to GeophysicsIn
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Previous Lecture
Gravity UnitsExample 1: Mars's twin moons
Example 2: The Valley and Ridge Province (1-5 mgals) Sensitivity: Density Distribution
Sensitive to changes in Mass (Density) changes Densities of Geological Materials On-Line Gravity Map Construction ToolColor table for standard Gravity map
Example 1: North Anatolian Fault Zone in the sea of Marmara
Example 2: Ghawar region of Saudi Arabia
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Parks and Plates,pp.27©2005 Robert J. LillieIn
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Parks and Plates
©2005 Robert J. Lillie
Parks and Plates
©2005 Robert J. LillieIntr
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Parks and Plates©2005 Robert J. Lillie
http://www.moorlandschool.co.uk/earth/tectonic.htm
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©19
77, M
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Parks and Plates©2005 Robert J. Lillie
Topography of the Ocean FloorIn
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Nat
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Spac
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Two Forms of Isostatic Uplift
pp.28 Parks and Plates ©2005 Robert J. LillieIntr
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Low density due to higher temperature
http://www.moorlandschool.co.uk/earth/tectonic.htm
Two Forms of Isostatic UpliftIn
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What is Isostatic Equilibrium?
• Crust floats on Mantle (hydrodynamic equilibrium)
• Surface elevation is function of underlying mass (thickness and density of crust)
• Surface elevation is compensated by mass below it
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Bouguer Anomaly
•Slightly positive over oceans•Slightly negative over continentsWhy?
• Because a uniform density was used in International Gravity Formula
2.67 g/cm3
3.3 g/cm3
Mantle
http://www.moorlandschool.co.uk/earth/tectonic.htm
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Example from Arabia
A
B
0
1
2
A B
Distance (km)
Dep
th (
km
)
0 500 1000 1500 2000
Bouguer Graviy
Mga
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0 500 1000 1500 2000
100
0
-100
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Example from Arabia
A
B
0
1
2
A B
Distance (km)
Dep
th (
km
)
0 500 1000 1500 2000
Data Source: Cornell University
Bouguer Graviy
Mga
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100
0
-100
• Bouguer anomalies that are negative over high areas and positive over oceans. •Near sea level, the average is close to zero.• The negative Bouguer anomalies areas due to the excess of low density material at depth in mountainous regions when compared to rocks at depth in areas of low relief near sea level.
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Homework, Due to May 17
Use the online tool for making a gravity map from the site http://www.itismolinari.mi.it/Gravity.htm.Make your gravity map and compare changes in the gravity to changes through regions of major tectonics? What is the gravity through the areas:
a) Red Sea b) Dead Sea Fault c) Iranian Sea d) Gulf of Aqaba e) Zagros Thrust Fold
33° E 60° E
10° N
37°N
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The quiz of the week
For the following equation, define the variables:
FAA= gobs – gth +FAC
BA = FAA – BC + TC
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