Rob

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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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Parks and Plates©2005 Robert J. Lillie

Topography of the Ocean FloorIn

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Nat

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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

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Distance (km)

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0 500 1000 1500 2000

Bouguer Graviy

Mga

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Example from Arabia

A

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Distance (km)

Dep

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0 500 1000 1500 2000

Data Source: Cornell University

Bouguer Graviy

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• 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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