magnetic anomaly number

age (Ma) from geomagnetic reversal chronology extrapolated in South Atlantic assuming constant rate of spreading

pale

onto

logi

cal a

geSeafloor ages from deep sea drilling versus geomagnetic reversal chronology

data for Atlantic ocean; similar data from older oceans permit reversal chronology to be calibrated back to 180 Ma

Chronology of geomagnetic field reversals recorded on ocean floor

Ocean floor age, millions of years (Ma), determined largely from deep sea drilling (ODP program)

Chronology of geomagnetic field reversals recorded on ocean floor

magnetic anomaly “number” is a convenient identifier of specific features of the magnetic anomaly profiles that have proven useful for correlation between different profiles.

Ocean floor age, millions of years (Ma), determined largely from deep sea drilling (ODP program)

Age of the ocean floor

Spatial correlation of magnetic anomalies produced by the seafloor spreading “tape recorder” of reversals of the geomagnetic field”

Chronology of reversals of geomagnetic field

Map pattern of magnetic anomalies and age of ocean floor

ODP drilling site

Map pattern of magnetic anomaly number

Map pattern of magnetic anomaly number: North Atlantic

5 6

10.9 120.4

anomaly no. 1813 25 31 3421 M16M10N

M4M0 M21 M25

154.3 180

From Muller, et al., 1997

Map pattern of ocean floor age

Continental rifting

continental shelf

Mid-ocean ridge: rift valley

Fracture zones: transform faults

7000

3500

1500

-250

0

-150

0

-500

0 500

-550

0

-450

0

-350

0

-650

0

South Atlantic Mid-Ocean Ridge

7000

3500

1500

-250

0

-150

0

-500

0 500

-550

0

-450

0

-350

0

-650

0

South Atlantic Mid-Ocean Ridge

inactive fracture zone

active transform faultactive ridge crest

Global topography and bathymetry

Bathymetry of world’s oceans: close correlation with ocean floor age shown in next slide

seafloor age, Ma

Ocean floor age from seafloor spreading tape recorder

seafloor age, Ma

Seafloor bathymetry

Close correlation with ocean floor age and bathymetry

spreading ridge

heat flow by conduction

Heat flow by convection

heat flow by conduction

Simple thermal model for spreading ridge showing

• conductive thermal “boundary layer” (= lithosphere)

• convecting mantle beneath.

• (oceanic crust is not shown)

dep

th

sea water

temperature

spreading ridge

heat flow by conduction

Heat flow by convection

dep

th

heat flow by conduction

pro

file

A

profile A

dep

th

The temperature profile B (temperature versus depth) at the ridge crest has a very rapid increase from sea bottom temperatures near 0 C to temperatures of 1300 C near the top of the hot, convecting mantle

sea water

spreading ridge

heat flow by conduction

Heat flow by convection

temperature

dep

th

heat flow by conduction

pro

file

B

profile B

At some distance, x, away from the ridge crest, the seafloor has an age = x/v (where v is half the spreading rate).

During the time since the plate formed at the spreading ridge, the plate has cooled to form a conductive “boundary layer”.

The thickness of the conductive boundary layer increases with time.

dep

th

sea water

v v

x

temperature

spreading ridge

heat flow by conduction

Heat flow by convection

dep

th

temperature

dep

th

heat flow by conduction

pro

file

A

pro

file

B

profile A

profile B

dep

th

The decrease in temperature due to cooling of the plate as it moves away from the ridge crest leads to• thermal contraction and• consequent deepening of the ocean bottom.

Thus the depth of the ocean increases with the age of the sea floor.

sea water

0 10 20 30 40 50 60 70 80 90 100-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

TIME, MY

- D

EP

TH

, KM

OCEAN DEPTH VERSUS TIMEocean depth

vrs

age of ocean floor

See thermal.pdf for derivation of these curves

.

0 10 20 30 40 50 60 70 80 90 100

10

20

30

40

50

60

70

80

90

100

110

120L

ITH

OS

PH

ER

E T

HIC

KN

ES

S, K

M

AGE, MY

LITHOSPHERE THICKNESSVERSUS AGE

0

Thickness of conductive boundary layer (lithosphere)

vrs

age of ocean floor

Subducted oceanic

lithosphere

Subdu

cted

ocea

nic

litho

sphe

re

Oceanic crust“island arc”

subduction zone

What happened to the sub-oceanic plates older than 200 Ma?

Subduction!

Continental Margin subduction zone

trench

trench

volcanic arc

seafloor age, Ma

ocean age from seafloor spreading tape recorder

0 Ma

NAM

SAM

AFR

EUR

20 Ma

40 Ma

60 Ma

80 Ma

100 Ma

120 Ma

140 Ma

160 Ma

180 Ma

The reconstructions are from C.R. Scotese’s PALEOMAP Project

Plate motions: the movieClick on the above to play the the “movie”, a file called SFS.MOV. This is a large file (38+ mb); at first it will be slow as the file is downloading, but then you can run it back and forth quite rapidly. It will play faster if you first right-click on the link above, download the file, and then run it from your own machine. Alternatively, you can find the same file (SFS.MOV) on our class folder in the computer lab. The CD that comes with the class text (Stanley) has a lower resolution version of the same reconstruction.

The file shows accurate reconstructions of plate positions based on fitting the map patterns of ocean floor ages decoded from the map patterns of magnetic reversals recorded by the seafloor spreading “tape recorder”.The reconstructions are from the PALEOMAP Project (C.R. Scotese) at the University of Texas in Austin.