Your Federal Quarterly Tax Payments are due April 15th Get Help Now >>

Geology 8000 by fuv20424


									                                                                           Geology 8000
                                                                     Magnetic Anomalies
                                                                             Ajay Tripathi
                                                                      October 14th, 2003


In the mid-1960’s magnetometer surveys at the sea disclosed some intriguing
characteristics of marine magnetic anomalies. Most magnetic anomalies at sea are
arranged in bands that lie parallel to the rift valley of the mid oceanic ridge. Alternating
positive and negative anomalies form a stripe like pattern parallel to the ridge crest. (see
figure below)

Description of Concepts

Testing the Sea-Floor Spreading Hypothesis

The volcanic rocks which make up the sea floor have magnetization because, as they
cool, magnetic minerals within the rock align to the Earth's magnetic field. During the
study of these rocks, the scientists found out that the intensity of the magnetic field they
measured was very different from the intensity they had calculated. These magnetic
anomalies (or differences in the magnetic field from place to place) were referred to be
either positive or negative. Positive magnetic anomalies are places where the magnetic
field is stronger than expected. Positive magnetic anomalies are induced when the rock
cools and solidifies with the Earth's magnetic north pole in the northern geographic
hemisphere. The Earth's magnetic field is enhanced by the magnetic field of the rock.
Negative magnetic anomalies are magnetic anomalies that are weaker than expected.
Negative magnetic anomalies are induced when the rock cools and solidifies with the
Earth's magnetic north pole in the southern geographic hemisphere.

                                                                                Page 1 of 4
The resultant magnetic field is less than expected because the Earth's magnetic field is
reduced by the magnetic field of the rock.

When mapped, the anomalies produce a zebra-striped pattern of parallel positive and
negative bands. The pattern was centered along, and symmetrical to, the mid-ocean
A hypothesis was presented in 1963 by Fred Vine and Drummond Matthews to explain
this pattern. They proposed that lava erupted at different times along the rift at the crest
of the mid-ocean ridges preserved different magnetic anomalies.

For example, lava erupted in the geologic past, when the north magnetic pole was in the
northern hemisphere, preserved a positive magnetic anomaly.

                                                                               Page 2 of 4
In contrast, lava erupted in the geologic past, when the north magnetic pole was in the
southern hemisphere, preserved a negative magnetic anomaly.

Lava erupting at the present time would preserve a positive magnetic anomaly because
the Earth's north magnetic pole is in the northern hemisphere.
Vine and Matthews proposed that lava erupted on the sea floor on both sides of the rift,
solidified, and moved away before more lava was erupted. If the Earth's magnetic field
had reversed (changed from one geographic pole to the other) between the two
eruptions, the lava flows would preserve a set of parallel bands with different magnetic
properties. The ability of Vine and Matthews' hypothesis to explain the observed pattern
of ocean floor magnetic anomalies provided strong support for sea floor spreading.

                                                                            Page 3 of 4
Two British geologists, Fred Vine and Drummond Mathews, made several observations
about magnetic anomalies. They recognized that the pattern of magnetic anomalies was
symmetrical about the ridge crust. That is, the pattern of magnetic anomalies on one
side of the ridge was a mirror image of the pattern on the other side (refer to figure on
page 1) Vine and Mathews also noticed that the same pattern of magnetic anomalies
exist over the different parts of the mid oceanic ridge. The pattern of anomalies over the
northern Atlantic Ocean is the same as the pattern over the ridge in the southern Pacific
Ocean. The most important observation that Vine and Mathews made was that the
pattern of magnetic reversals already known from studies of lava flow on the continents.

Question for Discussion
How does the observed pattern of ocean floor magnetic anomalies explain sea floor

Learning Objectives
   - Understanding positive and negative Magnetic anomalies.
   - Understanding the reason of deviations from normal magnetic intensity.
   - To learn about the use of magnetic anomalies in the study of sea floor spreading.

Before being widely accepted, a new hypothesis must be tested. One test for the sea-
floor-spreading hypothesis involved magnetic patterns on the sea floor.
In the late 1950's, scientists mapped the present-day magnetic field generated by rocks
on the floor of the Pacific Ocean.


“volcano world” website of University of North Dakota:

“Evolution of the Earth” (7th Edition) Donald R. Prothero & Robert H. Dott, Jr.

“Physical Geology” (6th Edition) Charles C. Plummer & David McGeary

                                                                                  Page 4 of 4

To top