Name __________________________________
Clark
Science Period __________
Date _________
Magnetic Reversals through the Ages
LAB11
TSWBAT: analyze the evidence that lithospheric plate movements occur.
Problem (scientific question):
How are magnetic strips at either side of mid-ocean ridges evidence that there is lithospheric plate movement?
Prior Research: please read and annotate the following.
Most basalt magmas contain abundant molten iron. As magma starts to harden into rock, iron-rich minerals
solidify first. Their crystals are pulled into alignment by the Earth’s magnetic field, just like a compass needle is
pulled towards magnetic north. Once the basalt cools completely into solid rock, the alignment of the iron minerals
is fixed. Thus, basalts preserve a permanent record of the strength and direction, or polarity, of the planet’s
magnetic field at the time the rocks were formed.
When geologists studied the polarity of ancient rocks, they were stunned to discover that in many of them, iron
minerals were aligned toward the south magnetic pole, not the north. Scientists have concluded that the Earth’s
magnetic field has reversed itself again and again throughout the ages. When the field points toward the north
magnetic pole, as it does today, the field and the rocks that record it have “normal” polarity. When the field points
toward the south magnetic pole, opposite of its current behavior, the field and the rocks that record it have
“reversed” polarity. All rocks of the same age have the same polarity.
In the 1970's, scientists sailed back and forth across the world’s oceans, measuring the magnetic signatures
emanating from the oceanic crust beneath their ships. These surveys revealed a series of invisible magnetic
“stripes” of normal and reversed polarity in the sea floor, like that shown in the figure on the back of this page. The
patterns reflect the creation and spreading of oceanic crust along the mid-oceanic ridges. Basalt forming at the
ridge crest picks up the existing magnetic polarity. As the plates move apart, the swath of fresh crust is moved
away from the ridge. As long as the magnetic field remains constant, the polarity “stripe” widens. When the
Earth’s magnetic field reverses, a new stripe, with the new polarity, begins.
Such magnetic patterns led to recognition of the occurrence of sea-floor spreading, and they remain some of the
strongest evidence for the theory of plate tectonics.
Please answer the following questions in complete sentences. Cite your source by putting the paragraph
number in parentheses after you answer.
1. What do basalt magmas contain?
2. What happens to the crystals as the magma hardens?
3. After the basalt cools completely, what does it preserve?
4. What did scientists discover and then conclude about Earth's magnetic field?
5. What do the patterns reflect?
How often does Earth's magnetic field reverse itself? The diagram below shows the record geologists have put
together for the last 65 million years or since the last dinosaurs became extinct. Each dark band represents a
"normal" magnetic field, as it is today. Each light band represents a reversed magnetic field.
6. Based on the diagram, was Earth's magnetic field "normal" or reversed 65 million years ago?
7. About how long ago was the last time Earth's magnetic field reversed?
Materials:
paper model of the sea floor and magnetic strips
Procedures (Please round your numbers to the nearest tenth.):
Step 1: Use the paper model to explore how these magnetic strips form on the ocean floor by threading the
magnetic strip in the center of "mountains" or the mid-ocean ridge.
Step 2: Pull the strip slowly in both directions and observe the size of the black and white stripes.
8. Can you see any pattern as you observe Earth's magnetic field reversals as they come out of the “mid-ocean
ridge”? Give reasons for your answer.
9. Why are the magnetic stripes on the sea floor parallel to and symmetrical across the mid-ocean ridge? (Please
include the terms mid-ocean ridge and sea-floor spreading.)
Albert Einstein called the generation of Earth’s magnetic field one of the greatest mysteries in physics. Scientists
are still not sure how the field is generated, though it is clearly related to the movement of molten iron in the liquid
outer core. Even more puzzling is why the field switches back and forth between normal and reversed polarity.