Earth's Magnetic Field — Strength, Polarity & Pole Flips
GED Science Practice — Reading graphs, tables, and scientific passages
What is Earth's magnetic field?
Many people think of Earth's magnetic field as a stationary bubble that protects humans from harmful radiation and helps guide ships and airplanes. In actuality, the field is dynamic — meaning it undergoes continuous change or activity.
Key facts from the passage:
Earth's overall magnetic field strength has decreased by nearly 10% since it was first regularly measured around 1900
The current magnetic poles are moving — they are not fixed in place
Some scientists believe this signals the beginning of a magnetic pole flip
Pole flips have happened many times in Earth's past — there is solid geological evidence
Earth's magnetic field — animated diagram
Earth's magnetic field lines extend from pole to pole, forming a protective magnetosphere
Dynamic
In the passage, "dynamic" means continuous change or activity. The field is NOT stationary — it shifts in strength and direction over time.
Magnetosphere
The region around Earth controlled by its magnetic field. It shields us from harmful solar radiation — like an invisible force field.
Polarity (N/R)
Normal (N) = current direction. Reversed (R) = opposite direction. A "pole flip" changes N to R or R to N over thousands of years.
10²³ AM²
The unit for magnetic field strength in the data table. A very large number — these measurements describe the total strength of Earth's entire magnetic field.
Magnetic field strength — 20th century (1900–2000)
Tap any point on the chart to explore. This graph shows the data from the passage.
Magnetic Field Strength (10²³ AM²) measured from 1900 to 2000
What does the graph show?
Look carefully at the shape of the line. The GED question asks which statement correctly describes the graph.
1900–1940
The line drops steeply from ~0.84 to ~0.81. A rapid initial decline.
1940–1970
The decline slows noticeably. The line becomes less steep — a gentler slope.
1970–2000
The decline steepens again, dropping sharply to ~0.77. A faster rate of decrease.
Overall
The rate of decline is NOT constant — it varies throughout the century. This is the key observation.
Watch out: The field only decreases — it never increases during this period. Any answer saying it "increased and decreased" is wrong. The rate of change varies, but the direction is always downward.
GED Question 1 — Graph interpretation
"Which statement describes the graph's representation of magnetic field strength during the twentieth century?"
WRONG
"The strength declined at a constant rate." The line is NOT a straight diagonal — it changes slope at different periods. A constant rate would be a perfectly straight line.
WRONG
"Decreased much more rapidly over the last half of the twentieth century." The steepest drop is actually in the first half (1900–1940), not just the last half. The last quarter (1970–2000) is steep again, but "last half" is too broad.
CORRECT
"The strength had variable rates of decline during the twentieth century." The rate of decline changes — steep early, slower in the middle, steep again at the end. "Variable rates" perfectly describes a line that isn't straight but always goes down.
WRONG
"The strength increased and decreased during the twentieth century." The graph shows a consistent decline. The field strength never increases during this period — the line only goes downward.
Earth's Magnetic Field Polarity Through Time
Data for the last 2 million years (my), sampled at 100,000-year intervals. N = Normal polarity. R = Reversed polarity.
Age of Rock (my)
Mag. Field Strength (10²³ AM²)
Polarity
0.1
0.8
N
0.2
0.9
N
0.3
0.8
N
0.4
0.8
N
0.5
0.6
N
0.6
0.8
N
0.7
0.6
N
0.8
0.4
R
0.9
0.8
R
1.0
0.4
N
1.1
0.6
R
1.2
0.2
R
1.3
0.6
R
1.4
0.6
R
1.5
0.8
R
1.6
0.8
R
1.7
0.8
R
Reading the table: The most recent rocks (0.1–0.7 my) all show N polarity — that's 700,000 years of normal polarity. Then from about 0.8–1.7 my ago, the polarity was mostly R (reversed). This confirms the field has flipped multiple times in Earth's history.
How minerals record polarity
When volcanic rocks cool, minerals inside them are "frozen" into alignment with Earth's magnetic field at that moment — like a tiny compass needle locked in rock. By measuring these rocks, scientists can reconstruct Earth's magnetic history millions of years back.
Low strength at flips
Notice rows 0.8 and 1.0 my have strength 0.4 — relatively low. The field weakens during a polarity transition before rebuilding in the new direction.
700,000 years of N
The most recent 700,000 years have been Normal polarity — an unusually long period. This is one reason some scientists think a flip may be approaching.
What is a magnetic pole flip?
A magnetic pole flip (also called a geomagnetic reversal) is when Earth's magnetic north and south poles swap positions. This process:
Takes place over the course of a few centuries to thousands of years
During the flip, the field weakens significantly before rebuilding in the opposite direction
Has happened hundreds of times in Earth's 4.5-billion-year history
The last full reversal was about 780,000 years ago
There is solid geological evidence in rock mineral alignment that this has occurred many times
GED Question 2 — Evidence for pole flip hypothesis
"Which piece of evidence supports the hypothesis that Earth is currently at the beginning of a magnetic pole flip?"
WRONG
"The magnetic polarity has been normal for the past 700,000 years." This is a fact from the table, but it does NOT support the hypothesis that a flip is beginning — if anything, it shows stability. Long normal periods can precede a flip, but this alone is not evidence a flip is starting now.
WRONG
"Minerals in rocks are imprinted with the strength and direction of Earth's magnetic field." This explains HOW we know about past flips — it's a method of measurement, not evidence that a flip is currently beginning.
WRONG
"During a pole flip, the field will diminish then reestablish in opposite polarity." This describes what happens during a flip — it's a definition, not evidence that one is happening right now.
CORRECT
"The overall strength has decreased by nearly 10% since 1900, and the poles are moving." This directly supports the hypothesis. A weakening field + moving poles are the two observable signs that scientists associate with the beginning of a pole flip. This is current, measurable evidence — not historical data or definitions.
GED Question 3 — Vocabulary in context
"What is the meaning of dynamic as it relates to the magnetic field in paragraph 1?"
WRONG
"Vigorous energy" This is one definition of "dynamic" in everyday use, but the passage contrasts "dynamic" with "stationary" — so it's about change and movement, not energy level.
CORRECT
"Continuous change or activity" The passage says the field "is dynamic" right after saying people wrongly think of it as "stationary." The contrast makes the meaning clear: dynamic = always changing, not fixed.
WRONG
"A force that produces motion" While the magnetic field does exert force, this doesn't fit the context. The passage is describing the field's nature (changing vs. fixed), not what force it creates.
WRONG
"A force that affects stability" Again, this misses the stationary vs. changing contrast the passage is making. The word "dynamic" here specifically contrasts with "stationary bubble."
GED strategy — passage + graph + table questions
Step 1: Read the passage carefully — underline key facts (10% decrease, poles moving, pole flips)
Step 2: Read the graph — identify the overall trend AND whether the rate is constant or variable
Step 3: Read the table — note what N and R mean and what pattern you see over time
Step 4: For vocabulary questions — use context clues (what word is it contrasted with?)
Step 5: For evidence questions — ask "does this show something is CURRENTLY happening?" vs. just defining or explaining
All three correct answers summarized
Q1 (Graph): "The strength had variable rates of decline during the twentieth century." — The line steepens, flattens, then steepens again. Not constant. Never increases.
Q2 (Evidence): "The overall strength has decreased by nearly 10% since 1900, and the poles are moving." — Current observable data directly supporting the pole-flip hypothesis.
Q3 (Vocabulary): "Continuous change or activity." — The passage contrasts "dynamic" with "stationary," making the meaning clear from context.
How to spot wrong answers on GED science questions
Too extreme
Answers that say "always," "never," "only," or make broader claims than the data supports are usually wrong.
Half right
Some wrong answers mix a true fact with a wrong conclusion. Always check the WHOLE statement, not just the first part.
Method ≠ evidence
Describing HOW we measure something is not the same as evidence that something is currently happening. Watch for this distinction.
Context first
For vocabulary questions, ignore what a word "usually" means. Focus on the contrast or comparison in the paragraph.
