SME
Grade 12 Science · Problem type
Seafloor spreading from magnetic stripes
Use this for “Rock 100 km from the ridge is 4 million years old. How fast is the seafloor spreading?”
Diagram
The
rock
100
km
from
the
ridge
is
4
million
years
old:
the
plate
moved
100
4
=
25
km
per
million
years,
25
mm
a
year.
The two plates part at 2 × 25 = 50 mm a year.
The stripes match on both sides: rock cooling at the ridge records the field of its time.
This is an example. Type your own values to start a new problem.
v (mm/yr)
=
x (km)
÷
t (million years)
w
Full spreading rate
worked out (not typed) · mm/yr
Formulas
v
=
x
t
25
=
100
4
w
=
2
×
v
50 = 2 × 25
Assumptions
•
New seafloor forms at the ridge and moves away on both sides at the same rate.
•
Cooling rock records the direction of Earth’s magnetic field, which has flipped many times, so the seafloor is striped the same on both sides.
•
A kilometer per million years is a millimeter per year.
Step-by-step
WE KNOW
x = 100 km, t = 4 million years
FIND
spreading rate, one side (v), full spreading rate (w)
Step 1 · Find spreading rate, one side (v)
Use v = x ÷ t
The
rock
moved
x
kilometers
in
t
million
years;
a
kilometer
per
million
years
is
a
millimeter
per
year.
v
=
100
4
v
=
25
mm/yr
Step 2 · Find full spreading rate (w)
Use w = 2 × v
The
plates
move
apart
on
both
sides,
each
at
v
.
w
=
2
×
25
w
=
50
mm/yr
Check
Put the numbers back in. Both sides should match.
25
=
100
4
✓
50 = 2 × 25 ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Refresh
Related lessons
Earthquakes, seismic waves and Earth's interior
Main lesson
Locating an epicenter from three stations
Use this for “Why are three seismic stations needed to find an epicenter?” and for the distances from each lag.
Comparing earthquakes by magnitude
Use this for “A magnitude 6 quake and a magnitude 4 quake: how much more shaking, and how much more energy?”
P, S or surface wave?
Use this for “Which seismic wave cannot travel through the liquid outer core?”
Which waves reach a station
Use this for “Which seismic waves reach a station 120° from an earthquake, and why do no S waves arrive past 104°?”
Earth’s heat or the Sun’s?
Use this for “Which of these is powered by Earth’s internal heat, like hot springs?”