km 0
NORTH
AMERICA
How fast does the Atlantic Ocean grow? To find out, we can calculate the rate of sea-floor
spreading. The map shows the age of the ocean floor and three rock samples. For example,
rock B is 55 million years old. So is any rock on the red line labeled "55. Rock B formed
55 million years ago at the ridge, then traveled to its current location. Rock C is now at
the ridge. To find the rate of spreading, first estimate the distance between rocks B and
C by using the scale bar. Divide that by the difference in their ages. In other words, rate is
distance divided by time (age difference): r
200,000,000 cm
55,000,000 y
or about 3.6 cm
Ages of Sea-Floor Rock Samples
1,000 2,000
Do the Math
Calculate the Rate of Sea-Floor Spreading
156
118
r=.
[=
84
156
A
55
118
Sora-
84 55
35
C
55 35200 2035 55
20
cm
y
ATLANTIC
OCLAN
Rock sample location
mi 0
1,000
2,000
Credit: From "Age, spreading rates, and spreading asymmetry of the world's ocean crust" by R. Dietmar Müller, et al, from Geochemistry
Age of sea floor rock (in millions of years)
Geophysics, Geosystems, April 3, 2008. Copyright © 2008 by John Wiley and Sons. Used with permission of Copyright Clearance Center.
(for example, 55 on the map = 55,000,000 years)
9.
0
2035 551
STEP 5 Divide and round to the nearest tenth.
cm
84
Mid-Atlantic Ridge
84
118
Find the rate using rocks A and C. Express your answer in centimeters per year
STEP 1 Use the scale bar to estimate the distance from rock A to rock C.
d=
118 156
STEP 4 Plug in the values for distance and time into the rate equation r
=
156
km
STEP 2 Convert the distance from km to cm. 1 km = 100,000 cm. So multiply by 100,000.
d=₁
km x 100,000 cm
km
cm
STEP 3 Find the age difference between rocks A and C.
t=118,000,000 y-0y=_
AFRICA
N
WIE
S
Houghton Mifflin Harcourt