SME
Oceanography · Problem type
Seafloor depth from its age
Use this for “Seafloor 1,225 km from the ridge spreads at 25 mm/yr. How old is it, and how deep?”
Chart
d(t) = 350√t + 2,500
The vertical axis grows downward.
This is an example. Type your own values to start a new problem.
t (Myr)
=
x (km)
÷
u (mm/yr)
d (m)
=
2,500
+
350
×
t
u is how fast this side of the ridge moves away from it, half the rate at which the two plates part.
Formulas
t
=
x
u
49
=
1,225
25
d
=
2,500
+
350
×
√
t
4,950
=
2,500
+
350
×
√
49
Assumptions
•
The plate cools and shrinks as it ages: d = 2,500 + 350√t, with t in Myr and d in m.
•
The square-root fit holds to about 80 Myr; older floor flattens near 5,500 to 6,000 m.
•
1 km per million years is 1 mm per year, so km ÷ (mm/yr) reads straight as Myr.
Step-by-step
WE KNOW
x = 1,225 km, u = 25 mm/yr
FIND
age (t), depth (d)
Step 1 · Find age (t)
Use t = x ÷ u
The
floor
moved
x
kilometers
from
the
ridge
at
u
:
distance
over
speed.
t
=
1,225
25
t
=
49
Myr
Step 2 · Find depth (d)
Use d = 2,500 + 350 × √t
Start at the ridge crest’s 2,500 m and add 350 m for each √Myr of age.
d
=
2,500
+
350
×
√
49
d
=
2,500
+
350
×
7
d
=
2,500
+
2,450
d
=
4,950
m
Check
Put the numbers back in. Both sides should match.
49
=
1,225
25
✓
4,950
=
2,500
+
350
×
√
49
✓
Topic check
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