Calculate the velocity of tectonic plate divergence.
Distance from the mid-ocean ridge to the magnetic anomaly or sample.
The geological age of the rock sample in millions of years (Ma).
Half-Spreading Rate (One Side):0.00 cm/yr
Full Spreading Rate (Total):0.00 cm/yr
Rate in km/Myr:0.00 km/Myr
function calculateSpreadingRate() {
// 1. Get input values
var distanceInput = document.getElementById('distanceKm').value;
var ageInput = document.getElementById('ageMyr').value;
// 2. Validate inputs
if (distanceInput === "" || ageInput === "") {
alert("Please enter both distance and age.");
return;
}
var distance = parseFloat(distanceInput);
var age = parseFloat(ageInput);
if (isNaN(distance) || isNaN(age)) {
alert("Please enter valid numeric values.");
return;
}
if (age <= 0) {
alert("Age must be greater than zero.");
return;
}
if (distance < 0) {
alert("Distance cannot be negative.");
return;
}
// 3. Calculation Logic
// Rate (km/Myr) = Distance (km) / Age (Myr)
var rateKmPerMyr = distance / age;
// Convert to cm/yr
// 1 km = 100,000 cm
// 1 Myr = 1,000,000 years
// Factor = 100,000 / 1,000,000 = 0.1
var rateCmPerYr = rateKmPerMyr * 0.1; // This is the half-rate (one plate moving)
// Full spreading rate is usually double the half-rate (two plates moving apart)
var fullRateCmPerYr = rateCmPerYr * 2;
// 4. Display Results
document.getElementById('halfRateCm').innerHTML = rateCmPerYr.toFixed(2) + " cm/yr";
document.getElementById('fullRateCm').innerHTML = fullRateCmPerYr.toFixed(2) + " cm/yr";
document.getElementById('rateKm').innerHTML = rateKmPerMyr.toFixed(2) + " km/Myr";
// Show result area
document.getElementById('result-area').style.display = "block";
}
How to Calculate Seafloor Spreading Rate
Seafloor spreading is a geologic process in which tectonic plates—large slabs of Earth's lithosphere—split apart from each other. Seafloor spreading and other tectonic activity processes are the result of mantle convection. As the plates move away from the ridge axis, new crust is formed. Calculating the rate at which this occurs is fundamental to understanding plate tectonics.
The Basic Formula
The rate of seafloor spreading is calculated using the basic physics formula for velocity:
Rate = Distance / Time
Where:
Distance: The distance of a specific point (usually a magnetic anomaly or a rock sample) from the mid-ocean ridge axis.
Time: The age of the oceanic crust at that specific distance.
Understanding the Units
Geologists typically measure large distances in kilometers (km) and deep time in millions of years (Myr or Ma). However, because plate movement is slow, the final rate is often expressed in centimeters per year (cm/yr).
1 km = 100,000 cm
1 Myr = 1,000,000 years
Consequently, a rate of 10 km/Myr is mathematically equivalent to 1 cm/yr.
Half-Rate vs. Full-Rate
When calculating spreading rates, it is crucial to distinguish between the half-rate and the full-rate:
Half-Spreading Rate: The velocity at which one plate moves away from the ridge axis. This is calculated by measuring the distance from the ridge to a point on one side.
Full-Spreading Rate: The rate at which two plates move away from each other. Assuming symmetrical spreading, this is double the half-rate.
Example Calculation
Let's look at a real-world scenario involving the Mid-Atlantic Ridge.
Suppose a research vessel collects a basalt sample 45 kilometers away from the ridge axis. Radiometric dating determines the rock is 2.5 million years old.
Step 1: Apply the formula: Rate = 45 km / 2.5 Myr.
Step 2: Calculate the result: 18 km/Myr.
Step 3: Convert to cm/yr: 18 × 0.1 = 1.8 cm/yr.
This result represents the half-rate. The full spreading rate for the ridge would be approximately 3.6 cm/yr.
Why Measure Seafloor Spreading?
Calculating these rates helps scientists reconstruct the past positions of continents (paleogeography). Fast-spreading ridges, like the East Pacific Rise (up to 15 cm/yr), have different topography and thermal profiles compared to slow-spreading ridges like the Mid-Atlantic Ridge (2-5 cm/yr).
Using Magnetic Anomalies
While rock samples are useful, geologists most often use marine magnetic anomalies. As new crust cools, iron minerals align with Earth's magnetic field. Since Earth's magnetic polarity flips periodically, the seafloor acts as a magnetic tape recorder. By matching these magnetic "stripes" to the known Geomagnetic Polarity Time Scale (GPTS), scientists can determine the age of the crust without physically sampling every kilometer.