The Rate of Seafloor Spreading Can Be Calculated by

Seafloor Spreading Rate Calculator

Enter the distance between the ridge axis and the magnetic anomaly.

Enter the age determined by radiometric dating or magnetostratigraphy.

function calculateSpreadingRate() { // Get input values var distanceKm = parseFloat(document.getElementById('distanceKm').value); var ageMa = parseFloat(document.getElementById('ageMa').value); var resultBox = document.getElementById('sfsResult'); // Validation if (isNaN(distanceKm) || isNaN(ageMa)) { alert("Please enter valid numbers for both Distance and Age."); resultBox.style.display = "none"; return; } if (ageMa <= 0) { alert("Age must be greater than 0."); resultBox.style.display = "none"; return; } // Calculation Logic // Formula: Rate = Distance / Time // 1. Calculate rate in km / Million Years (km/Ma) var rateKmMa = distanceKm / ageMa; // 2. Calculate rate in cm / year // Conversion: // 1 km = 100,000 cm // 1 Ma = 1,000,000 years // (dist * 100,000) / (age * 1,000,000) = (dist / age) / 10 var rateCmYr = (distanceKm * 100000) / (ageMa * 1000000); // 3. Full rate (separation of two plates) is double the half-rate var fullRateCmYr = rateCmYr * 2; // Display Results document.getElementById('resCmYr').innerText = rateCmYr.toFixed(2); document.getElementById('resKmMa').innerText = rateKmMa.toFixed(2); document.getElementById('resFull').innerText = fullRateCmYr.toFixed(2); resultBox.style.display = "block"; }

How the Rate of Seafloor Spreading Can Be Calculated

Seafloor spreading is a fundamental process in plate tectonics where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Understanding the rate at which this occurs helps geologists reconstruct the history of Earth's continents and ocean basins.

The Core Formula

The rate of seafloor spreading is calculated using a simple velocity formula: Velocity = Distance / Time.

To apply this to geology:

• Distance ($d$): Measured from the center of the Mid-Ocean Ridge (where new crust is born) to a specific point on the seafloor (usually a magnetic anomaly).
• Time ($t$): The age of the rock at that specific distance, usually determined by paleomagnetism or radiometric dating.

The calculation typically looks like this:

$$ \text{Rate} = \frac{\text{Distance from Ridge (km)}}{\text{Age of Rock (Millions of Years)}} $$

Understanding Units: cm/year vs km/Ma

Geologists often express spreading rates in centimeters per year (cm/yr) because it relates to human-scale speeds (roughly the speed at which fingernails grow). However, the inputs are often measured in larger scales: kilometers for distance and millions of years (Ma) for time.

Mathematically, a rate of 10 km/Ma is exactly equivalent to 1 cm/year.

Half-Rate vs. Full-Rate

When calculating the rate of seafloor spreading, it is crucial to distinguish between the half-spreading rate and the full-spreading rate.

• Half-Rate: The speed at which a single tectonic plate moves away from the ridge axis. This is what you calculate when you measure the distance from the ridge to a point on one side.
• Full-Rate: The speed at which two diverging plates separate from each other. This is simply the half-rate multiplied by two (assuming symmetrical spreading).

Example Calculation

Imagine a ship tows a magnetometer and detects a magnetic reversal anomaly known to be 4 million years old. The ship is currently 80 kilometers away from the mid-ocean ridge axis.

1. Input Distance: 80 km
2. Input Age: 4 Ma (Million Years)
3. Calculation (km/Ma): $80 / 4 = 20 \text{ km/Ma}$
4. Conversion to cm/yr: Since $10 \text{ km/Ma} = 1 \text{ cm/yr}$, then $20 \text{ km/Ma} = 2 \text{ cm/yr}$.

This indicates the plate is moving at 2 cm/year (half-rate), and the ocean basin is widening at 4 cm/year (full-rate).

Why Do We Calculate This?

Calculating seafloor spreading rates allows scientists to:

• Reconstruct Past Geography: By rewinding these rates, we can determine where continents were positioned millions of years ago (e.g., Pangea).
• Understand Thermal Subsidence: The age of the seafloor correlates with its depth; faster spreading ridges behave differently thermally than slow spreading ridges.
• Analyze Plate Boundaries: Fast spreading ridges (like the East Pacific Rise) have different topography compared to slow spreading ridges (like the Mid-Atlantic Ridge).