How to Calculate Half Spreading Rate

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Half Spreading Rate Calculator

Determine the tectonic plate movement speed based on seafloor age and distance from a mid-ocean ridge.

Geological Results

Half Spreading Rate:
Full Spreading Rate (Total Opening):
Velocity in km/Million Years:
function calculateSpreadingRate() { var distance = parseFloat(document.getElementById('distance_km').value); var age = parseFloat(document.getElementById('crust_age').value); var resultsDiv = document.getElementById('spreading-results'); if (isNaN(distance) || isNaN(age) || age <= 0 || distance < 0) { alert('Please enter valid positive numbers for distance and age.'); return; } // Calculation Logic: // Rate = Distance / Time // 1 km = 100,000 cm // 1 Million Years (Ma) = 1,000,000 years // (km / Ma) * (100,000 / 1,000,000) = cm / year // Simplified: (km / Ma) * 10 = cm / year var rateKmMa = distance / age; var halfRateCmYr = rateKmMa * 10; var fullRateCmYr = halfRateCmYr * 2; document.getElementById('half_rate_cm').innerHTML = halfRateCmYr.toFixed(2) + " cm/year"; document.getElementById('full_rate_cm').innerHTML = fullRateCmYr.toFixed(2) + " cm/year"; document.getElementById('rate_km_ma').innerHTML = rateKmMa.toFixed(2) + " km/Ma"; resultsDiv.style.display = 'block'; }

How to Calculate Half Spreading Rate

In plate tectonics, seafloor spreading occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. The Half Spreading Rate is the measure of how fast a single tectonic plate moves away from the ridge axis.

The Half Spreading Rate Formula

To calculate the rate, you need to know the distance the crust has traveled from the ridge where it was born and the time (age) it took to travel that distance. The basic formula is:

Rate = Distance / Time

Step-by-Step Calculation Example

  1. Identify the Distance: Suppose a sample of basalt is found 80 kilometers away from the Mid-Atlantic Ridge.
  2. Determine the Age: Magnetic striping data indicates the crust at that location is 4 million years old.
  3. Apply the Math:
    80 km / 4 million years = 20 km/Ma.
  4. Convert to Standard Units (cm/yr): Since 1 km = 100,000 cm and 1 million years = 1,000,000 years, multiplying km/Ma by 10 gives you centimeters per year.
    20 * 0.1 (simplified) or 20 * 10 / 100? No, let's stick to the direct conversion:
    20 km/Ma = 2 cm/year.

Half Rate vs. Full Rate

It is important to distinguish between the two types of spreading rates used by geologists:

  • Half Spreading Rate: The speed of one plate relative to the ridge. (Example: The North American Plate moving west).
  • Full Spreading Rate: The speed at which two plates move away from each other (The total rate of basin growth). This is always double the half spreading rate.

Why is this important?

Calculating spreading rates allows scientists to reconstruct past continental positions, predict future plate locations, and understand the thermal evolution of the lithosphere. For instance, the East Pacific Rise is considered a "fast-spreading" ridge (rates > 9 cm/yr), while the Mid-Atlantic Ridge is a "slow-spreading" ridge (rates < 5 cm/yr).

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