Mid-ocean ridges are long underwater mountain chains where tectonic plates move apart and new ocean crust forms. Seafloor spreading explains how magma rises, cools, and records evidence of plate motion. This cheat sheet helps students connect ridge features, rock ages, magnetic patterns, and plate movement.
It is useful for understanding plate tectonics, ocean basin formation, and evidence for continental drift.
The most important idea is that new crust forms at the ridge and moves away on both sides. Ocean crust is youngest at the ridge and becomes older, cooler, and denser farther away. Matching magnetic stripes on both sides of a ridge show that Earth’s magnetic field has reversed many times.
Spreading rate can be found with rate = distance divided by time, using consistent units.
Key Facts
- A mid-ocean ridge is a divergent plate boundary where two oceanic plates move apart and magma rises to form new basaltic crust.
- Seafloor spreading is the process in which new ocean crust forms at a ridge and moves outward away from the ridge over time.
- The age pattern of ocean crust follows this rule: youngest crust at the ridge, older crust farther from the ridge.
- Magnetic stripes form when iron-rich minerals in cooling basalt align with Earth’s magnetic field, creating bands of normal and reversed polarity.
- Magnetic stripes are usually symmetrical on both sides of a mid-ocean ridge because crust spreads outward in opposite directions.
- Spreading rate is calculated with rate = distance / time, and common units include cm/yr or km/million years.
- Total spreading rate across both sides of a ridge is total rate = rate on one side x 2 when spreading is symmetrical.
- Oceanic crust is recycled at subduction zones, so ocean crust is generally much younger than continental crust.
Vocabulary
- Mid-ocean ridge
- A long underwater mountain range where tectonic plates pull apart and new ocean crust forms.
- Seafloor spreading
- The process by which new oceanic crust forms at a ridge and moves away from it.
- Divergent boundary
- A plate boundary where two tectonic plates move away from each other.
- Magnetic reversal
- A change in Earth’s magnetic field when magnetic north and magnetic south switch positions.
- Basalt
- A dark, fine-grained igneous rock that commonly forms new oceanic crust at mid-ocean ridges.
- Subduction zone
- A plate boundary where one plate sinks beneath another into the mantle.
Common Mistakes to Avoid
- Thinking old crust forms at the ridge is wrong because magma creates the youngest oceanic crust directly at the ridge axis.
- Forgetting to double a one-side spreading rate can give the wrong total rate because plates move away from both sides of a ridge.
- Mixing units in rate = distance / time is wrong because km, cm, years, and million years must be converted consistently before calculating.
- Assuming magnetic stripes are random is wrong because matching stripe patterns on both sides of a ridge are evidence of symmetrical seafloor spreading.
- Confusing ridges with trenches is wrong because ridges create new crust at divergent boundaries, while trenches are linked to crust destruction at subduction zones.
Practice Questions
- 1 Ocean crust 120 km from a ridge is 6 million years old. What is the one-side spreading rate in km/million years?
- 2 A ridge spreads symmetrically at 3 cm/yr on one side. What is the total spreading rate across both sides of the ridge?
- 3 A magnetic stripe is found 40 km east of a ridge and the matching stripe is 40 km west of the ridge. What does this symmetry show about seafloor spreading?
- 4 Explain why ocean crust is youngest near a mid-ocean ridge and generally gets older as distance from the ridge increases.