Question

Imagine you are studying seafloor spreading along two different oceanic ridges. Along the first ridge the magnetic stripes are uniformly narrow. Along the second ridge they are wide near the ridge crest, but they become narrower as you move away from the crest. What can you say about the history of motion in each example?

Short answer

First ridge has consistent spreading; second ridge shows variable rates.

Step-by-step solution

Understanding Magnetic Stripes

Magnetic stripes on ocean floors represent the historical record of Earth's magnetic field reversals, captured as magma solidifies at mid-ocean ridges. These stripes are symmetrically aligned around the ridge.

Analyzing Narrow Magnetic Stripes on First Ridge

Uniformly narrow magnetic stripes suggest that the seafloor has been spreading at a consistent rate over time. The regularity in stripe width indicates a stable geological environment with constant magma supply and spreading rate.

Examining Wide to Narrow Stripes on Second Ridge

On the second ridge, stripes that are wide near the ridge crest and narrow as you move away suggest varying rates of seafloor spreading. Initially, there was a rapid spread (wide), slowing down (narrowing) as you move away from the crest point, indicating changes over time possibly due to variations in magma supply or tectonic activity.

Concluding the Motion History

For the first ridge, the history reflects a steady seafloor spreading. The second ridge indicates a more dynamic history, with periods of fast spreading followed by a slowing of the process.

Key concepts

Mid-Ocean Ridges

Mid-ocean ridges are underwater mountain ranges formed by plate tectonics where new oceanic crust is created. They are found at divergent or constructive plate boundaries, where tectonic plates pull apart. This separation creates a gap that is filled by magma from the mantle, forming new crust as it solidifies. Mid-ocean ridges are the site of continuous creation of new seafloor.

These ridges extend across the ocean floor, sometimes for thousands of kilometers. The process of seafloor spreading occurs at these ridges as magma rises to fill the space created by diverging plates. Over time, this results in the landmasses on either side of the ridge moving away from each other.

• The ridges account for nearly 20% of Earth's surface.
• New crust formation at mid-ocean ridges contributes to the cycle of crust generation and destruction in plate tectonics.
• The presence of mid-ocean ridges is associated with geological phenomena such as earthquakes and volcanic activity.

Magnetic Field Reversals

Earth’s magnetic field is not static; it has switched its polarity at irregular intervals throughout geological time. This phenomenon is known as magnetic field reversal. These reversals are recorded in oceanic crust, specifically in the magnetic stripes on the seafloor.

As magma emerges and solidifies at mid-ocean ridges, the iron minerals within align according to Earth's magnetic field at the time. Through studying these magnetic stripes, scientists can understand when reversals have occurred and at what rates seafloor spreading happened.

• Magnetic field reversals help geologists date different sections of the ocean floor.
• Reversals are one key methodology for further understanding plate tectonics.
• The alternating pattern of normal and reversed polarity stripes are crucial for analyzing the history and rate of seafloor spreading.

Tectonic Activity

Tectonic activity refers to the movement and interaction of Earth's tectonic plates. This phenomenon is crucial for understanding many geological processes, including the formation of mid-ocean ridges. When tectonic plates move apart at divergent boundaries, seafloor spreading occurs, and new crust is formed at mid-ocean ridges.

The study of tectonic activity helps explain the dynamics of Earth's surface. Changes in tectonic activity can lead to variations in the rate of seafloor spreading, as observed in the difference in magnetic stripe widths along mid-ocean ridges.

• Tectonic activity can cause earthquakes, volcanic eruptions, and the formation of mountain ranges.
• It provides insight into the movement of continental and oceanic plates over millions of years.
• Understanding tectonic activity is essential for determining past and present geology and observing how Earth's surface evolves over time.

Magma Supply

Magma supply refers to the molten rock that is produced beneath the Earth's crust and is crucial for the formation of new crust at mid-ocean ridges. This process of rising magma leads to seafloor spreading, an essential part of the tectonic cycle.

The consistency and amount of magma supply can influence the rate of seafloor spreading. For instance, a stable and ample magma supply results in uniform stripe patterns, indicating steady seafloor spreading. Variations in supply can lead to changes in spread rates and stripe widths.

• Magma is generated in the mantle, below Earth's crust.
• Consistent magma supply supports continuous formation of new crust as tectonic plates diverge.
• Fluctuations can signal changes in geological activity, such as increased seafloor spreading rates or volcanic activity.