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

The first ridge had steady seafloor spreading; the second ridge had variable spreading, becoming faster over time.

Step-by-step solution

Understanding Magnetic Stripes

When lava erupts at mid-ocean ridges and cools, it records Earth's magnetic field. Magnetic stripes on the ocean floor occur as the Earth's magnetic field reverses over time.

Interpreting Narrow Uniform Stripes

Along the first ridge with uniformly narrow stripes, this suggests that the spreading has been steady and consistent over time. Such stripes indicate a constant and relatively fast rate of seafloor spreading, as uniformity in width implies consistent tectonic activity.

Interpreting Wide to Narrow Stripes

On the second ridge, the wide stripes near the crest that become narrower further away suggest variable spreading rates. Wide stripes at the crest indicate periods of relatively slow spreading initially, which becomes faster, as seen by the narrowing of stripes.

Historical Spreading Rates

The consistent narrow stripes of the first ridge suggest a stable tectonic environment with constant spreading rate. The second ridge's pattern of wide to narrow stripes points to a history of increasing spreading rates over time.

Key concepts

Magnetic Stripes

Magnetic stripes on the ocean floor are one of the most fascinating evidences of seafloor spreading. These stripes are formed when lava emerges from mid-ocean ridges and cools, capturing the Earth's magnetic field in the process. The Earth's magnetic field has reversed many times over geologic history, and these magnetic stripes offer a record of those reversals.

They exhibit patterns on the seafloor that mirror each side of a mid-ocean ridge.

• **Records magnetic field reversal**: As new crust forms and spreads away from the ridge, it captures the magnetic direction of the current geologic epoch.
• **Symmetrical patterns**: These patterns are symmetrical on both sides of the ridge due to the nature of tectonic spreading.

By studying these magnetic stripes, scientists can decipher the past movement of tectonic plates and the history of Earth's magnetic field.

Mid-Ocean Ridges

Mid-ocean ridges are underwater mountain ranges formed by tectonic oceanic plates moving apart. They are the birthplace of new oceanic crust and are pivotal in the process of seafloor spreading. When tectonic plates diverge, magma rises from beneath the Earth's crust to fill the gap, solidifying to form new crust. This explains why mid-ocean ridges are considered geological features of incredible importance.

• **Crust Formation**: Mid-ocean ridges create new oceanic crust as lava emerges and solidifies.
• **Plate Movement**: Horizontal from the ridge generates symmetrical patterns on both sides known as magnetic stripes.
• **Tectonic Boundaries**: These ridges mark divergent boundaries of tectonic plates.

The continuous creation of crust at these ridges plays a crucial role in plate tectonics and the constant reshaping of Earth's surface.

Tectonic Activity

Tectonic activity refers to the movement of Earth's plates and the interactions that occur at their boundaries. This activity is responsible for phenomena such as earthquakes, volcanic eruptions, and the formation of geological features. Understanding tectonic activity is essential in studying seafloor spreading as it drives the process.

• **Movement of Plates**: Plates can move apart, together, or slide past one another, affecting the formation and renewal of oceanic crust.
• **Divergent Boundaries**: At mid-ocean ridges, these boundaries see plates moving away from each other, leading to seafloor spreading.
• **Consistent and Variable Movements**: Tectonic plates may spread consistently or at varying rates, influencing the width of magnetic stripes.

Studying tectonic activity helps scientists predict geological phenomena and understand past and future changes in the Earth's crust.

Spreading Rates

Spreading rates refer to how quickly oceanic crust moves away from a mid-ocean ridge. These rates can vary over time and can be detected by examining the width of magnetic stripes. In cases where spreading is consistent, magnetic stripes will have uniform widths, indicating a steady and reliable rate of spreading. Wider stripes may indicate slower initial spreading that accelerates over time.

• **Consistent Spreading**: Uniform stripes suggest a constant spreading rate, as seen in some ridge systems.
• **Variable Spreading**: Wider to narrower stripe patterns point to changing rates, with the potential for increased geological activity or volcanic eruptions.
• **Measuring Past Spreading**: By using the width of stripes and the known rate of magnetic pole reversals, historical spreading rates can be estimated.

Understanding spreading rates provides insight into how tectonic activity and oceanic crust generation have occurred over millions of years.