Science 10: Lesson 5 Seafloor Spreading

Summary

In this science lesson, Marian Soriano revisits the continental drift theory, explaining how continents once formed a single landmass called Pangaea and later split into Laurasia and Gondwanaland. She introduces Alfred Wegener's evidence for continental drift, including matching continental shapes and fossil records. The lesson then delves into the concept of seafloor spreading, a key process supporting continental drift, which involves the creation of new oceanic crust at mid-ocean ridges and its subsequent movement away from the ridge. Evidence for seafloor spreading includes the age and density of rocks, sediment thickness, and geomagnetic reversal patterns. The lesson concludes by emphasizing the balance between crust creation and destruction, maintaining Earth's shape and size.

Takeaways

• 🌏 The continental drift theory suggests that continents were once part of a single landmass called Pangaea, which later split into Laurasia and Gondwanaland.
• 🔍 Alfred Wegener proposed the continental drift theory, citing evidence such as matching continental shapes, fossil records, and ancient climates.
• 🚫 Despite the evidence, Wegener's theory was initially rejected due to a lack of a mechanism to explain how continents could drift.
• 🌊 Seafloor spreading was introduced as a concept by Harry Hess, who used sonar technology to study the ocean floor and its relation to continental drift.
• 🔥 Seafloor spreading theory posits that new oceanic crust is formed at mid-ocean ridges as magma rises and spreads sideways, creating new sea floor.
• 📏 Evidence for seafloor spreading includes the finding that the youngest rocks are near the ridges, while the oldest are found further away.
• 🌌 The ages, densities, and thicknesses of the oceanic crust increase with distance from the mid-ocean ridges.
• 🧲 Geomagnetic reversals provide strong evidence for seafloor spreading, with matching magnetic stripes on either side of mid-ocean ridges indicating past magnetic field changes.
• 🌍 Seafloor spreading disproves the idea that continents move through static oceans, showing that oceans are also sites of tectonic activity.
• ⚖️ The balance between seafloor spreading, which creates new crust, and subduction, which destroys old crust, maintains the Earth's shape and size.
• 📚 The lesson concludes that seafloor spreading occurs at divergent plate boundaries and is a key process in understanding the Earth's tectonic movements.

Q & A

What is the Continental Drift Theory?

The Continental Drift Theory, proposed by Alfred Wegener, suggests that continents are slowly drifting around the Earth and were once part of a single large landmass called Pangaea.

What were the two supercontinents formed after the division of Pangaea?

After the division of Pangaea, the supercontinent split into two parts: Laurasia, which drifted to the north, and Gondwanaland, which drifted to the south.

What evidence did Alfred Wegener identify to support the Continental Drift Theory?

Wegener identified several pieces of evidence, including the matching shapes of continental edges, fossil evidence, cold deposits in Antarctica, ancient climates, and glacier carvings.

Why was the Continental Drift Theory initially rejected?

The theory was initially rejected because Wegener could not explain the mechanism causing the continents to drift.

What is Seafloor Spreading?

Seafloor Spreading is a geological process where new oceanic crust is formed at mid-ocean ridges as magma rises, cools, and solidifies, pushing older crust away from the ridge.

Who was Harry Hammond Hess and what was his contribution to the understanding of the ocean floor?

Harry Hammond Hess was a professor of geology at Princeton University who used sonar to study the ocean floor during World War II. He is known for developing the idea of seafloor spreading.

What evidence supports the idea of seafloor spreading?

Evidence supporting seafloor spreading includes the presence of the youngest rocks near the ridge, progressively older rocks as one moves away from the ridge, thinner sediments near the ridge, and symmetrical magnetic stripes on either side of the mid-ocean ridges.

How do the ages, densities, and thicknesses of the oceanic crust relate to the mid-ocean ridge?

The ages, densities, and thicknesses of the oceanic crust increase with distance from the mid-ocean ridge, indicating that new crust is being created at the ridge.

What is the relationship between seafloor spreading and geomagnetic reversal?

Geomagnetic reversal is a phenomenon that provides evidence for seafloor spreading. The magnetism of the ocean floor, recorded in the basalt rock, shows symmetrical stripes on either side of the mid-ocean ridges, indicating past reversals of the Earth's magnetic field.

How does seafloor spreading disprove and support the Continental Drift Theory?

Seafloor spreading disproves the original idea that continents move through unmoving oceans. It supports the Continental Drift Theory by showing that the ocean floor is an active site of tectonic activity, with new crust being created and old crust being destroyed through subduction.

How do seafloor spreading and subduction maintain the shape and diameter of the Earth?

Seafloor spreading creates new crust, while subduction destroys old crust. These two processes roughly balance each other, keeping the shape and diameter of the Earth constant.