Overview
This lecture explains how discoveries about magnetic patterns on the ocean floor revolutionized our understanding of Earth's geology, leading to the theory of plate tectonics.
The Deep Ocean and Early Exploration
- The deep ocean is largely unexplored, with little known compared to outer space.
- Most deep ocean discoveries have been made in the last 60 years due to technological advancements.
Oceanic Crust and Magnetic Studies
- Oceans rest on oceanic crust; continents rest on continental crust.
- In the 1950s, scientists studied the magnetic properties of the oceanic crust.
- A magnetometer, developed during World War II for submarine detection, was adapted to map the ocean floor's magnetism.
Discovery of Magnetic Stripes
- Scientists expected random magnetic anomalies but found symmetrical stripes of positive and negative anomalies.
- These stripes are parallel and alternate in direction on either side of mid-ocean ridges.
Earth's Magnetic Field and Rock Formation
- Earth's magnetic field gradually shifts and sometimes reverses (magnetic pole reversal).
- As new volcanic rocks form at mid-ocean ridges, they record the current direction of Earth's magnetic field.
- Rocks with a matching magnetic direction show positive anomalies; those with the opposite direction show negative anomalies.
Seafloor Spreading and Plate Tectonics
- Symmetrical magnetic stripes show that oceanic crust forms at mid-ocean ridges and spreads outward over time.
- The age of oceanic rocks increases with distance from the ridge.
- This process, called seafloor spreading, proves the continents move—supporting the theory of plate tectonics.
- Plate tectonics explains continental drift, earthquakes, volcanoes, and Earth's geological evolution.
Key Terms & Definitions
- Magnetometer — an instrument used to measure magnetic anomalies.
- Magnetic anomaly — deviation from the expected Earth's magnetic field due to local rock properties.
- Mid-ocean ridge — underwater mountain range where new oceanic crust forms.
- Seafloor spreading — the process of new oceanic crust forming at ridges and moving outward.
- Plate tectonics — the scientific theory describing the movement of Earth's crustal plates.
Action Items / Next Steps
- Review the concept of magnetic pole reversals and their geological evidence.
- Read more on the history and development of plate tectonics theory.