How is magnetism used to support the theory of seafloor spreading?
The Earth's magnetic field will cause the newly formed seafloor to become magnetic. When the seafloor cools, the magnetism of the sefloor will become permanent.
Then if the Earth's magnetic field changes direction (happens every few hundred thousand years), the seafloor's magnetic field will still point in the direction the magnetic field pointed in when it was formed.
Magnetism is used to support the theory of seafloor spreading through the study of magnetic anomalies on the ocean floor.
1. Magnetic Anomalies: As magma rises from the Earth's mantle and solidifies into new crust at the mid-ocean ridges, it records the Earth's magnetic field at that time. The Earth's magnetic field periodically reverses its polarity, meaning that the North and South magnetic poles switch places. This creates a pattern of magnetic stripes on the ocean floor, with bands of normal (aligning with the present-day magnetic field) and reversed (opposite to the present-day field) magnetism.
2. Paleomagnetism: Scientists can analyze the magnetic properties of rocks to determine their magnetization direction and polarity at the time of their formation. Through various techniques like magnetometer measurements and core sampling, they can precisely measure and map the magnetic anomalies on the seafloor.
3. Symmetry of Magnetic Stripes: By comparing the magnetic stripes on either side of the mid-oceanic ridge, scientists noticed a remarkable symmetry. They found that similar patterns of magnetic stripes emerged on both sides of the ridge, implying that new oceanic crust was being formed continuously.
4. Age Dating: As seafloor spreading occurs, the newly formed crust moves away from the mid-ocean ridge, and older oceanic crust gets pushed sideways. By studying the width of the magnetic stripes and the distance from the ridge, scientists can estimate the age of the oceanic crust. This age dating method matches the predictions of seafloor spreading, with the youngest crust near the ridge and progressively older crust further away.
5. Confirmation of Plate Tectonics: Seafloor spreading, along with other evidence like the distribution of earthquakes and volcanic activity, supports the theory of plate tectonics. The movement of oceanic plates away from mid-ocean ridges, and the subduction of older crust at convergent plate boundaries, provides a mechanism for the continuous recycling of Earth's crust.
In summary, the study of magnetic anomalies on the seafloor confirms the theory of seafloor spreading by revealing symmetrical patterns of normal and reversed magnetism, age dating the oceanic crust, and supporting the broader concept of plate tectonics.