PT 1.3 magnetic striping

What is Magnetic Striping?

• the pattern of alternating bands of rocks with normal and reversed magnetic polarity

• found on both sides of a mid-ocean ridge

• forming a symmetrical zebra-like pattern.

Earth's Magnetic Field

Earth behaves like a giant magnet.

It has: Magnetic North and Magnetic South

Unlike the geographic poles, Earth's magnetic poles have reversed many times throughout Earth's history.

Types of polarity:

• Normal Polarity → Magnetic North points roughly towards Geographic North.

• Reverse Polarity → Magnetic North points roughly towards Geographic South.

Remember: Only the magnetic poles reverse. The geographic poles stay in the same position

How Does Magnetic Striping Form?

1. Plates diverge / move apart from each other

2. Lava erupts at the mid-ocean ridge. Lava cools into basalt.

3. Basalt records Earth's magnetic field.

4. New crust moves away from the ridge.

5. Earth's magnetic poles reverse after long periods.

6. New lava records the new magnetic direction.

7. Repeated eruptions produce alternating strips of normal and reversed polarity.

8. The same pattern forms on both sides of the ridge because plates move apart equally.

How do rocks 'record' Earth's Magnetic Field?

Oceanic crust is mainly made of basalt (Basalt: from cooled lava and contains iron-rich magnetic minerals)

When lava erupts at the mid-ocean ridge → Lava rises → Lava cools → Basalt forms

Magnetic minerals align with Earth's magnetic field → Once cooled, the magnetic direction is permanently recorded inside the rock.

Evidence of magnetic striping

Evidence 1: Rock Composition (rock material)

Evidence 1: Rock Composition (rock material)

• Basalt contains iron-rich magnetic minerals.

• These minerals align with Earth's magnetic field when the lava cools.

• Therefore, rocks permanently record Earth's magnetic polarity at the time they formed.

Evidence of magnetic striping

Evidence 2: Rock Patterns

Evidence 2: Rock Patterns

Scientists found: Alternating bands of normal and reversed polarity.

• The bands are symmetrical on both sides of the ridge, and this pattern is not random.

• This proves: New crust forms at the ridge.

• Crust moves away equally on both sides → Seafloor spreading is occurring.

Therefore... Magnetic Striping Supports the Plate Tectonic Theory (PTT) because it shows:

• New crust forms at mid-ocean ridges → Oceanic plates move apart.

• Magnetic polarity at that time is preserved in the rocks.

• This symmetrical pattern could only form if the plates have been moving over millions of years.