Science 8

Ms. Jamieson

Theory of Seafloor Spreading

Seafloor Spreading= The process in which magma rises to Earth's surface at spreading ridges, and as it continues to rise,pushes older rock aside

  • proposed by American geologist Harry Hess in 1960
  • provides a mechanism for Wegener’s Theory of Continental Drift

Example of Seafloor Spreading: Mid-Atlantic Ridge = a long mountain range running north to southdown the length of the Atlantic Ocean

Evidence for Seafloor Spreading:

  1. Age of ocean rocks: the youngest rocks are found closest to the ridge, indicating that new rock is being formed
  2. Sediment thickness: The layer of ocean sediment becomes thicker the farther it is from the ridge, indicating the sea floor is older the farther it is from the ridge
  3. Magnetic striping: an alternating pattern of normal and reversed magnetic fields found in rocks on both sides of the Mid-Atlantic Ridge Ridge, indicatingthat new rock is being laid down on the sea floor

Explanation for Magnetic Striping

As magma cools and hardens into rock, iron-containing mineralsin the magma align themselves with Earth’s magnetic field. Every couple of hundred thousand years, Earth’s magnetic field undergoes magnetic reversal. Rocks preserve the direction of the Earth’s magnetic field as it was at the time that the rocks were formed

Theory of Plate Tectonics

Plate Tectonic Theory= The Earth’s surface is broken into tectonic plates - large, slabs of rock that slide over a layer of partly molten rock. When tectonic plates move across Earth’s surface, they carry the continents with them

  • proposed by Canadian geologist J Tuzo Wilson in the 1960’s

Evidence for Plate Tectonics Theory:

Geological Hotspots= areas where magma (molten rock) has risen to the Earth’s surface and breaks through the crust to form volcanoes

  • Example: chains of volcanic islands, such as the Hawaiian Islands. The northernmost island (Kauai) has the oldest rocks and the southernmost island (Hawaii) has the youngest rocks.
  • providesevidence for tectonic plates moving across the Earth

Explanation:The islands were formed as a tectonic plate moved northward, and passed over a stationary hotspot. Magma pushed through the ocean floor several times to create a chain of volcanic islands

  • Plate Tectonics is considered theunifying theory in geology

Plate Tectonic Theory unifies the theories of Continental Drift and Seafloor Spreading