Intermission: Intermission: Plate Tectonics

National Oceanic and atmospheric Administration/National Geophysical Data Center

Alfred Wegener

Evidence: (1) Continents Fit Together

Evidence: (2) Rocks & Structures Match Up

Evidence: (3) Glacial Features

Evidence: (4) Fossils

Pangea

Animation Link

“If we are to believe Wegener’s Hypothesis, we must forget everything which has been learned in the last 70 years and start over again.” –Critic of Continental Drift in 1928

Harry Hess

Evidence: Seafloor

Seafloor Age

Seafloor Spreading

Earthquake Distribution

Volcanoes and Plate Tectonics

Plate Tectonics

Earth’s “surface” (lithosphere) is broken into plates Plates move on asthenosphere “Geology happens” where the plates interact with one another Basic Plate Tectonics

Crustal Properties CrustDensityCompositionThicknessAge continental ~2.8 g/cm 3 Felsic Thick: km Old: up to 4 Byrs oceanic ~3.2 g/cm 3 Mafic Thin: 2-10 km Young: <200 Mys

Divergent Boundary Results in the formation of Oceanic Crust

Examples:

Transform Boundary

Transform Example

Convergent Boundary: Subduction Results in the formation & growth of Continental Crust and destruction of Oceanic Crust Melting Produces More Felsic Magma

Example: Pacific Northwest

Example: Andes Mountains

Convergent Boundary: Collision Results in the growth of Continental Crust

What Drives Plate Tectonics? Internal Heat

Convection Models

Set the “Wayback Machine” to return to the Hadean…

Heat Generated by Radioactive Decay Hadean Earth was much hotter! Convection was more intense

Rapid Convection Created “Mini” Plates that were Quickly Subducted & Destroyed

Subduction Creates more Felsic Volcanic Islands

As Plates Move - Islands “Accrete” to Form Larger Islands

Greenstones & Tonalites (Granites)

Continental Crust Forms 3.96 Billion Year Old Gneiss

Archean Crust Oldest Rocks Oldest Minerals

Continental Evolution: One Reconstruction Up to 60% by the Proterozoic