Evolution of the Earth and Our Restless Planet: Chapter 33 Plate Tectonics Evolution of the Earth and Our Restless Planet: Chapter 33

Review: Interior

33.1 The Theory of Continental Drift Scientists in the early 20th century believed Oceans and continents were fixed in place The surface is a skin spread over a cooling interior Cooling caused contraction of the skin= wrinkles and bumps aka mountains and valleys

Alfred Wagner Thought Differently! He believed: The continents are slowly moving All the continents has once been attached in a supercontinent and this was called Pangaea “all land” Pangaea split and the continents moved apart The boundary of each continent is not at the shoreline but the continental shelf

Alfred Wegener cont. No one believed Wagner…..at least not yet!

Scientific Revolution Discovery of the Earth’s Magnetic Field gave a window into the past Paleomagnetism (magnetism from the geologic past…shows 3 major things) The position of the magnetic north versus the geographic north pole Direction to the magnetic pole at the time the rock was formed Magnetic latitude of the rock’s location at the time it was formed The movement of the magnetic pole over time(500 million years) suggests that either the pole moves around or the continents move

The Earths North and South pole have flipped many times These leaves magnetic ‘stripes’ in rock containing iron minerals

Seafloor Spreading: H. H. Hess 1950’s had detailed mapping of the ocean floor Figured out Some really deep parts are next to the continents and some shallower parts are out in the middle due to the mountains under the water Why?

Finally Wagner's’ Ideas became the: Theory of Plate Tectonics The earth’s outer shell, the lithosphere, is divided into 8 large plates and a number of smaller ones Most earthquakes, volcanoes, and mountains occur along the borders

All Based on Motion It turns out that the upper section of the mantle is stuck to the underside side of the crust to form what we call tectonic plates

Evidence of Tectonic Plates Our first evidence of tectonic motion is based on similar fossils and rock types on opposing sides of the ocean

Tectonic Plates Today plate boundaries are determined by examining the location of volcanoes and earthquakes. Volcanoes result from the friction (heat) of the plates motion. Earthquakes occur where plate rub against one another

Tectonic Plates Volcanoes

Tectonic Plates

Why do the Plates Move? No single idea explains everything but we can identify several forces that contribute to the movement of the plates. Slab pull The sinking of the cooled dense oceanic plates pulls on the rest of the plate Ridge rises The material deposited on the top of the ridge slides downs from the rise pushing on the plate Convection Movement within the mantle could be part of the driving force behind the motion of the plates.

Why do the Plates Move?

Why are they spreading? Why is the Atlantic still getting wider The plates are pulled apart by convection currents in the mantle below Caused by heat released from natural radioactive processes At the mid Atlantic ridge molten rock from below rises up to fill the gap with new basaltic rock

Plate Boundaries Convergent – plates move toward one another Divergent – plates move away from each other Transform-Fault – plate moves sideways from each other

Plate Boundaries

Convergent Plates

Convergent Plates Ocean-Continental Convergence Ocean-Ocean Convergence Continental-Continental Convergence

Divergent Plates Spreading centers on the ocean floor and on land (create rifts) These create the beginning of a new ocean basin

Transform Plates

Transform Plates San Andreas Fault

Mid-Plate Hotspots Hawaii

Pangaea

Pangaea

Pangaea The break up of Pangaea

Where are we going? We appear to be headed for another super continent as North America, South America, Asia and Australia converge in the ever shrinking Pacific Ocean