Updated Schedule

Vocabulary & Concepts

• Accretion • Differentiation • Out-gassing • Mafic • Felsic • Earth’s Interior: layers, properties • Dim Sun • Hadean • Archean • Theia • Solar Wind • Late Heavy Bombardment • Rock vs Mineral • Types of Rocks • Principles of Stratigraphy

Recap Blue Planet: Oceans and Atmosphere

Where Did Earth’s Water Come From? (TEDEd 3:52)

https://www.youtube.com/watch?v=RwtO04EXgUE&vl=en

Lecture 7 – Hadean Era: Gray Earth; Plate Tectonics Earth Science 23 - Historical Geology Columbia College Spring 2019

TERRA FIRMA??? Something seems to be moving! (some early insights) • Map makers notice the remarkable match between Africa and South America coastlines after exploring the New World.

• Sir Francis Bacon (1620) notes that continents look like they must have fit together.

• Benjamin Franklin (1782): “The crust of the Earth must be a shell floating on a fluid interior…Thus the surface of the globe would be capable of being broken and distorted by the violent movements of the fluids on which it rested.” Alfred Wegener (1880 – 1930)

In 1915, Alfred Wegener proposes the “continental drift” hypothesis.

Proposal: The continents are drifting about Earth’s surface. They were previously together in a supercontinent “Pangea” about 250 million years ago.

Evidence???

Exhibit A:

Consider the Fit of the continents

Exhibit B: Match of rocks and mountain ranges across oceans

Exhibit C: Distribution of animal and plant fossils Exhibit D: Correlation of Glacial deposits But … Wegener’s Continental Drift Doesn’t explain:

• What force drives this movement of continents?

• How can continents plow through the ocean floor or slide over it?

• How does rock “flow”?

Wegener’s ideas remained “on hold” for decades.

After WWII, sonar reveals:

• A Mid-Oceanic Ridge (40,000 mile long volcanic Mountain Range!)

• Ocean peaks are usually 1,000 feet below sea level (Exceptions include Iceland) Ocean Floor Map by Tharp and Heezen (1977) Deep sea trenches:

– always parallel to continental coastline – Always associated with a volcanic mountain range.

• If the trench is close to the coastline: we find a “Continental Arc” volcanic range

• If the trench is far offshore: we find a volcanic island chain. More interesting findings …

Paleomagnetism record in rock shows periodic reversal of Earth’s magnetic field.

• Continental magnetic evidence is puzzling. Alignment of minerals formed at same geologic time but different continents point in different directions. Were there many poles??? The evidence accumulates

Paleo-magnetism recorded in rock shows reversal of Earth’s magnetic field through time.

Ocean evidence is enlightening…there’s symmetric banding! Zebra stripes … A mirror image! Sea floor spreading hypothesis explains it… but where does the extra crust go? Possible Explanation - the ocean floor spreads outward from ridges then “subducts” into trenches Plate Tectonics About 1960s, the theory of Plate tectonics becomes widely accepted … about time!

Tectonic plates and boundaries

Earth has eight major and dozens of minor plates, each of which is composed of crust and upper mantle. Plate boundaries fall into several categories. Convergent boundaries include subduction zones where one plate dives beneath another, as occurs along the coast of South America, and continent-continent collisions, such as where India is colliding with Eurasia and raising the Himalayas. Divergent plate boundaries are where two plates are moving apart and new crust forms as magma rises up to the surface and solidifies, such as at the Mid- Atlantic Ridge. Transform boundaries, where plates slide past one another, occur in places such as the West Coast of North America along the San Andreas Fault. Also noted on this map are “diffuse boundary zones,” where wide swaths of crust are deformed due to a faraway boundary. Source: AGI/The Geoscience Handbook, 2016. Earth’s Tectonic Plates & Three kinds of Boundaries

Divergent Plate Boundaries

• Plates move away from each other…Diverge

• New lithosphere is created here

• Divergence between two plates: – Causes mid-ocean ridges – Seafloor spreading – Examples: Mid-Atlantic Ridge (Iceland), African Rift Zone

Convergent Plate Boundaries

• Where plates collide

• Collisions between continent and ocean plates: Subduction Zones – More dense ocean plates sink (subduct) and melt beneath “floating” continent. – Melted magma rises to form volcanoes.

• Collisions between two continental plates: Continental Collision Boundary – Neither plate subducts, instead the plates crunch together. – Form large mountains such as Himalayas. Transform Boundary

• Plates slide past one another

• Mostly occur in the ocean

• Ex.: San Andreas Fault – Separates the Pacific from the North American plate

Earthquakes mostly occur along Plate Boundaries Volcanoes usually occur along Plate Boundaries While continental crust that is billions of years old still exists on Earth’s surface, most oceanic crust is less than 200 million years old (Ma). Older oceanic crust, which is more dense than continental crust, has long since been recycled in the process of subduction. Credit: U.S. Geological Survey Theory of Plate Tectonics Explains so much:

• Oceanic Trenches • Mid-oceanic ridges • Mountain building • Faulting and earthquakes • Volcanoes

What Drives this Motion?

What Force Can Move these Plates? Sources of Earth Energy

• Solar Energy: radiant energy (40% reflected back to space 99.9% of energy to earth surface)

• Internal Energy: – radioactive energy (especially Potassium, Uranium, Thorium) – gravitational energy (pressure, heat)

Convection Cells

In 1919, Arthur Holmes speculates radioactive heat drives convection “cells” that carry Earth’s floating plates.

His supporting evidence required another 30 years (and the “Cold War”). Plate Tectonics - Key Points The earth’s crust is divided into about 12 major tectonic plates. The evidence supporting Plate Tectonic Theory has been accumulating for centuries. Evidence now includes: – Apparent match of continents across oceans. – Matching of rocks and mountain ranges – Matching of plant and animal fossils – Matching of glacial deposits – Discovery of long, volcanic mid-oceanic ridges – Discovery of deep oceanic trenches off continental coasts and paralleled by volcanoes – Relative youth of ocean floor – Relative dearth of sediments on ocean floor – Observed geologic activity along plate boundaries

Partial Melting Partial Melting – Silica & Temperature Partial Melting: Felsic/Mafic ~ Differentiation Archean – Gray Earth

As a consequence of mantel plumes and partial melting of the early basaltic crust, granite islands were created. On-going accumulation eventually led to granitic islands, island continents, and finally super- continent/breakup cycles.

Feature Presentation

Plate tectonics on small scale (3:02): https://www.nationalgeographic.com.au/videos/geologic-journey/african-rift-lava-lake-1298.aspx