Geologic Time

1. Deposition of the Barkley Sandstone2. Deposition of the Grin Sandstone3. Deposition of the Lemin Ashtone4. Deposition of the Cave Limestone5. Deposition of the Lewin Mudstone6. Folding7. Possible unconformity dividing upper

and lower sequences (only inferred)8. Deposition of the Jep Shale9. Deposition of the Peaty Shale10. Deposition of the Coal Measure11. Deposition of the Basin Conglomerate12. Deposition of the Deekin Sandstone13. Deposition of the Shallow Shale14. Deposition of the Park Claystone15. Faulting lower sequence into blocks16. Thrust faulting of upper sequence over

lower sequence17. Intrusion of Basalt18. Continuation of thrust faulting

A little quantitative exercise:

• A radioactive isotope A has a half life of T1/2 = 10,000 years and decays into a stable daughter isotope B. Att = 0, the rock cools just below its blocking temperature and begins to trap the daughter isotope B, which would otherwise be lost.

• On the provided sheet of graph paper carefully plot the relative abundance of A and B from t = 0 to t = 5 T1/2.

• The analysis of an igneous rock specimen yields 12,500 isotopes of A and 87,500 isotopes of B. How old is the rock?

A brief history of oceans, continents and Connecticut

• oceans : mostly basalt• continents : mostly granite

If Earth started out as one molten blob – how do we separate crust into continents and oceans ?

Magmatism of Subduction Zones

Which magma types are produced ?

(wet) partial melting in subduction zones can produce

•andesitic

•basaltic

•rhyolithic magma

image: USGS volcano hazards programMt. Spurr, AK

image: ETH

Mt. Stromboli, Italy

Clark AFB, Philippines, image: USGS volcano hazards program

http://www.elams.org/Places/Yosemite%202001/Half%20Dome%20with%20Royal%20Arches.jpg

Early Continents

• small island arcs (e.g., Japan)• have core of granitic rocks (light, won’t be

subducted)• are pushed across surface of earth by plate

tectonics• combine with other small (micro)continents• form accreted terranes

A Closer Look at Subduction Zones

• most of the oceanic slab goes down • some sediment and oceanic rock will be

caught in subduction zone to for accretionary prism

• small microcontinents will not be subducted• accreted terranes

Early Terranes of North America

More Recent Additions

From: Bell, 1985

The Wilson Cycle

• continents are hard to destroy → float on top of crust

• they are bound to crash into each other• form super continent (Pangea)

• what drives plate movement?• what makes continents break up?

The Elusive Mechanism:What Drives Plate Tectonics ?• Ridge-Push forces

from: NOAA's National Geophysical Data Center

Why do Ridges Stand High?

• temperature of rocks ?• density of rocks ?• what’s below the lithosphere ?

• what are the physical properties of the asthenosphere?

• hot rocks are less dense• buoyant• float high on asthenosphere• become denser as they cool

• oceanic lithosphere slides down the ridges

Temperature Distribution in Subducted Slab

Which Processes are Important?

• how would you investigate importance of slab pull and ridge push forces?

• where does ridge push occur?• where does slab pull occur• what is easier to move: a small or a large

plate?• which plates should move the fastest?

from: Cox and Hart, 1986

What Makes Continents Break up?

• Continents are thick, good thermal insulators• How will temperature change under thick

continental crust?• How will buoyancy change?

• How might this affect stability of continent?

The Wilson Cycle

• continents are pushed together by plate tectonics

• can form super continent• thick layer of insulating rocks• base of continent heats up• softens rocks, makes them buoyant• continents can drift apart

from: Bell, 1985