terra firma? - uc berkeley seismology...

1

Environmental Geology – Terra firma?

Terra firma?

Reading: This week: Ch 3Next week: Ch 4

Internal processes:

Environmental Geology – Terra firma?

Toward a unifying theoryPlate tectonics:

Plate tectonics: Recognizes that rigid plates are moving across the Earth’s surface.

A unifying theory: A single theory that explains a large body of evidence.

Why did it come so late (1960s)?

The idea was around long before substantial evidence

Environmental Geology – Terra firma?

A continental jigsaw

1620: Francis BaconAfrica and South America fit together

1655: Antonio SniderPublished a sketch

Mid 1800’s: First proposed that continents did fit together

Toward plate tectonics:

Environmental Geology – Terra firma?

Glacial striationsToward plate tectonics:

Glacial evidence in the tropics!

Also, coal deposits in Antarctica

2

Environmental Geology – Terra firma?

1912: Alfred Wegener – Continental Drift

1. Similar fossils: N America and EuropeS America and Africa

2. Similar rock formations across boundaries

Conclusion: There used to be a supercontinent - Pangaea

Flora and faunaToward plate tectonics:

MesosaurusGlossopteris

Environmental Geology – Terra firma?

Mantle convection?Toward plate tectonics:

1928: Arthur Holmes

Proposed that thermal convection in the mantle could be the driving force for plate tectonics.

However, this was a speculative idea. There was no observational evidence supporting mantle convection.

Environmental Geology – Terra firma?

SkepticsToward plate tectonics:

Wegener’s ideas were only discussed for a decade and then dropped.

• The coastlines don’t match perfectly

• Were the rock formations really the same?

Alternative: land bridges between the continents could explain the fossil record

Environmental Geology – Terra firma?

BathymetryToward plate tectonics:

3

Environmental Geology – Terra firma?

Magnetic stripesToward plate tectonics:

QUESTION

Environmental Geology – Terra firma?

The Earth’s magnet

…and magnetic rocks

Environmental Geology – Terra firma?

Magnetic volcanoes

Environmental Geology – Terra firma?

Seafloor spreading?Toward plate tectonics:

QUESTION

4

Environmental Geology – Terra firma?

Seafloor ageToward plate tectonics:

Environmental Geology – Terra firma?

Identifying the platesToward plate tectonics:

Environmental Geology – Terra firma?

Subduction at the trenchesToward plate tectonics:

Environmental Geology – Terra firma?

Subduction at the trenchesToward plate tectonics:

5

Environmental Geology – Terra firma?

Identifying the platesToward plate tectonics:

Environmental Geology – Terra firma?

Reconstructing plate motion

Plate tectonics:

The magnetic tape recorder:

Given the oceanic record we can wind back the plates to see how the plates have been moving.

This shows us how Pangaea broke up.

Environmental Geology – Terra firma?

Continental magnetizationPlate tectonics:

Environmental Geology – Terra firma?

Plate motion further back in timePlate tectonics:

6

Environmental Geology – Terra firma?

The formation of PangaeaPlate tectonics:

When we add the continental plate motion recoded we can extend plate reconstruction back beyond 200Ma

i.e. prior to the break-up of Pangaea

Environmental Geology – Terra firma?

The unifying theoryPlate tectonics:

Plate tectonics: Recognizes that rigid plates are moving across the Earth’s surface.

A unifying theory: A single theory that explains a large body of evidence.

Other examples:

Physics – theory of relativityChemistry – nature of a chemical bondAstrophysics – the Big BangBiology – DNAGeology – plate tectonics

Environmental Geology – Terra firma?

Properties of geological materialsEarth’s interior:

How can plates move across the Earth’s surface?

The Earth is not a rigid solid, but a plastic solid.

Environmental Geology – Terra firma?

Stress and strainEarth’s interior:

7

Environmental Geology – Terra firma?

Stress-strain diagramEarth’s interior:

Environmental Geology – Terra firma?

Brittle and ductile rocksEarth’s interior:

Brittle failure

Ductile flow

Environmental Geology – Terra firma?

An overviewEarth’s interior: