This Friday's Geology Seminar speaker: - PowerPoint Presentation

Slide1

This Friday's Geology Seminar speaker:

Dr. Justin Dodd

Northern Illinois University

"High latitude marine climate proxies: Oxygen isotopes, biogenic silica, and diagenesis"

Friday, 1:00 p.m. Mudd 218

Slide2

Where we left off .....Slide3

NO ONE HAS ACTUALLY SEEN

THE MANTLE OR THE CORE

What we believe about them is based on:

1. Meteorites

There's a whole display case of real meteorites down the hall from the lab, outside Mudd 212!Slide4

2. inclusions in volcanic rocks called

xenolithsSlide5

3. rare rocks (fragments of uppermost mantle) exposed in mountains where upper parts of the oceanic crust and mantle are folded, buckled & pushed up ( called

ophiolites

).

Part of an ophiolite block in Newfoundland

Slide6

Olivine-rich peridotite

Slide7

4. behavior of

seismic waves

from earthquakes & large explosions (e.g., nuclear tests)

5. theoretical studiesSlide8

BUT for MOST of the semester, we're going to be focusing on the part we know the best - the crust.

(

Be it ever so humble, there's no place like home!

)Slide9

Plate tectonics ~

A revolution hits the Earth Sciences

This has

REVOLUTIONIZED

the Earth Sciences

much the way Darwin and Natural Selection did to

biology in the 19th century.Slide10

The earliest mapmakers noticed the apparent

close fit of continental margins.

As map making improved, the possible fits of the continents were noted by Francis Bacon (1561- 1626), George Buffon (1707-1788), and Alexander von Humboldt (1769-1859)! This isn't new!Slide11

BUT...

With better, modern mapping - the fit got even better yet! - about 96% perfect!Slide12

Alfred Wegener

proposed in 1912 that the continents were not only moving, but actually plowing through the sea floor.

The leading physicists of his day, however, were able to show that this was physically not possible. The proposed

mechanism

was thus discredited, so the entire hypothesis was set aside (by most!) as yet more wishful thinking.Slide13

Now, however, plate tectonics is accepted by virtually all of the scientific community as beyond significant doubt.

BASIC PREMISE

:

The Earth's surface is comprised of 7 major plates, and about 20 minor plates, that behave as relatively brittle slabs, called

LITHOSPHERE

. They move over slow flow in an underlying plastic layer, the

AESTHENOSPHERE

.Slide14

The plates of the

LITHOSPHERE

are ~100-150 km thick, but rigid

.

The

AESTHENOSPHERE

is plastic (soft and deformable) and generally 200-400 km thick.

NOTE that these lithospheric plates are mostly mantle, with the crust carried along on top.

LITHOSPHERIC mantle

is rigid. The

AESTHENOSPHERIC mantle

is plastic, and deformable – like warm cheese. Slide15

Note that Iceland is split between TWO plates!Slide16

SO, what's the

EVIDENCE

????

1.

Matching geology across areas of apparent fit Slide17

This even includes matching mountain belts!Slide18

Lystrosaurus, a primitive Triassic reptile

Glossopteris, a primitive seed fern, ancestor of flowering plants

specimen from Africa

specimen from Australia

2.

Matching fossils distributed across areas of proposed fit.Slide19

MADAGASCARSlide20

This is significant because

the MODERN natural biotas are

completely

different!



Monkeys that have prehensile tails are found

only

in the New World…. while the Great Apes



are

only

found in the Old World.



For

example

, the plant family Cactaceae (the cacti) is native

only

in the Western Hemisphere, while the shrubs and trees of

Eucalyptus

are found naturally

only

in Australia

Slide21

3.

Glacial geologic matching in ancient rocks - of glacial patterns that make absolutely

no

sense

unless the continents WERE connected in the past, as matching margins suggest they were.

[ Oorlogskloof Nature Reserve,

South Africa ]Slide22

4.

Paleomagnetic signatures in rocks of the sea floor...

... supplemented by demonstrably greater sea-floor ages the farther one is from the center.Slide23

5.

"Polar wandering" curves - those from North America

and Europe are very different. BUT there's only one north

pole in the world!

This apparent discrepancy ONLY makes sense if you allow the continents to move.Slide24

6.

We now even have very accurate GPS measurements that show how fast the plates are actually moving - DIRECTLY!

http://xenon.colorado.edu/spotlight/index.php?action=kb&page=3Slide25
Slide26

This is driven by CONVECTION in the mantle, produced by heat from deep within the Earth.

Soooo, how does this

work

?Slide27

If we look at the ocean floor, we see:

This mountain range is called the

Mid-Oceanic Rise and Ridge System (MORRS)Slide28

If we look closely at the MORRS, we see this topography Slide29

The increased age of rocks from the center outwards implies the ridge in the center is expanding outwards, pushing the continents apart as it grows.

This is called a

Zone of Divergence

of lithospheric plates.Slide30

BUT

,

if this is happening, either ...

… the Earth must be getting larger and larger,

OR …….Slide31

Something has to be destroyed somewhere else!

This occurs in

Zones of Plate

Con

vergence

Type I: oceanic lithosphere is

subducted

beneath a continental margin (an

Andean-type subduction zone

)

"Subduction leads to orogeny."Slide32
Slide33

This process also generates a great deal of heat, which melts rocks, producing

magma

. We'll come back to this.

This subduction process creates earthquakes, defining a plane called a

Benioff-Wadati Zone

.Slide34

Earthquake origins off the coast of Kamchatka also show this pattern, including one on February 10th, 2004, marked by the green star.Slide35

FRIDAY:

The drama continues:

WHY? HOW?

mountain building,

earthquakes, & volcanoes - oh my!