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Ch2. The Sea Floor #1 Why geology of the oceans?

Marine habitats are directly shaped by geological processes

The form of the coastlines

The depth of the water

Type of bottom (muddy, sandy, rocky)

#2 Geological times

are very different to

biological times

60% water

80% water

#3 Unequal distribution of the Ocean Basins

Pacific Ocean equals all the others

Oceans work as a big interconnected system

A) Earth’s structure Big Bang: 14,000 millions years ago

The Earth originated about 4,500 millions years ago

its orbit… allows water to exist in a liquid state!

The Earth was probably molten

because of the amount of heat

generated

This allowed materials to arrange in

layers according to their density

Density = Mass / Volume

Earth’s Internal structure

Inner and Outer Core

Mixture or alloys of iron

Inner core is solid, but outer core is liquid

Swirling motion in the outer core: earth’s magnetic field

Lower and Upper Mantle

Near melting point of the rocks

Mantle flows very slowly, almost like a liquid

Crust

“Very thin”: divided in Oceanic and Continental Crust

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A.1) The Earth’s Crust is very thin!

(USGS) 5 x 5 degree gridded crustal model CRUST 5.1 (Mooney et al., 1998)

http://earthquake.usgs.gov/research/structure/crust/index.php

Contour lines (isolines, iso=same) are in Km

Probably less than

30 km (18 miles)

on average!

A.2) Continents and sea floor have different composition

It is the nature of the rocks that determines the elevation of a particular

area of the earth’s crust, and therefore whether or not it is covered by water

Both, oceanic and continental crust

are less dense than the mantle, but they have differences in density

between them

Sea floor Continents

Basaltic rocks

(basalt, a type of mineral, dark in

color)

Granite rocks

(lighter in color )

More density Less density

Geologically young

(200 millions)

Geologically old

(3,800 millions)

These differences will matter when considering how the

different Earth plates interact!

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Evidence that continents had been

once joined ---> slowly accumulated

Coal deposits and other geological

formations are in both sides

Fossils collected in opposite sides

of the Atlantic also matched

B.1) Sir Francis Bacon, 1620

Coasts of America and Africa fit

together like pieces of a puzzle

B) Geological History of the Earth

Earth is not static and unchanging

B.2) Continental Drift (Alfred Weneger, 1912)

The continents move!

How is that possible? The process includes the entire earth surface

But no mechanism, no experiment, no facts

could prove that at that time!

• Only one “super” continent: Pangaea

• Only one ocean: Phantalassa

• A shallow sea, Tethys, separated Africa (1)

from Eurasia (2), will eventually become

the Mediterranean sea!

(2)

(1)

Tethys sea

Sinus Borealis

Pangaea began to break up about 225-200

million years ago

• North Atlantic Ocean (3)

• Laurasia: North America + Eurasia

• Gondwana: Africa, South America, India,

Antarctica, Australia

(3)

http://pubs.usgs.gov/gip/dynamic/historical.html

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http://pubs.usgs.gov/gip/dynamic/historical.html

• Indian ocean appears

• India “migrates” towards Asia

• Africa still attached to South America

• Antarctica still attached to Australia

• South Atlantic ocean appears

• India still “migrating” towards Asia

• Antarctica still attached to Australia

• The collision between India and Asia

created the Himalayas

• Australia is the last continent to separate

from Antarctica

B.3) Plate Tectonics (1950 and up)

Mid-Ocean ridge system

Submarine chain of mountains

that encircle the world

Occasionally the mountains

rise so high that give origin to

islands

e.g. Iceland, Galapagos

and the Azores islands

Trenches

Especially common in the Pacific Ocean

Sonar allowed mapping the sea floor

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Plates limits are determined by the location of ridges and trenches

1 2

3

4 5

1 Caribbean plate

2 Cocos plate

3 Juan de Fuca plate

4 Philippine plate

5 Arabian plate

What are the Mid-ocean ridge system and the trenches important?

Plates move ones against the others

Important geological activity at the ridges and trenches

Oceanic Crust disappears (is destroyed) at the trenches

New seafloor material is originated at the ridges

Plates = Crust (oceanic & continental) + upper mantle = Lithosphere (60 mi)

B.4) Seafloor spreading

Cracks or rifts

Molten material rises from the mantle

Mid-ridge

system

Seafloor at the ridge system: bands of material with

“normal” and inverse magnetization

Bands are symmetric at each side of the mid-

ocean ridge

Young rocks are located

near the ridge

Almost no sediment near

the ridge

Sediment gets

thicker and older

moving away

from the ridge

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B.5) Plates interactions 1: Activity at the Trenches

Trenches originate by the collision of two plates, one at least has to be oceanic

Continental-oceanic plate

Is always the oceanic plate (denser) the one that descends into the mantle

Subduction zone

Earthquakes

Occur as the plate sinks and breaks up

Rising magma as a result

of melting mantle

Oceanic-oceanic plate

One of the plates dips

beneath the other to

form the trench.

* The plate melts

* Some material returns to the

surface

* Again, this associated with

earthquakes and volcanoes

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Volcanoes

Concentrate around the world at the trenches

Continental-continental plate

Two plates of low density, none is subducted

Both plates weld and rise

The Himalayas!

Continental-continental plate

NO collision

The plates slide past each other

San Andreas fault!

B.6) Plates interactions 2: continental plates

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C) The Record in the Sediments 2 main types of sediments in the sea

Diatoms (algae)

Coccolithophorids (algae)

Lithogenous

(Geological)

Physical and chemical

breakdown (weathering) of

rocks (mostly continents)

Biogenous

(Biological)

“Skeletons” and shells of

marine microorganisms

Radiolarians (protozoa)

Foraminifera (protozoa)

Radiometric dating methods

• Methods based on the known,

regular decay of certain radioactive

elements (isotopes) into other

isotopes or "daughter products."

• By measuring the amount of

"parent" and "daughter" products in

a rock sample, its approximate age

may be calculated

http://paleo.cc/kpaleo/fossdate.htm

D) Geological Provinces in the Oceans

Shallowest part of the continental margin

Ranges from 0.6 to 450 mi

8% of the Ocean’s surface

accounts for the richest diversity of

organisms of the ocean and the best fishing

Edge between

the continental

shelf and the

continental

slope

Limit between continental

and oceanic crust

Thick layer of

sediment piled up on

the sea floor

Is the real oceanic crust.

Most of the deep-sea floor

lies at a depth of 10,000 to

16,500 ft

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Active vs. Passive Continental Margins

Intense

geological activity Minimal

geological activity

Steep slopes Gentle slopes

Narrow shelf Wide shelf

Presence of

trenches

NO continental rise

Flat coastal

plains

Continental rise