THE SEA FLOOR

about 4.6 billion years old

Geology: the science that deals with the dynamics and physical history of the earth, the rocks of which it is composed, and the physical, chemical, and biological changes that the earth has undergone or is undergoing

In most cases we will discuss geology in terms of habitats, the places in which organisms live

Oceans cover 72% of the globe and 80% of that is still considered undiscovered◦ 2/3 of the earth’s land mass is in the Northern

Hemisphere, which is only 61% ocean

◦ 80% of the Southern Hemisphere is ocean

1. Pacific - Largest and deepest◦ almost as large as the

other 3 combined

2. Atlantic - Second largest◦ similar to Indian in avg.

depth

3. Indian - Third largest◦ similar to Atlantic in avg.

depth

4. Arctic - Smallest and shallowest

All four basins are connected

This connection is most obvious when the world is viewed form the south pole ◦ Fig 2.2

Oceanographers often speak of one world ocean

World Ocean: Refers to the continuous body of water that surrounds Antarctica as the Southern Ocean

The Earth and our solar system is thought to have originated about 4.6 billion years ago from clouds or clouds of dust.

Big Bang: A great cosmic explosion that is estimated to have occurred 13.7 billion years ago in which dust particles collided with each other merging into larger particles that again collided with each other eventually building up the earth and other planets

Density: the mass of a given volume of a substance; Mass/Volume

When the Earth was formed it was most likely molten◦ This allowed the

interior of the Earth to form based on density

Fig 2.3

Core: Innermost layer of the Earth that is composed mainly of iron◦ Pressure at the core is more than a million times

the pressure on the Earth’s surface

◦ Made up of a solid inner core and a liquid outer core

◦ It is thought that the swirling motions of the liquid material in the iron-rich outer core produce the earth’s magnetic field

Mantle: The layer outside the earth’s core◦ most of it is thought

to be solid, but very hot - near the point of melting rock

◦ much of the mantle slowly flows almost like liquid

Lithosphere: The fairly rigid layer of the earth’s surface composed of the crust and upper-most part of the mantle.◦ About 100 km (60 mi) thick

◦ Means “rock sphere”

◦ Broken up into a number of plates called lithospheric plates

Lithospheric Plates: A part of the lithosphere that can contain continental crust, oceanic crust, or both

Asthenosphere: The denser, more plastic layer of the upper mantle on which the lithosphere floats

The distinction between the lithosphere and the asthenosphere is based on how easily the rock flows

Crust: The outermost layer of the earth◦ extremely thin in comparison to the other layers of

the earth

◦ Earth’s skin

The geological distinction between ocean and continents results from physical and chemical differences in the rock that makes up the crust

Oceanic crust: The earth’s crust that makes up the sea floor, generally made up of the mineral basalt◦ Denser than

continental crust

◦ Thinner than continental crust

Continental crust: The earth’s crust that makes up the land portions of the world, most continental rocks are made of granite◦ Older than the oceanic crust

There is constant geological change

Sir Francis Bacon ◦ noted that the coasts of the continents on opposite

sides of the Atlantic fit together like pieces of a giant puzzle

◦ Evidence in coal deposits, geological formations, and fossil findings

Alfred Wegener◦ German geophysicist

◦ proposed the first detailed hypothesis of continental drift in 1912

◦ suggested that all the continents had once been a single “supercontinent” he named Pangaea

biggest obstacle was his inability to explain how this had occurred

Continental drift: The movement of continental masses on the surface of the earth

Pangaea: The single large landmass, or supercontinent, that broke up to form today’s continents (Fig 2.16a)

One giant sea called Panthalassa which is the ancestor of the modern Pacific

Theory of plate tectonics: The process involved in the movement of large plates on the earth’s crust

Mid-ocean Ridge: A continuous chain of submarine volcanic mountains that encircles the globe like the seams on a baseball◦ The largest geological feature on earth

◦ Displaced at locations by cracks known as faults

◦ Occasionally these mountains rise so high that they break the surface to form islands such as Iceland and the Azores

◦ Earthquakes tend to cluster near the sites of ridges

Fault: A crack in the earth’s crust usually formed when two pieces of crust are moving past each other

Mid-Atlantic Ridge: a chain of submarine volcanic mountains that runs down the center of the Atlantic Ocean, closely following the curves of the opposing coastlines. ◦ The ridge forms an inverted Y in the Indian Ocean

and runs up the eastern side of the Pacific

◦ The main section of ridge in the Eastern Pacific is called the East Pacific Rise.

Surveys of the sea floor also revealed a system of trenches

Trench: A deep depression in the sea floor◦ especially common in the Pacific

◦ Usually many volcanoes nearby

◦ Lithosphere is destroyed

A trench is formed when two plates collide and one of the plates sips below the other and sinks back down into the mantle

Subduction: The downward movement of the plate into the mantle

Trenches are also known as Subduction Zones

The nature of the sea floor is related to the mid-ocean ridge

Sea floor rock right near the ridge is very young and the rock gets progressively older moving away from the ridge

Sediment: Loose material like sand and mud that settles to the bottom, layers get thicker at greater distance from the ridge

From time to time the earth’s magnetic field reverses direction◦ Reversals happen every 700,000 years on average

◦ thought to be related to movements of material in the earth’s outer core

Many rocks contain tiny magnetic particles

Magnetic Anomalies: Magnetic bands or “stripes” in the sea floor running parallel to the mid-ocean ridge

Rifts: Cracks in the oceanic crust separating at the mid-ocean ridges

releases some of the pressure form the underlying mantle

allows hot mantle material to melt and rise up through the rift

magma pushes up the oceanic crust around the rift to form the mid-ocean ridge (Fig. 2.9)

Sea-floor Spreading: The entire process by which the sea floor moves away from the mid-ocean ridges to create new sea floor

Also known as spreading centers

explains crust and sediment build up, and magnetic anomalies

Continental margins: are the boundaries between continental crust and oceanic crust

The shallowest part of the continental margin is the continental shelf

consists of the shelf to shelf break, continental slope, continental rise to abyssal plains

Continental shelf

make up about 8% of the ocean’s surface area

biologically richest part of the ocean◦ most life and best fishing

◦ composed of continental crust

The continental shelf ends at the shelf break where the slope abruptly gets steeper

Two types of margins

Active Margins: zones of intense geological activity, including earthquakes and volcanoes◦ ex. South America’s western coast

Passive Margins: zones of little geological activity with flat coastal plains, wide shelves and gradual continental slopes◦ ex. South America’s eastern coast

Ocean sediment holds many clues to the earth’s past

Most marine sediments are of two basic types:

lithogenous sediment: which is derived from weathering and is usually deposited near coasts

biogenous sediment: which consists of the shells and skeletons of marine animals

some biogenous sediment is composed of calcium carbonate (CaCO3)

This is called calcareous ooze

Uses the isotope C14 to age a fossil

Half-life of 5,700 C14 to tell the age of the fossil, the percentage of C12 remains constant

Can date something up to 50,000 - 60,000 years old

Global warming is the increase in the average temperature of the Earth’s near surface air and oceans since the mid-20th century, and its projected continuation

Increasing global temperature is expected to cause sea levels to rise, an increase in the intensity of extreme weather events, and significant changes to the amount and pattern of precipitation

Likely leading to an increase in tropical areas and increased rates of desertification, changes in agricultural yields, modification of trade routes, glacier retreats, mass species extinctions and increases in the ranges of disease vectors