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 THE SEA FLOOR

 

 

Introduction

 

The Earth is covered by 71% ocean.

 

Most of what we know about the ocean floor was discovered after 1950, when advances in technology permitted its exploration.

 

We study the ocean floor using:

1)            Sonar

2)           Core drilling

3)           Submersibles

4)           Gravity and magnetic surveys

 

The ocean floor consists of sediment lying on top of basaltic crust

 

Therefore oceanic crust (basaltic) is compositionally and structurally simpler than continental crust (chiefly granitic). 

 

 

 

Structure of the Ocean Floor

 

 

Continental shelf – An underwater platform of continental crust at the edge of a continent.  It is inclined very gently seaward at an angle of less than 1°.

 

On the Atlantic coast of the US, the shelf is 500 km wide.  On the Pacific coast, it is only a few kilometers wide. 

 

The shelf is covered with young, loose, sediment derived from the land via rivers.

 

Continental slope – A relatively steep (~4-5°) slope extending from a depth of 100-200 meters at the edge of the continental shelf down to the deep ocean floor.

 

This is where the continental crust transitions into oceanic crust.

 

Abyssal plain – The very flat region of the deep-ocean floor, consisting of oceanic (basaltic) crust and overlying sediments.

 

The plain starts at the base of the continental slope.  The water depth is about 5 km.

 

This is the flattest feature on the Earth (overlying sediment “fills in” the rugged volcanic oceanic crust). 

 

Submarine Canyons- V-shaped erosional canyons incised in the continental shelf and slope, and end at the abyssal plain.

 

Sediment transported within these canyons is deposited in fan-shaped features called abyssal fans (analogous to alluvial fans on land).

 

These underwater canyons may have initially been carved by rivers during the most recent period of glaciation when sea level was lower.

 

Currents related to tides move up and down the canyons aiding in the transport of sediment and the erosion of the canyon.

 

Also, turbidity currents (underwater landslides triggered by earthquakes or strong storms) contribute to the formation of these canyons.

 

 

 

Types of Margins

 

Passive Continental Margins (East Coast of US)

 

Passive continental margin- A margin that connects continental crust to oceanic crust without any tectonic boundaries.

 

This is a geologically "quite" boundary without volcanoes, earthquakes, or young mountain belts.  The main activity is sediment deposition.

 

Passive margins include a large continental shelf, a continental slope, an abyssal plain, and a “continental rise”.

 

Continental rise- A wedge of sediment that lies at the base of the continental slope on passive margins.  It connects the continental slope to the abyssal plain, and has a gentler slope than the continental slope (~0.5°).

 

Active Continental Margins (West Coast of US)

 

Active continental margins are characterized by tectonic boundaries, volcanoes, earthquakes, and young mountain belts.

 

They include a continental shelf and continental slope.

 

The continental rise is typically absent.

 

Instead, oceanic trenches are present.

 

Oceanic trenches are the deepest (8-10 km) parts of the ocean.  They parallel the edge of a continent and are related to a subduction zone.

 

Trenches are characterized by earthquakes associated with the subducting slab of oceanic crust (the Benioff zone).  Volcanoes are produced above the subduction zone on the continent.

 

The continental slope occurs on the landward side of the trench.  The continental slope angle changes from 4-5° on the upper part to 10-15° or more near the bottom of the trench.

 

The Mid-Oceanic Ridge

 

 

Mid-ocean ridges are giant undersea mountain ranges.  There are 49,700 miles of mid-ocean ridges on earth.  They are 930-1550 miles wide and 1.2-1.8 miles high.

 

The crests of the mid-ocean ridges are rift valleys: normal fault-bounded, down-dropped areas where the crust is undergoing extension.  They are about the size of the Grand Canyon.

 

Mid-ocean ridges are characterized by:

1. Basalt eruption (pillow basalts)

2. Shallow earthquakes

2. High heat flow

4. Black smokers (sulfide minerals) and associated exotic organisms (that survive toxic chemicals, high temperatures, high hydrostatic pressure, and total darkness).  These organisms may give some evidence for how life first evolved on earth.

 

 

 

Sediments of the Sea Floor

Sea floor sediment varies in thickness but can be up to thousands of meters thick in spots.

 

Terrigenous sediment – sediment derived from land that finds its way to the sea floor (via turbidity currents).

 

Pelagic sediment – sediment that settles slowly from ocean water.  It is made of:

a)            Fine grained clay- washed to deep sea.

b)           Volcanic ash- airborne fallout

c)            Skeletons of microscopic organisms (foraminifera and radiolarian).

 

Sediment thickness increases away from the mid-oceanic ridge.  Why?

 

 

 

Oceanic Crust and Ophiolites

Oceanic crust is about 7 km thick on average and has three distinct layers:

 

Top layer – marine sediment  (~0.5 km thick)

Middle layer – pillow basalts  overlying vertical, sheeted, basaltic dikes. (~ 1.5 km thick).

Lower layer – sills of gabbro (~5 km thick)

 

Ophiolites are slices of oceanic crust that have been “obducted” onto the continent during subduction events.  Ophiolites show a specific stratigraphy:

Top:        -Thinly-bedded chert

                        -Pillow basalts

                        -Sheeted basaltic dike complex

                        -Gabbro intrusions and sills

          Bottom:   -Peridotite/Serpentinite

 

 

 

The Age of the Seafloor

The age of oceanic crust and seafloor sediments do not exceed 200 million years in age (~ Jurassic).  In contrast, the oldest crustal rocks are 3.7 - 4.3 billion years old.

 

The young ages reflect recycling of dense oceanic crust at subduction zones.

 

The seafloor's age gets older as you go away from the mid-oceanic ridge.  Why?