Oceanography 120 Lecture Outline –

Seafloors and Marine Sediments

I. Lecture Content

Introduction – Earth's Hidden Surface - The Seafloor

Seafloor Topography - Undersea Bottom Relief

Seafloor Physiology - Major Features of the Sea bottom

Continental Margins - Where Ocean Meets Continent

Deep-Sea Floors - Basaltic Crust Conveyor Systems

Marine Sedimentation - Mucky Clays and Oozes

Islands, Reefs and Atolls - Interesting Places

Life and Death of an Ocean Basin - The Wilson Cycle

Sea Bottom Resources - Natural Treasure

II. Introduction

A. The Earth is Truly a Water Planet - "Oceania"

1. Seafloors cover nearly 3/4's of Earth's surface.

2. The seafloor is the final receptacle for vast

amounts of terriginous and organic material.

3. Ocean basins form wide barriers between the continents

B. The Ocean Realm is Earth's Last Frontier - Why?

1. Remote and inhospitable - It’s a harsh "inner" space.

· Conditions at sea can be very uncomfortable

· Water pressure is tremendous at great depths

· Deep sea world is very wet, cold, and in complete darkness

C. Earth's Seafloors are Vast, Rugged & Resource-rich

1. A highly variable topography, which includes, rugged

mountain ranges, very deep trenches, amazing island

chains, ocean plateaus, seamounts, and deep canyons

that rival anything seen on land.

III. Seafloor Topography - Underwater Bottom Relief

A. Earth has Two Distinctive Topographic Regions

· Continental Highlands - Continents

· Oceanic Lowlands - Ocean basins

B. The Earth's Seafloors are Rugged in Appearance and

Have Considerable Topographic Relief.

· See Figures

· Much more topographic relief than the continents

· Seafloors have distinctive topographic features

· Seafloors look much different than dry continents

C. Earth's Seafloor is Divided into Two Major Provinces

1. Continental Margins

· Submerged shallow platforms

· Floored mostly by granitic rock

· Varies greatly width, depth, and topo relief

· Vast majority of marine life concentrated there

2. Deep-Ocean Basins

· Starts at base of continental margins

· Deep seafloor consists primarily of:

1. High-standing mid-ocean ridge systems and

2. Low-standing sediment-covered abyssal plains

3. The two-province division is based upon the major

inherent differences between continental and oceanic

crust.

· Composition (density)

· Thickness

· Isostatic equilibrium

IV. Continental Margins

A. Shallow Seafloor Rims of Ocean Basins

1. Continental margins - the submerged edges of continents

2. Continental margins are underlain by faulted blocks of

granitic crust , overlying sediment piles, and possible

accreted subduction zone material

B. Continental Margins are Classified into Two Types

1. Passive Margins = Atlantic Ocean style

§ Situated within a plate

§ Develops after continental rifting and opening of a new ocean basin opening

§ Typically broad (avg. 100's km) with a very thick pile of

accumulating sediments

§ Lacks much seismic or volcanic activity

2. Active Margins = Pacific Ocean style

· Situated at the leading edge of a continental plate

· Develops after initiation of subduction

· Typically narrow with rugged topography

· Outer edge typically forms inner wall of ocean trench

· Regionally unstable with much seismicity & volcanism

C. Physiological Features of a Continental Margin

· Continental Shelf

· Continental Slope

· Submarine Canyons

· Continental Rise

D. The Continental Shelf

· Shallow, submerged edge of continent between the

shoreline and continental slope (shelf-slope break)

· Has a very low sloping angle (<< 1degree)

· Typically shallow water depths (avg. = 75 m = 250 ft)

· Greatly influenced by fluctuations in sea level

· Shelf sediments are mainly influenced by waves and

tidal currents

· Site of abundant mineral resources and sea life

E. Continental Slope and Rise (See Figs )

· Deeper, steeper, outermost edge of continent between the continental shelf and the deep ocean floor

· A continental rise may separate the continental slope

from the deep ocean basin along passive margins

ü A continental rise forms a thick pile of sediments

that have accumulated at the base of the

continental slope

· The shelf-slope break marks the abrupt transition between the slope and the shelf

· Location of Earth's greatest depository of sediments

ü Roughly 70% of Earth's sediments

· Slope and Rise sediments are mainly influenced by

gravity, and are transported down-slope via strong

turbidity currents and deposit as submarine fans.

· Submarine canyons and fan deposits are present on all continental slopes and rises, and on some continental shelves

· Submarine fan deposits grade into deep-ocean deposits

V. Deep-Ocean Basins - True Oceanic Seafloor

A. Ocean Basins are Classified by Size and Extent

· Oceans - broad, large, and globally extensive

Examples: Pacific, Atlantic and Indian

· Seas - narrow, smaller, and regionally limited

Example: Mediterranean, South China, & Red

B. Deep-ocean basins are underlain by basaltic crust

1. Ocean Crust - A typical cross section (See Figs .)

· Layered basaltic crust covered by sediments

· Rugged volcanic surface covered by layers upon

layers of very fine pelagic sediment

ü Pelagic clays

ü Silica and carbonates Oozes

· Oceanic igneous crustal column is also layered

Ø Pillow lava basalt

Ø Sheeted gabbroic dikes

Ø Massive gabbro (intrusions)

Ø Layered gabbro (intrusions)

Ø Layered Peridotite

· Oceanic crustal sections found on land are

termed an ophiolite suite

C. Ocean Basins are Relatively Young Earth Features

· Oldest part of ocean basins is 180 million years old

· Average age of deep ocean seafloor is 60 million y.o.

· Age distribution pattern of deep-ocean crust is striking

· See Figure 3.27 - Ocean crust age map

D. Deep-ocean basins are rugged with variable relief, and

have a wide variety of distinctive physiological features

Ø See Figure 4.30 - Seafloor Topographic Map

1. Mid-ocean ridges

2. Mid-ocean ridge fractures

3. Hydrothermal vents

4. Abyssal plains and Abyssal hills

5. Seamounts and Guyouts

6. Oceanic island chains

7. Oceanic plateaus

8. Trenches and Island arcs

E. Most Deep Ocean Features are the Result of Seafloor

Spreading Processes Occurring at Mid Ocean Ridges

1. Seafloor spreading processes create:

ü Mid-ocean rift valleys and ridge flanks

ü Vast expanses of ocean crust (abyssal plains)

ü Chains of volcanoes (seamounts and islands)

ü Transform fracture systems

ü Hydrothermal systems (black smokers)

VI. Marine Sedimentation - Pelagic Clays and Oozes

A. Sediments - Defined

1. Sediments are particles that accumulate on the sea floor. Sediments have three major sources: lithogenous, biogenous, and hydrogenous.

B. Nearly All Deep Ocean Sediment is Very Fine Grained

· No mechanisms to transport coarse-grained material

ü Exception is icebergs

· Vast majority of deep seafloor sediment is deposited via

vertical settling of suspended material

· Sediment deposited from suspension is termed pelagic

C. Sediments Sources

1. Sediments have four major sources:

· Lithogenous

· Biogenous

· Hydrogenous

· Cosmogenous

2. Classification Chart of Marine Sediments – See Table 5.2

D. Lithogenous Sediments

1. Lithogenous Sediments come from Rocks.

· Lithogenous sediments are produced by weathering processes, which break up rocks into smaller fragments and, sometimes, change their chemical composition.

· Clays and quartz are major end products of weathering.

· Most lithogenous sediments are found near land, since nearly all lithogenous sediment comes from the continents.

2. River Sediment via Turbidity Currents

· Turbidites and Submarine Canyons

3. Windblown dust and volcanic ash

· Continentally derived

· Deep-sea clays are termed pelagic clay

4. Accumulation Rates of Lithogenous Sediments

· In river deltas, sediments can accumulate at >800 cm/year. Tidewater glacial fjords also have extremely high sedimentation rates.

· In estuaries and bays, such sediments accumulate at about 0.5 cm/year. These often "trap" the sediment carried by rivers and streams.

· On the continental shelf and slope, sediments accumulate at about 10-40 cm/1000 years. Some shelves presently accumulate no sediment at all.

· Large areas of the deep sea, deeper than about 4500 m, are also covered with lithogenous sediment, called red or brown clay. Most of this sediment reaches the mid-ocean regions where it is found via wind transport.

· The deep ocean has extremely low sedimentation rates, <1 cm/1000 years.

5. Lithogenous Sediment is Sorted by Size

· Lithogenous sediments tend to be made up of progressively smaller particles with increasing distance from shore. This is because the larger particles sink more quickly, and so can't be transported as far by ocean currents.

· An exception to this pattern is rocks and gravel rafted by glaciers.

E. Biogenous Sediments

1. Defined: Sediments consisting of at least 30% of the remains of marine plankton (small marine plants and animals) are termed biogenous oozes.

2. Two Categories of microscopic marine organism skeletons

· Carbonate hard parts = calcareous ooze

· Silica hard parts = siliceous ooze

3. Calcareous oozes are made up of the remains of tiny shells or tests consisting of calcium carbonate.