Chapter 11

 

THE SEAFLOOR

 

Introduction

Gases derived for the earth interior were releases during volcanic eruption and probably resulted in the origin of the atmosphere and the surface waters in a process called outgassing (fig 11.3).

 

Now 71% of earth covered by bodies of saltwater called oceans (table 11.1) and seas (fig. 11.4).

 

Early Exploration

Scientific studies initiated by British in late 1700's, HMS Challenger in late 1800's.

Results of these studies indicate that the sea Floor is not flat and featureless- trenches, ridges, broad plateaus, hills, and vast plains.

 

Exploration today

 NSF Funded Glomar Challenger in 1968, and JODIES Resolution  (fig. 11.7) in 1985 to present.

 

Techniques used in ocean exploration include rock dredge, corer, drill, submersible (Alvin), echo sounder, seismic profiling (fig. 11.8), gravity and magnetic surveys.

 

Continental Margin

Submerged zone separating continental areas from oceanic areas (fig. 11.9).

 

Continental Shelf- shallow platform sloping ~0.1˚ seaward or about 2 m/km; width vary from less than 1 km to more than 1000 km; at shelf-slope break (depth of ~100-200 m), slope abruptly increases.

 

Continental Slope- average slope of ~4˚ (ranges from 1˚ to 25˚); depth of ~100-200 m to a few km; continental to oceanic crust transition occurs beneath.

 

Continental Rise- wedge-shaped deposit of sediment that extends from continental slope to deep ocean floor; slope of ~0.5˚; on oceanic crust entirely.

 

Continental margin sedimentation landward of shelf-slope break, sediment affected by wave and tidal action, whereas seaward of this boundary sediments affected only by gravity; most sediment eroded from continents ends up on cont. slope & rise; deposited by turbidity currents

 

Turbidity Currents- sediment-water mixtures denser than seawater that flows rapidly down submarine canyons (deep, steep-sided canyons present on continental shelves and slopes generated mainly by turbidity currents) to flat sea floor where it slows and is deposited as graded beds in submarine fans (figs. 11.9, 11.10).

 

 

 

Types of continental margins:

Active- at convergent plate boundaries where oceanic lithosphere is being subducted; narrow cont. shelf, no cont. rise, and a deep oceanic trench (fig. 11.12); no abyssal plain because turbidity current sediments are trapped in trench.

Passive- broad cont. shelf, slope, rise, and an abyssal plain (fig. 11.12).

 

Deep Ocean Basin

Abyssal plain- very flat area ocean ward of continental rises (fig. 11.14); rugged oceanic crust (basalt) covered by continent derived and turbidity-current deposited sediment (fig. 11.14).

 

Oceanic trench- long narrow depression at active continental plate margins related to subduction (fig. 11.14); only 2% of the seafloor; greatest ocean depths (>11,000 m) here.

 

Oceanic ridge- large underwater mountain range that extends around Earth; ~2 to 3 km higher than abyssal plains; large valley (rift valley) down center of ridge; offset by fractures perpendicular to ridges (fig. 11.14).

 

Submarine Hydrothermal Vents- hot springs near spreading ridges generated when seawater seeps down into the oceanic crust through cracks and fissures, heated, rises, and discharged onto seafloor.  Organisms live near them and are interesting for their economic potential (fig. 11.17).

 

Fractures n the Seafloor- Fractures oriented approximately normal to the ridge axes.  They abruptly terminate and offset the ridges (fig. 11.18).

 

Seamount- underwater or mostly underwater volcanic mountain (fig. 11.19).

 

Guyot- flat topped seamount originally above sea level; flattened by wave erosion (fig. 11.19).

 

Aseismic ridges- linear chains of seamounts or guyots; not associated with seismic activity like oceanic ridges are (figs. 11.14).

 

Deep Sea Sediments

Pelagic sediment- very fine-grained sediment slowly settled out of suspension far from land (coarse material too heavy to be transported far from land); consists of windblown dust, volcanic ash, and shells of microscopic organisms (fig. 11.20).

 

Structure and composition of oceanic crust- some oceanic crust not subducted; thrust up onto continents during mountain building; present on land today (fig. 11.25).

Ophiolite- sliver of oceanic crust now on continental crust; a typical sequence from top to bottom includes pelagic sediments, pillow basalt, basalt dikes, gabbro, and peridotite (mantle); boundary between gabbro and peridotite is the Moho (fig. 11.25).