Chapter 4

Continental Margins and Basins

Continental Margins• These are the areas of the edges of the

continents that are under water– Passive margins face edges of diverging plates and

have little earthquake/volcanic activity. Ex: East Coast of USA

– Active margins face edges of converging plates and have lots of activity. Ex: West Coast of USA

• 2 subdivisions: Continental Shelf and Slope

Continental Shelf• Shallow extension of the continent – Can be narrow, quickly becoming steep– Or can be broad, gently sloping downward

• They look much like the continent from which they extend: hills and depressions

• The sediment on the shelf is eroded dirt and debris from the dry land above it– Sediment is rich in minerals, oil and gas

Continental Slope• The transition between the shelf and the

deep-ocean floor– The shelf break marks the transition– Very steep

• Continental rise is past the slope– It is oceanic crust covered with sediment from

the land

Activity

• Draw, label, and color figure 4.8 on pg 93

•Draw, label, and color figure 4.10 on pg 94

Submarine Canyons• These canyons cut into the continental shelf and

slope, ending in a deep-sea fan• Sometimes they extend from the mouth of a river

that drains sediment and fast flowing water out to sea– Created by erosion

like canyons on land• Sometimes they are

created by earthquakes that cause an “avalanche” of sediments

Deep-Ocean Basins• The seafloor is a blanket

of sediment covering the basalt rock

• Consists of oceanic ridge systems and plains

• The plains are scattered with islands, hills, trenches, active and extinct volcanoes, and seafloor spreading

Oceanic Ridges• A chain of mountains formed during seafloor

spreading– Composed of “new” basaltic rock with no

sediment– If they extend above the water, they are called

islands

•

Seamounts and Guyots

• Seamounts are volcanoes that are below ocean surface– From hot spots or spreading

centers– Found alone or in groups

• Guyots are flat-topped seamounts that once went above the ocean surface – flattened by waves and erosion

Atla

ntic

Oce

an D

epth

(m)

Distance from North America (Km)

0

4,6004,8005,0005,2005,4005,6005,8006000

015

030

045

060

075

090

01,

050

1,20

01,

350

5,70

0

39o N

Trenches and Island Arcs

• Trenches are deep pits in the ocean where the converging plate is subducted– Recall the cold ocean crust is sinking into the

upper mantle of the continental plate– V-shaped due to spherical shape of Earth

• Parallel to the side of the trench, the molten rock from the subducted plate is coming back up through volcanoes called an island arc.– Example is Caribbean islands

• Cartography – the science of making maps– Dry land forms are

mapped using topography.

– Underwater landforms are maps using bathymetry.

Mapping the Oceans

Alvin explored the Titanic (HOV)

The Trieste explored the Mariana’s Trench in 1960 (HOV)

Hercules ROV

SONAR

• Sound Navigation and Ranging provided a rapid method of looking through water to identify features in the water beneath a vessel and on the sea floor.