CONTINENTAL MARGINS AND THE SEAFLOOR
Chapter 4
• Geology in the oceans is like flying in a balloon over the continents on a cloudy day and periodically dropping a bucket over and pulling things up. • Imagine what you would think if you only got one sample from Vermont and one from Florida. • The surface of Venus has been mapped in more detail than the bottom of the ocean.
Techniques:
• 1) Drilling
– deep cores they can be >1km long, and collected in water depths > 5 km – Short cores, generally < 10 m of sediment on the bottom
• 2) Dredging, the "bucket" analogy
• 3) submersibles and ROVsubmarines manned and unmanned- can make observations and collect samples (Box 4.1)
• 4) echo soundingdetailed bottom depths- similar to "fish locators"- measures the travel time for sound waves through water.(1500 m/s)(fig. 4.2)
• 5) Seismic measurementsidentical in theory to echo sounders, but they are higher energy so the sound penetrates into the sediment • 6) Satellite altimetry – measures sea level variations (fig. 4.4)
II. PROFILES OF THE OCEAN FLOOR:
• A. Atlantic Ocean If you could drive on the ocean floor from North America to Europe, you would see the following major features: (fig. 4.5)
Continental shelf
• extends from shoreline to tens to hundreds of kilometers offshore. Depths range from 0 m to about 130 m at the edge (fig. 4.10)
• (2) Continental slope- extends from edge of shelf (130 m water depth) to depth of about 2000 to 3000 m. It is fairly narrow, around 50 km wide. • On average the slope is about 4 degrees, or about 70 m vertical for every 1000 m horizontal. Cutting across the slope are submarine canyons deep valleys eroded into the slope (fig. 4.16-17)
• (3) Continental rise- a place where the slope gradually becomes less steep, this areas grades into very flat places called abyssal plains. • They are the flattest places on earth. Occur at depths ranging from 4000 to 6000 m.
• (4) Abyssal hill- Water depth gradually becomes less deep as we move up the Mid-Atlantic ridge. (fig. 4.21-4.22) • Go up the side of the ridge until reach a steep cliff. this cliff marks the edge of the rift valley- a place where the plates are separating. • All of these features are found in a mirror image on the other side of the rift valley.
B. Pacific Ocean
• Now imagine driving from South America to Australia. Some of the same features, but different scales, and there are some new features. • (1) Continental shelf and slope- these are still present, but they are much more narrow. • The base of the slope is much deeper than the Atlantic (~8000 m) and there is no rise present.
• (2) Trench- long narrow and deep depressions on the seafloor.(fig 4.284.29)
• These are the deepest parts of the ocean, they are generally narrow and reach depths down to 11,000 m • The deepest is the Mariana's Trench in the western Pacific.
• (3) East Pacific Rise- This is similar to the midAtlantic ridge- it is a place where seafloor spreading occurs. • There are some differences, it is broader, not as deep and the central rift valley is not as large. • As you continue the journey, you would cross another trench (the Tonga trench) and then cross the continental margin of Australia. • It would look very similar to the Atlantic margin.
• From these descriptions, it would appear that there are two types: • the Atlantic type with a spreading center in the middle and no deep trenches, and the Pacific type that has trenches surrounding it. • Look at figures 4.5 to see the differences in these basins. Realize that the vertical exaggeration is very great- in other words, none of the places are as steep as these figures imply.
III. CONTINENTAL MARGINS
• There are two types of continental margins- active and passive. • Active margins have trenches on the ocean side and mountains on the continent side. • They are continental margins that also mark plate boundaries. (fig 3.14) • An example is the west coast of South America.
• Passive margins are the edge of the continent, but do not also mark the edge of a plate. • An example is the east coast of North America.
• Continental margins are shaped by sediment deposition. • Much sediment builds up on the continental shelfsediment that is eroded from the continents. • This sediment will not move over the shelf edge until disturbance (such as an earthquake) shakes the sediment loose • Sediment and water mixture flows downhill in a dense slurry of water and sediment called a turbidity current (fig. 4.18-4.19).
• There is a special kind of sedimentary deposit that is called a turbidities that forms from turbidity currents • The best studied example of a turbidity current happened after an earthquake on the Grand Banks of Newfoundland.
• The Turbidities covered nearly 100,000 km2- This is about the size of Florida. • It was ~1 m thick. The total volume was ~100 km3 • This large amount of sediment easily erodes the bottom of the seafloor. • It is one of the processes that form submarine canyons.
Mid-Ocean Ridge- (represent divergent boundaries)
• Mountain range that stick up above the surrounding abyssal plains. • It is the largest single feature on the earth- 80,000 km long, 1,500 to 2,500 km wide and 2-3 km above the abyssal plains. • In certain places it is characterized by a rift valley- A feature that is 1-2 km deep and up to ~10 km wide- approximately like the Grand Canyon
• There are many earthquakes along the rift valley, but they are shallow- generally < 20 km deep • Another important feature of rift valleys is they are the location of numerous hot springs. • Water circulates into the crust, it is heated by the magma chambers present at these places, the water rises to the surface of the crust and is expelled to the ocean.
• These places, along with rivers, are what control the chemistry of seawater. • They are also the first places where organisms were found that do not rely on energy from the sun to survive. • Look at at Box 4.2 for description of the hydrothermal systems at the ridges.
Seamounts and Guyots
• Seamounts- Conical mountains formed of basalt and thought to have once been volcanoes• Other evidence is that now there are a few seamounts that are volcanoes
• Guyots- These are round, flat-topped structures that are less than 1000 m above the surrounding seafloor. • They often are capped by coral, and generally are made of volcanic rock- basalt • Where the water is warm enough, coral reefs form around the edges of seamounts. • When the volcanoes stops growing, it begins to subside (i.e. sink) back into the ocean.
• The rate which the volcano sinks is about the same rate that coral can grow upward. • the coral reef usually forms a ring several km in diameter surrounding a shallow lagoon. • These places are called Atolls. • Look at Fig. 12.27 to see the origin of Atolls