Chapter 13 Concept Check

Explain what happened when the spreading center that generated the Farallon plate collided with the North American plate.

A transform boundary was created

How does an accretionary wedge form?

An accretionary wedge forms at an active continental margin when the subducting oceanic slab scrapes pieces of itself onto the more buoyant continental slab.

Why does the lithosphere thicken as it moves away from the ridge as a result of seafloor spreading?

Because it is cooled from above as seawater circulates through the pore spaces and fractures in the rock.

Why are abyssal plains more extensive on the floor of the Atlantic than on the floor of the Pacific?

Because the Atlantic Ocean has the most extensive abyssal plains because it has fewer trenches to act as traps for sediment carried down the continental slopes

How are deep-ocean trenches related to plate boundaries?

Deep-ocean trenches are sites of plate convergence where an oceanic plate subducts under another plate.

Name a modern example of a continental rift

East African Rift Rio Grande Rift (Southwestern United States) Baikal Rift (South-Central Siberia) Rhine Valley Rift (Northwestern Europe)

Briefly describe the four layers of the ocean crust.

Layer 1 - Upper layer is a sequence of deep sea sediments or sedimentary rocks. Sediments are very thin near the axes of oceanic ridges but may be several kilometers thick next to continents. Layer 2 - Below the layer of sediments is a rock unit composed mainly of basaltic lavas that contain abundant pillowlike structures called pillow basalts. Layer 3 - The middle rocky layer is made up of numerous interconnected dikes that have a nearly vertical orientation, called the sheeted dike complex. These dikes are former pathways where magma rose to feed pillow basalts on the ocean floor. Layer 4 - The lowest unit is mainly gabbro, the coarse grained equivalent of basalt, which crystallized deeper in the crust without erupting. Ophiolite complex- fragments of oceanic crust and mantle on land.

Although oceanic ridges can be as tall as some mountains found on the continents, list some ways in which the features are different

Most mountain ranges form through the collisions of two continents, whereas oceanic ridges form when material upwells from the mantle and generates new oceanic crust.

How does a sheeted dike complex form?

New dikes intrude older dikes, which are still warm and weak, to form a sheeted dike complex.

What is the primary reason for the elevated position of the oceanic ridge system?

Newly created oceanic lithosphere is hot and therefore less dense than cooler rocks of the deep ocean basin.

Explain why oceanic lithosphere subducts even though the oceanic crust is less dense than the underlying asthenosphere

Oceanic lithosphere subducts because its overall density is greater than that of the underlying mantle.

How does a flat-topped seamount, called a guyot, form?

Over time, guyots lose their original shapes and sizes through the process of weathering, after moving away from their original locations over hot spots.

Describe the differences between active and passive continental margins and give a geographic example of each

Passive continental margins are geologically inactive regions located some distance from plate boundaries. Active continental margins are located along convergent plate boundaries, where oceanic lithosphere is being sub-ducted beneath the leading edge of a continent.

How are active continental margins related to plate tectonics?

Prolonged plate subduction can produce massive accumulations of sediment along active continental margins.

What is meant by subduction erosion?

Rather than sediment accumulating along the front of the overriding plate, sediment and rock are scraped off the bottom of the overriding plate and transported down into the mantle by the subducting plate

Assuming the average speed of sound waves in water is 1500 meters per second, determine the water depth if the signal sent out by an echo sounder requires 6 seconds to strike bottom and return to the recorder.

Since the velocity of sound waves in water is 1500 meters per second and the total travel time is 6 seconds, the water depth is 4500 meters.

Compare and contrast a slow spreading center such as the Mid-Atlantic Ridge with one that exhibits a faster spreading rate, such as the East Pacific Rise.

Slow develops rift valley along ridge crust. Intermediate shallow rift valley Fast spreading valleys are absent with elevated ridge axis called swell=more gradual profile.

Compare spontaneous subduction and forced subduction. Provide examples of places where each operates

Spontaneous Subduction- Cold , dense lithosphere, steep angles (Mariana) Forced Subduction- Younger, hotter and less dense lithosphere that dips at shallower angles. (Peru-Chile)

Describe how satellites orbiting Earth can determine features on the seafloor without being able to directly observe them beneath several kilometers of seawater

The satellite measures the variation in the sea-surface elevation by bouncing microwaves off of the water's surface, which mimics the shape of the seafloor.

How does hydrothermal metamorphism alter the basaltic rocks that make up the seafloor? How is seawater changed during the process?

This alteration causes the dark silicates (olivine and pyroxene) in basalt to form new metamorphic minerals such as chlorite and serpentine. It also changes its chemistry. Hydrothermal fluids dissolve ions (silca) and various metals and precipitate them on the seafloor.

Where do rift valleys form along the oceanic ridge system?

Tranform Fault.Along the axis of some segments of the oceanic ridge system

What role do mineral phase changes play in plate subduction?

When an oceanic slab reaches 400 km within the mantle, the mineral olivine changes to its more dense spinel structure. This increase in density helps pull the slab farther down into the mantle.

What role do hot spots and mantle plumes play in the breakup of a supercontinent?

When hot mantle plumes reach the base of the lithosphere, they weaken the lithosphere and create three rifts in the lithosphere in a process called doming. Continents generally break apart along two of these rifts.

What is a black smoker?

a particle-filled cloud resulting from metallic-rich solutions gushing form the seafloor into the ocean water

What features on the ocean floor most resemble flood basalt provinces on the continents?

oceanic plateaus

What are the three major topographic provinces of the ocean floor?

oceanic ridges, continental margins, and deep-ocean basins

What is bathymetry?

the topography of the ocean floor and the measurement of ocean depths

Briefly describe the oceanic ridge system.

winds through all of the major oceans on Earthis transected along its length by transform faults at irregular intervals