Plate Tectonics Hw - Wang

Although new crust is being formed at divergent boundaries, Earth remains the same size. Which of the following statements provides the best reason for this?

An equal volume of crust is being consumed at convergent boundaries.

Use your measurements from the Part B instructions to calculate the annual rate of spreading at placemark A. Assume that the rift began opening 20 million years ago. (Answer to the nearest centimeter per year.)

0.98 centimeters/year *Measurements near placemark A return values of 19 to 20 million centimeters, indicating spreading rates of 0.9 to 1.0 centimeters per year. This is relatively slow, with other spreading centers operating at up to 10 centimeters/year.

Use your measurements from the Part C instructions and assume that the rift began opening 20 million years ago. What is the spreading rate for the region of placemark B?

1.5 centimeters/year

What percentage of earthquakes occurs at active plate boundaries?

95%

How are normal polarity and reverse polarity of Earth's magnetic field different?

A compass points toward the North Pole during normal polarity and to the South Pole during reverse polarity.

A continental volcanic arc is formed when which of the following conditions exists?

A piece of oceanic crust is subducted below continental crust.

In general, what is our best evidence that the orientation of Earth's magnetic field has changed over time?

A record of Earth's magnetic field is recorded in oceanic rocks, which show a clear pattern of changes in Earth's magnetic polarity.

A cross-section through a plate boundary shows earthquakes in groups at shallow, intermediate, and deep locations descending at an angle away from the boundary. What feature do these earthquakes delineate?

A subducting oceanic lithospheric plate

Which of the following statements about Earth's magnetic field is most accurate?

Earth's magnetic field switches polarity at irregular time intervals.

How do reversals in the Earth's magnetic field support plate tectonic theory?

Mirror images of matching magnetic reversals in oceanic crust generated at spreading centers

fastest spreading rate to slowest spreading rate .....

Pacific Ocean to South Atlantic Ocean to North Atlantic Ocean

The Red Sea presents a simple geometry of an elongate oceanic basin. Volcanic rocks along both shores of the Red Sea display similar ages of ~20 million years. Examine the shape of the shoreline of Red Sea. What type of tectonic environment is present at the Red Sea?

The Red Sea is a divergent plate boundary, where new rocks are being created at the center of the Red Sea as the Arabian peninsula and Africa are pulling apart.

When oceanic crust is subducted beneath continental crust, partial melting can occur. Which of the following best explains why the resultant magma rises through the crust toward the surface?

The magma is less dense than the surrounding solid rock.

At a convergent boundary, what is the relative motion of plates on each side of the boundary?

The plates move toward each other.

The formation of the Himalaya and the Tibetan Plateau was caused by which of the following events?

There was a collision of two pieces of continental crust at a subduction zone.

What type of plate boundary forms where two plates slide past one another horizontally?

Transform

the spatial distribution of earthquakes at a subduction zone indicates the

direction of plate motion

Which type of plate boundary is most closely associated with the formation of new ocean floor?

divergent

How often does the Earth's magnetic field switch polarity?

every 100,000 to several million years

Where would you expect to see alternating bands of rock with different magnetic polarities?

on both sides of the mid-ocean ridge

North Atlantic Ocean

spreading rate of 2.6 cm/yr basin is 269 million years old basin is 7000 km wide

South Atlantic Ocean

spreading rate of 3.6 cm/yr basin is 125 million years old basin is 4500 km wide

When oceanic and continental crust collide at a subduction zone, one plate is forced under the other. The angle at which the subducting plate descends is due to which of the following factors?

the age of the plate being subducted and the density of the plate being subducted

The timing of plate tectonic motions can be calculated by all of the following except __________.

the maximum depth of earthquakes at a subduction zone

As oceanic crust is subducted, it melts, forming a body of magma. When this magma rises and interacts with the overlying continental crust, what change occurs in the composition of the magma?

the silica content of the magma increases.

As oceanic crust is subducted below continental crust, which of the following factors most directly drives the melting that will fuel volcanoes at the surface?

water