5: Plate Motions A) The origin and fate of oceanic rocks. Figure 6A and B is map

Question

5: Plate Motions A) The origin and fate of oceanic rocks. Figure 6A and B is map of the sea floor age of the world derived from paleomagnetic stripes and radiometric dating of oceanic rocks. 1) What type of plate boundary is associated with the youngest oceanic crust? 2) What is the age of the oldest oceanic crust in the North Atlantic on the east coast of the US? What does this tell us when the North Atlantic started to form? 3) What is the age of the oldest oceanic crust in the Eastern Pacific to the west of South America? What happened to oceanic crust older than this age on this plate? 4) The oldest continental rocks are 3.8 billion years old whereas the oldest oceanic rocks are less than 200 million years old. Why is the continental crust so much older than oceanic crust? B) Calculating The sea floor age maps in Figure 6 can be used to calculate the relative rate of spreading across a mid ocean ridge. Each color band on Figure 6 represents a duration of 20 million years. The width of each band is therefore proportional to the spreading rate relative sea floor spreading rates. total width of new lithosphere time required to create new lithoshpere spreading rate = Geoscientists measure seafloor spreading rates either as half spreading rotes or whole spreading rates The half spreading rate is calculated by measuring the width of an age band on one side of the ridge. The whole spreading rate is the rate of one point on one side of the ridge to a point on the other side of the ridge and is twice the half spreading rate On figure 12, measure the width in km of the section lines A-A' and B-B. From the time duration of each section line, calculate the half and whole spreading rate Section Duration Width of Half spreading Whole Spread section rate (m/y Rate (mm/yr) rate X 2 of section 1 km 10mm 1 MY 10 yr Width/Time A-A North Atlantic B-B SE Pacific 1 yr

Explanation / Answer

1. a) diveregent boundaries or Mid oceanic ridges. It is also know as constructional or extensional boundaries.

b) around 180 Mya . This would also be the time when the atlantic started forming because , as the spreading continues older and older rocks would move away from the central ridge.

c) approximately 20 Mya . Pacific ocean is a closing ocean that is the plate boundary involved is a destructive boundary or a subduction is happening. The older oceanic crust of this area is consumed during the subduction process.

d) continental crust is basically lighter or less denser than the oceanic crust. It is because of subduction: oceanic crust tends to get colder and denser with age as it spreads off the mid-ocean ridges. It gets so dense, that it sinks in the upper mantle (subduction). This is like a giant recycling system for oceanic lithosphere! Because continental crust is lighter than oceanic crust, continental crust cannot subduct. We therefore still have some very old continental rocks at the surface of the Earth. This would mean that any oceanic crust present before what is in record was never preserved.

B , assuming the scale bars are applicable for both ocean plots.

section AA'

width 1500 Km

duration 80 Mya

half spreading rate = 1500 x 106mm / 80 x 106 yr

= 18.75 mm/ yr

full spreading rate = 2 x 18.75

= 37.50 mm/yr

section BB'

width = 4000 Km

time= 40 My

half spreading rate = 4000 x 106 mm / 40 x 106 yr

= 100 mm/ yr

full spreading rate = 200 mm / yr

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