Analyze the sea-floor part of the map in Fig. A2.5.1 , which depicts the area ju
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Question
Analyze the sea-floor part of the map in Fig. A2.5.1, which depicts the area just off the Pacific Coast, west of California, Oregon, Washington, and southwest Canada. The colored bands are marine magnetic anomalies. Colored bands are rocks with a a positive (+) magnetic anomaly, so they have normal polarity, like now. The white bands are rocks with a negative (-) magnetic anomaly, so they reversed polarity. Different colors indicate the ages of the rocks in millions of years as shown in the magnetic polarity scale provided.
1. Using a pencil, draw a line on the seafloor to show where new ocean crust and lithosphere is forming now (zero millions of years old). Using Figs. 2.1, 2.3, and 2.12 as guides, label the segments of your line that are Juan de Fuca Ridge and Gorga Ridge (divergent plate boundaries). Then label the segments of your line that are transform fault plate boundaries. Add half arrows to the transform fault boundaries to show the motion of the two plates relative to the transform fault.
2. What has been the average rate and direction of sea-floor spreading in cm per year (cm/yr) west of the Juan de Fuca Ridge, From B to A? Show your work.
3. What has been the average rate and direction of sea-floor spreading in cm per year (cm/yr.) east of the Juan de Fuca Ridge, From B to C? Show your work.
4 Notice that rocks older than 11 million years are present west of the Juan de Fuca Ridge but not east of the ridge. What could be happening to the sea-floor rocks along line segment C-D would explain why rocks older than 11 million years no longer exist on the seafloor east of the ridge?
Figure A2.5.1
O 50 100 200 400 miles rankin 0-50100 Glacier voicanMount Meager 500 Kilpmet Mount Cayley : Mount Garibaldi CANA A-M. Baker· WASHINGTON MONTANA 0 Magnetic polarity time scale: ages are in millions of years ago (Mya) Glacier Mt. Rainier 2 Mountst Mt. Adams colors indicate rocks of normal magnetic polarity (+), like now The north-seeking end of a compass needle would have pointed in the general direction of the geographic Helens Mt. Hood Mt. Jefferson Three Sisters 4 Crest of Juan de Fuca Ridge 6 North Pole. Newberry olcano White areas have reversed magnetic polarity (). At those times, the north and south poles exchanged positions. The north-seeking end of a compass would have pointed in the Crater Lake OREGON 8 Mount Medicine Shasta Lake Volcano volcano Lassen Peak trench 11 general direction of CALIFORNIA the geographic South : Crest of. Gorda Ridge 2 Sea-floor spreading west spreading eastExplanation / Answer
Ans 1 could not be answered due to lack of information(fig 2.1 2.3 2.12)
Ans 2) Rate of movement= Distance/Time
= 250 km/ 9 Ma
= 250×1000×100/9×10^6
= 2.78 cm/year
Direction is north west.
Ans 3) Rate = 250 km/8 Ma
= 3.125 cm/year
Direction is South east.
Ans 4) Along the axis C- D, the plate is getting subducted under the North American plate. This being an converging plate boundary results in the formation of subduction zone where the heavier oceanic plate gets subducted and therefore melted. This is why no rocks older than 11 Ma is found.
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