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Canada (509,297)
EESA06H3 (563)
Nick Eyles (495)
Lecture

Lec 4

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Department
Environmental Science
Course
EESA06H3
Professor
Nick Eyles
Semester
Summer

Description
Three types of plate boundaries: 1. Divergent plate boundary – plate boundary moving APART 2. Convergent plate boundary – plates moving TOWARDS each other 3. Transform plate boundary – move HORIZONTALLY past each other How do we know that plates move? Paleomagnetic Evidence Magnetic reversals – changes in the polarity of the magnetic field, when the north and south magnetic pole exchange places Normal polarity – magnetic lines flow from the South Pole to the North Pole and our compass needles point to the north Reversed polarity – magnetic lines run from North Pole to South Pole and our compass needles point to the South Paleomagnetism – study of magnetic fields recorded in rocks helps us understand the magnetic fields in the past Magnetic polarity time scale – this is constructed by using stacked continental lava flow to record the pattern of magnetic reversals over time Magnetometer – an instrument that measures the strength of the Earth’s magnetic field. It is taken over land surface or sea. Marine Magnetic Anomalies  Magnetic anomalies on the sea floor are arranged in bands that lie parallel to the rift valley of the mid-oceanic ridge  Positive and negative anomalies form a stripelike pattern parallel to the ridge crest The Morley-Wine-Matthews Hypothesis  The pattern of magnetic anomalies at sea matches the pattern of magnetic reversals of lava flows on the continents 1. Opening of cracks within the rift valley on the mid-oceanic ridge crest 2. They are filled by basaltic magma 3. Cools and forms dikes i. Cools ii. Magnetized iii. Forms new dikes How fast do plates move?  On each side of the ridge the sea floor is moving away from the ridge crest 1cm to 10cm per year Predicting Sea-Floor Age  Predict the age of the igneous rock of the sea floor by measuring the magnetic anomalies at the sea surface o Matching the measured anomaly pattern with the known pattern of anomalies Another Test: Fracture Zones and Transform Faults  Studying the seismicity of fracture zones  Transform Fault – the portion of a fracture zone between two offset portions of ridge crest o Earthquakes o Rock is moving away from the ridge crest What happens at plate boundaries? Divergent Plate Boundaries – plates move AWA
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