Plate Tectonics II

Modern discoveries supporting Plate Tectonic Theory

• Symmetry of magnetic polarity across mid-ocean ridges

• Mid-ocean ridges – underwater mountain chains that circle the globe and often mimic the shape of the coastline

• Distribution of earthquakes and volcanoes• Relatively young age of the oceanic crust (less

than 180 million years) and lack of deep-ocean sediment

• Deep mantle plumes (aka hot spots).

The Earth’s Magnetic Field

The earth has a magnetic field, caused by a complex interaction between the molten outer core and the solid inner core.

The Earth’s Magnetic FieldThe earth acts like a

giant (but pretty weak) bar magnet. Refrigerator magnets won’t stick, but a compass needle will be deflected.

The lines are the standard way to depict the (invisible) magnetic field. The arrows point in the same direction as a compass needle.

Paleomagnetism: study of ancient magnetic fields

• Lava is rich in iron-bearing minerals.• As lava cools and solidifies on the earth’s surface, those minerals orient themselves towards the north pole like little compass needles. • Once the lava becomes solid, the iron minerals are “stuck” in place.• Ancient lavas tell us the strength and direction of the earth’s magnetic field during geologic history.

Magnetic polarity reversals

•At irregular time intervals, the “magnet turns around”, due to changes in the inner and outer core. •Lava that solidified during these reversals allows us to determine the date of these reversals.•Volcanic rocks dated to 760,000 years ago in several locations, including the ski area up at Mammoth Lakes, California, show evidence of reversed magnetic polarity. Reversed Normal

Recent time scale of Earth’s magnetic polarity

Magnetic polarity reversals on the ocean floor

•Development of ocean-going magnetometers allowed remote mapping of the magnetic field of the ocean crust.

•Symmetrical pattern at mid-ocean ridges could best be explained by the sea-floor spreading hypothesis, which was still being debated at the time.

polarity reversals recorded by basaltic flows at mid-ocean ridges

Seafloor Spreading and Seafloor Magnetization

Oceanic Ridge System•The Mid-Atlantic Ridge was first discovered in 1872.•By the 1950s, extensive mapping showed a system of underwater mountains that circled the globe• The existence of the ocean ridge system led Harry Hess and others to formulate the sea-floor spreading hypothesis, which led to our present-day theory of plate tectonics.

Distribution of earthquakes and volcanoes

The next 2 slides show the world distribution of volcanoes and earthquakes. In both cases, scientists observed that they occur in the same places.

This supported the idea of separate plates, with interacting boundaries.

Volcano distribution

Earthquake distribution

Evidence from ocean drilling

Lack of sediments at mid ocean ridges supports idea of seafloor spreading.

Evidence from ocean drilling• Next slide shows youngest ocean crust

(red) at oceanic ridge system, and oldest ocean crust (blue) farthest from the ridge. This supports sea-floor spreading (note symmetry).

• Oldest ocean crust is about 180 million years old (myo), while oldest continental crust is close to 4000 myo (that’s 4 billion!). Where could the old stuff have gone?

Age of the sea floor

Hot spots and mantle plumes

• Caused by rising stationary plumes of mantle material

• Volcanoes can form over them (Hawaiian Island chain)

• Originate at great depth, perhaps at the mantle-core boundary

• Since plate moves, but plume does not, it allows us to measure rate of plate motion.

Figure 15.18

What drives plate motion?

Plate Mantle Convection• Convective flow in the mantle is the underlying driving

force for plate movement.• Mantle convection and plate tectonics are part of the

same system. Warm, buoyant rock rises and “cold” dense rock, e.g. subducting plates, sink.

• Plate tectonic movements are ultimately due to unequal heat distribution in the earth’s interior.

Slab-pull, ridge-push, slab suction• Descending oceanic crust pulls the plate in direction of

subduction. This results in suction which pulls the two sides closer. Slab-pull is likely the main mechanism of plate motion.

• Elevated ridge system pushes the plate away from ridge.

Convection and Tectonics

Two possible hypotheses of plate-mantle convection– 2-layer model– whole mantle

convection model