Mon. April 7 Do Now What is the principle of superposition?

Plate Tectonics The theory of plate tectonics explains the movement of continents and other geological events like earthquakes and volcanoes. The term tectonics means construction or building. The theory of plate tectonics, stated in 1965, refers to the movement of giant pieces of solid rock on Earth’s surface called tectonic plates.

Continental Drift In 1915, Alfred Wegener (1880-1930), a German meteorologist, wrote a book titled The Origin of Continents and Oceans.

Continental Drift Wegener gathered evidence that supported his idea that all the continents had once been connected.

Sea Floor Spreading In the early 1960s, Henry Hess (1906-69), a geologist and former commander of a Navy ship equipped with an echo sounder, used the profile of the sea floor to propose that it was spreading at the mid- ocean ridges. An echo sounder makes a profile of the sea floor.

Sea Floor Spreading Around the same time, Robert Dietz (1914-95), a scientist with similar ideas, coined the term sea-floor spreading. Sea-floor spreading describes the sea floor on either side of a mid-ocean ridge as moving away from the ridge & creating a rise or valley.

Sea Floor Spreading Hot fluid from the mantle (called magma) enters the rise or valley and cools, creating new sea floor (also called oceanic crust).

Magnetic Patterns Over geologic time, the magnetic polarity of Earth has switched. Scientists believe the poles switch because of a magnetic interaction between the planet’s inner and outer core.

Magnetic Patterns In the 1950s and 1960s, scientists discovered that the rocks of the sea floor have a very interesting magnetic pattern. Stripes of rock with a north-south orientation (normal) alternate with stripes of rock with a south-north orientation (reversed).

Describing Plate Boundaries There are three main kinds of plate boundaries: divergent convergent transform

Plate Tectonics The tectonic plates that cover Earth’s surface are pieces of the lithosphere that fit together and float on the asthenosphere (a part of the mantle). There are a number of large tectonic plates on Earth’s surface, and smaller plates are being identified all the time.

Can you identify which of the plates are only made of oceanic crust?

Divergent Plate Boundaries Diverging plates move apart and new crust forms.

Divergent Plate Boundaries Divergent boundaries are sites of earthquakes and volcanic activity. Mid-ocean ridges and associated sea-floor spreading occur at divergent plate boundaries. In effect, a mid-ocean ridge is like a very long volcano.

Convergent Plate Boundaries Convergent plate boundaries occur where two plates approach each other. One result of two plates converging is subduction. A deep oceanic trench marks the boundary between a subducting and an overriding plate at a convergent boundary.

Movement of Plates The movement of tectonic plates is related to the distribution of heat by convection currents in the mantle.