The document summarizes key aspects of plate tectonic theory: 1) Plate tectonics explains the movement of continents and geological events like earthquakes and volcanoes. Early theorists like Alfred Wegener and Harry Hess contributed to developing this theory in the early 20th century. 2) Sea floor spreading was proposed by Hess and coined by Robert Dietz, describing how new ocean crust forms along mid-ocean ridges as the plates spread apart. 3) Magnetic patterns in ocean crust provided evidence that Earth's magnetic poles switch polarity over time, recording a history of plate movements.

1. Mrs. Neistadt Physical Science

2.  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.

3.  In 1915, Alfred Wegener (1880-1930), a German
meteorologist, wrote a book titled The Origin of Continents
and Oceans.
 *Alfred Wegener gathered evidence that supported his idea
that all the continents had been connected.
 *This was the Theory of Continental Drift. 1915

5.  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 is used
to make a profile of the
sea floor.

6. *Harry Hess
 *An American
geologist who studied
mid-ocean ridges.
 *He suggested that the
ocean floors move like
conveyor belts, carrying
the continents along
with them.

8.  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.

9.  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 and
creating a rise or valley.

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

12.  In the 1950s and 1960s,
scientists discovered that the
rocks of the sea floor have a very
interesting magnetic pattern.
 *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.

13.  Stripes of rock with a
north-south orientation
(normal) alternate with
stripes of rock with a
south-north orientation
(reversed).
 The blue and white stripes
you see in the figure are an
interpretation of a
magnetic profile.

15.  The Earth is like a bar magnet, It has a North
and South pole.
Sketch
Bar magnet. Iron filings
line up with magnetic
field.
Concept

16.  The Geographic North and Magnetic North
are not in the same place.
The asterisk marks
Magnetic North or True
North.

17.  *The Earth’s magnetic poles switch direction.
 The south pole becomes the north pole, and the
north pole becomes the south pole
 *This switch in direction is known as Magnetic
Reversal.
 Such reversals are caused by changes in Earth’s
magnetic field.
 *Earth’s Geographic Pole never changes place.

18.  *Each Magnetic Reversal is recorded in the
sea-floor rock.
 As the molten material rises and cools, iron
particles line up with Earth’s magnetic field.
 When the material hardens, these particles
stay in place like compass needles that are
stuck in position forever.

19.  *The records of magnetic reversals line up like stripes
in a rock
 Evidence of plate movement is found by finding the
age of the rock.
 The youngest rock shows the last magnetic reversal
occurred about 760,000 years ago.
 The oldest reversal we have found happened 150
million years ago.

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

22.  Use usgs.gov to find earthquakes in real-time to map
the tectonic plates.

23.  There are three main kinds of plate boundaries:
 divergent
 convergent
 transform

24.  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.

25.  Diverging plates move apart and new crust forms.

26.  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.

27.  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.

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

31. Key Question:
 What will Earth look like in 50 millions
years?
*Read text section 28.2 BEFORE Investigation 28.2