Ics1 sem1 block 2 Plate tectonics
Name_________________________per___
ICS 1 Block 2 Plate Tectonics
Ocean Floor Characteristics, Types / Properties of Plate Boundaries, Physical / Chemical Conditions of Rocks
1.
2.
3.
4.
5.
6.
Cover sheet / Agree or Disagree
WCW- your paper
Standards/crossword
Vocabulary
Notes: Ocean floor
Continental Drift Handout
Content Objective
To understand the different types of plate boundaries, characteristics of the objects located at each, and the
conditions that form rocks close and far from them.
Language Objective
Describe the three types of plate boundaries and give examples of what types of land features and rock forms at
each.
Explain why the sea floor is spreading apart; discuss the reason for the “zebra pattern of polarity” on the ocean
floor.
Discuss the conditions that are required to form the three types of rocks, explain the chemical composition and
physical formation of each.
To be accepted for grading it must be organized and signed
Student: Print_______________________________Sign________________________________________Date_______
Parent/Guardian: Print_________________________________Sign______________________________________Date_______
Ics1 sem1 block 2 Plate tectonics
On your own paper
WCW- Warm-up, Critical Thinking, and Wrap-up: copy and answer all
For- 8/27/12 odd or 8/28/12 even
Warm-up
Sunlight can’t reach the Ocean Floor.
The ocean floor looks like….
Life that exists there is…
Critical
Thinking
Sonar helped in the mapping of the Earth’s
Ocean Floor by…
Ocean floor look like…
Wrap-up
Discuss the process of creating and recycling the ocean floor.
Where does it occur? How does it occur?
Explain how sonar helped in the mapping of the Earth’s Ocean
Floor.
What does the Ocean floor look like? Why is it like that?
8/29 odd 8/30 even
Warmup
 The seven continents are…
 The 4 major oceans are…
 The reason for different Land
forms is …
Critical Some coastal Mountain ranges are
Thinking farther from the convergent boundaries
that construct them because…
Ics1 sem1 block 2 Plate tectonics
Wrap-up Most Earthquakes and Volcanic
eruptions occur along or near
____Why?
Ics1 sem1 block 2 Plate tectonics
Ics1 sem1 block 2 Plate tectonics
Notes: Ocean Floor
Harry Hess
Pacific mtn range
Sonar-mid-atlantic ridge
12,000 miles long
Ocean floor 4-5km deep
Mid-ocean ridge~2.5 km high
Sea-floor
spreading
Old floor-far from ridge (near shore)
Young new floor-near ridge (mid-ocean)
Before WW1 thought crust moved
Now we know its like an escalator
Ics1 sem1 block 2 Plate tectonics
Standards Fill in the blank
ES 3. Plate tectonics operating over geologic time have changed the patterns of land, sea, and mountains on Earth’s surface.
As the basis for understanding this concept:
ES 3. a. Students know features of the _____________ (_______________, ___________, and ______________) provide evidence of
plate tectonics.
Much of the evidence for _________________ came from the seafloor rather than from the continents themselves. The longest
topographic feature in the world is the ___________________, a chain of volcanoes and rift valleys about 40,000 miles long that rings
the planet like the seams of a giant baseball. A portion of this system is the Mid-Atlantic Ridge, which runs parallel to the coasts of
Europe and Africa and of North and South America and is located halfway between them. The _________ system is made from the
youngest rock on the ocean floor, and the floor gets _______________, symmetrically, on both sides of the ridge. No portion of the
ocean floor is more than about ________________. _____________ is thin on and near the ridge. Sediment found away from the
ridge thickens and contains progressively older fossils, a phenomenon that also occurs _____________.
Mapping the _________________ anywhere across the ridge system produces a striking pattern of high and low fields in almost
perfect symmetrical stripes. A brilliant piece of scientific detective work inferred that these “___________” arose because lava had
erupted and cooled, locking into the rocks a __________________ whose direction matched that of Earth’s field when cooling took
place. The magnetic field near the rocks is the sum of the residual field and Earth’s present-day field. Near the lavas that cooled during
times of ___________, the residual field points along Earth’s field; therefore, the total field is high. Near the lavas that cooled during
times of ___________, the residual field points counter to Earth’s field; therefore, the total field is low.
The “stripes” provide strong support for the idea of ____________ because the lava in these stripes can be dated independently and
because regions of reversed polarity correspond with times of known _________________. This theory states that new seafloor is
created by volcanic eruptions at the mid-oceanic ridge and that this erupted material continuously spreads out convectively and opens
and creates the ocean basin. At some continental margins _______________ mark the places where the oldest ocean floor sinks back
into the mantle to complete the convective cycle. Continental drift and seafloor spreading form the modern theory of plate tectonics.
ES 3. b. Students know the principal structures that form at the three different kinds of ____________.
There are three different types of plate boundaries, classified according to their ___________: ___________ boundaries;
___________ boundaries; and ___________, or parallel slip, boundaries. Divergent boundaries occur where plates are ____________.
Young divergence is characterized by thin or thinning crust and ____________; if divergence goes on long enough, mid-ocean ridges
eventually develop, such as the Mid-Atlantic Ridge and the East Pacific Rise. Convergent boundaries occur where plates are moving
___________ other. At a convergent boundary, material that is _____________, such as oceanic crust, may __________back into the
mantle and produce a deep ocean trench. This process is known as ________. The sinking material may partially melt, producing
_________________, such as the Aleutian Islands and Japan. If the Subduction of denser oceanic crust occurs underneath a continent,
a _________________ chain, such as the Andes or the Cascades, is formed. When two plates collide and both are too light to
________, as when one continent crashes into another, the crust is __________and __________ to produce great mountain chains,
such as the relatively young Himalayas or the more ancient Appalachians. The third type of plate boundary, called a transform, or
parallel slip, boundary, comes into existence where two plates move ________________, parallel to the boundary. The San Andreas
fault in California is an important example. Marking the boundary between the North American and Pacific plates, the __________
from the Gulf of California northwest to Mendocino County in northern California.
ES 3. c. Students know how to explain the properties of _________ based on the ________ and _________ conditions in which they
formed, including plate tectonic processes.
Rocks are classified according to their _________________ and ____________. The composition reflects the chemical constituent’s
__________ when the rock was formed. The texture is an indication of the conditions of ___________and ____________ under
which the rock formed. For example, many ______________, which cooled from molten material, have interlocking crystalline
textures. Many _________________ have fragmental textures. Whether formed from cooling magma, created by deposits of sediment
grains in varying sizes, or transformed by heat and pressure, each rock possesses identifying properties that reflect its origin.
______________ processes directly or indirectly control the _____________of different rock types. _____________, for example,
takes rocks from close to the surface and drags them down to depths where they are __________________________. Tectonic
processes also uplift rocks so that they are exposed to lower ____________________ and to the _______________ effects of the
atmosphere.
Ics1 sem1 block 2 Plate tectonics
California State Standards with Framework
ES 3. Plate tectonics operating over geologic time have changed the patterns of land, sea, and mountains on Earth’s surface.
As the basis for understanding this concept:
ES 3. a. Students know features of the ocean floor (magnetic patterns, age, and sea-floor topography) provide evidence of plate
tectonics.
Much of the evidence for continental drift came from the seafloor rather than from the continents themselves. The longest
topographic feature in the world is the mid-oceanic ridge system, a chain of volcanoes and rift valleys about 40,000 miles long that
rings the planet like the seams of a giant baseball. A portion of this system is the Mid-Atlantic Ridge, which runs parallel to the coasts
of Europe and Africa and of North and South America and is located halfway between them. The ridge system is made from the
youngest rock on the ocean floor, and the floor gets progressively older, symmetrically, on both sides of the ridge. No portion of the
ocean floor is more than about 200 million years old. Sediment is thin on and near the ridge. Sediment found away from the ridge
thickens and contains progressively older fossils, a phenomenon that also occurs symmetrically.
Mapping the magnetic field anywhere across the ridge system produces a striking pattern of high and low fields in almost perfect
symmetrical stripes. A brilliant piece of scientific detective work inferred that these “zebra stripes” arose because lava had erupted
and cooled, locking into the rocks a residual magnetic field whose direction matched that of Earth’s field when cooling took place.
The magnetic field near the rocks is the sum of the residual field and Earth’s present-day field. Near the lavas that cooled during times
of normal polarity, the residual field points along Earth’s field; therefore, the total field is high. Near the lavas that cooled during
times of reversed polarity, the residual field points counter to Earth’s field; therefore, the total field is low.
The “stripes” provide strong support for the idea of seafloor spreading because the lava in these stripes can be dated independently
and because regions of reversed polarity correspond with times of known geomagnetic field reversals. This theory states that new
seafloor is created by volcanic eruptions at the mid-oceanic ridge and that this erupted material continuously spreads out convectively
and opens and creates the ocean basin. At some continental margins deep ocean trenches mark the places where the oldest ocean
floor sinks back into the mantle to complete the convective cycle. Continental drift and seafloor spreading form the modern theory of
plate tectonics.
ES 3. b. Students know the principal structures that form at the three different kinds of plate boundaries.
There are three different types of plate boundaries, classified according to their relative motions: divergent boundaries; convergent
boundaries; and transform, or parallel slip, boundaries. Divergent boundaries occur where plates are spreading apart. Young
divergence is characterized by thin or thinning crust and rift valleys; if divergence goes on long enough, mid-ocean ridges eventually
develop, such as the Mid-Atlantic Ridge and the East Pacific Rise. Convergent boundaries occur where plates are moving toward each
other. At a convergent boundary, material that is dense enough, such as oceanic crust, may sink back into the mantle and produce a
deep ocean trench. This process is known as subduction. The sinking material may partially melt, producing volcanic island arcs, such
as the Aleutian Islands and Japan. If the subduction of denser oceanic crust occurs underneath a continent, a volcanic mountain chain,
such as the Andes or the Cascades, is formed. When two plates collide and both are too light to subduct, as when one continent
crashes into another, the crust is crumpled and uplifted to produce great mountain chains, such as the relatively young Himalayas or
the more ancient Appalachians.
The third type of plate boundary, called a transform, or parallel slip, boundary, comes into existence where two plates move laterally
by each other, parallel to the boundary. The San Andreas Fault in California is an important example. Marking the boundary between
the North American and Pacific plates, the fault runs from the Gulf of California northwest to Mendocino County in northern
California.
ES 3. c. Students know how to explain the properties of rocks based on the physical and chemical conditions in which they formed,
including plate tectonic processes.
Rocks are classified according to their chemical compositions and textures. The composition reflects the chemical constituents
available when the rock was formed. The texture is an indication of the conditions of temperature and pressure under which the rock
formed. For example, many igneous rocks, which cooled from molten material, have interlocking crystalline textures. Many
sedimentary rocks have fragmental textures. Whether formed from cooling magma, created by deposits of sediment grains in varying
sizes, or transformed by heat and pressure, each rock possesses identifying properties that reflect its origin.
Plate tectonic processes directly or indirectly control the distribution of different rock types. Subduction, for example, takes rocks
from close to the surface and drags them down to depths where they are subjected to increased pressures and temperatures. Tectonic
processes also uplift rocks so that they are exposed to lower temperatures and pressures and to the weathering effects of the
atmosphere.
Ics1 sem1 block 2 Plate tectonics
Vocabulary
1. Sea-floor topography the relief features or surface configuration of an area
2. Plate tectonics theory in which the lithosphere is divided into pieces, each move on the plastic
asthenosphere to collide with, slide under, or move past adjacent pieces
3. Continental drift the lateral movement of continents resulting from the motion of crustal plates
4. Mid-ocean ridge extend through the Atlantic, Indian, and South Pacific oceans: each is hypothesized to
be the locus of seafloor spreading
5. Rift valleys chasm extending along the crest of a mid-ocean ridge, locus of the magma upwelling that
accompany seafloor spreading
6. Sediment mineral or organic matter deposited by water, air, or ice
7. Fossils remains, impression, or trace of a living thing of a former geologic age, as a skeleton, footprint,
etc
8. Magnetic field near a magnet, electric current, or moving charged particle in which a magnetic force acts
on any other magnet, electric current, or moving charged particle
9. Zebra stripes pattern created by the changing Earth’s magnetic field on the ocean crust
10. Polarity the property or characteristic that produces unequal physical effects at different points in a
magnet or storage battery
11. Lava the molten, fluid rock that issues from a volcano or volcanic vent
12. Seafloor spreading process in which new ocean floor is created as molten material from the earth's
mantle rises in margins between plates or ridges and spreads out
13. Ocean Basin a large geologic areas that are below sea level
14. Deep Ocean trench any long, narrow, steep-sided depression in the ocean bottom in which maximum
oceanic depths (24,000–36,000 ft)
15. Convection cell a volume of circulating material, under gravity, that is heated from below and cooled
from above
16. Divergent boundary type of tectonic boundary where two plates are moving in the opposite direction.
17. Convergent boundary type of tectonic boundary where two plates are moving toward each other
18. Transform boundary type of tectonic boundary where two plates are moving past each other in opposite
directions
19. Oceanic crust part of Earth's lithosphere that covers the ocean basins
20. Continental Crust the layer of igneous, sedimentary, and metamorphic rocks which form the continents
21. Mantle is a highly viscous layer between the crust and the outer core
22. Subduction one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the
plates converge
23. Volcanic island arcs generally they result from the subduction of an oceanic tectonic plate under another
tectonic plate, and often parallel an oceanic trench
24. Volcanic mountain chain when oceanic crust subducts beneath continental crust on an adjacent plate,
creating an arc-shaped mountain belt
25. San Andreas Fault continental transform fault that runs a length of roughly 810 miles (1,300 km)
through California
26. Chemical composition is the simplest whole number ratio of atoms of each element present in a
compound.
27. Igneous rock formed through the cooling and solidification of magma or lava
28. Crystalline texture the distribution of crystallographic orientations of a polycrystalline sample
29. Sedimentary rock minerals and/or organic particles settle and accumulate to form a new material
30. Metamorphic rock the transformation of an existing rock type to a new rock type through heat and/or
pressure
Ics1 sem1 block 2 Plate tectonics
Earth Science ocean floor, rock type, Plate tectonics
Across
3. the transformation of an existing rock type to a new rock type through heat and/or pressure
5. a volume of circulating material, under gravity, that is heated from below and cooled from above
7. formed through the cooling and solidification of magma or lava
8. the molten, fluid rock that issues from a volcano or volcanic vent
9. remains, impression, or trace of a living thing of a former geologic age, as a skeleton, footprint, etc
10. pattern created by the changing Earth’s magnetic field on the ocean crust
Ics1 sem1 block 2 Plate tectonics
15. the lateral movement of continents resulting from the motion of crustal plates
21. type of tectonic boundary where two plates are moving toward each other
22. when oceanic crust subducts beneath continental crust on an adjacent plate, creating an arc-shaped mountain
belt
23. process in which new ocean floor is created as molten material from the earth's mantle rises in margins
between plates or ridges and spreads out
24. generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often
parallel an oceanic trench
25. the relief features or surface configuration of an area
26. ridge extend through the Atlantic, Indian, and South Pacific oceans: each is hypothesized to be the locus of
seafloor spreading
27. mineral or organic matter deposited by water, air, or ice
28. the property or characteristic that produces unequal physical effects at different points in a magnet or
storage battery
29. type of tectonic boundary where two plates are moving in the opposite direction.
Down
1. theory in which the lithosphere is divided into pieces, each move on the plastic asthenosphere to collide with,
slide under, or move past adjacent pieces
2. part of Earth's lithosphere that covers the ocean basins
3. is a highly viscous layer between the crust and the outer core
4. is the simplest whole number ratio of atoms of each element present in a compound.
6. minerals and/or organic particles settle and accumulate to form a new material
11. one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge
12. chasm extending along the crest of a mid-ocean ridge, locus of the magma upwelling that accompany
seafloor spreading
13. near a magnet, electric current, or moving charged particle in which a magnetic force acts on any other
magnet, electric current, or moving charged particle
14. any long, narrow, steep-sided depression in the ocean bottom in which maximum oceanic depths (24,000–
36,000 ft)
16. a large geologic areas that are below sea level
17. the distribution of crystallographic orientations of a polycrystalline sample
18. type of tectonic boundary where two plates are moving past each other in opposite directions
19. the layer of igneous, sedimentary, and metamorphic rocks which form the continents
20. continental transform fault that runs a length of roughly 810 miles (1,300 km) through California
Ics1 sem1 block 2 Plate tectonics
Notes: Ocean Floor Characteristics
Standard(s): ES 3. a. Students know features of the ocean floor (magnetic patterns, age, and sea-floor
topography) provide evidence of plate tectonics.
Objective (s): Understand the “zebra pattern” on the ocean floor and how this is evidence for sea-floor
spreading
Ics1 sem1 block 2 Plate tectonics
Continent drift and Plate tectonics
http://pubs.usgs.gov/gip/dynamic/developing.html
Continental drift was hotly debated off and on for decades following Wegener's death before it was largely dismissed as being
eccentric, preposterous, and improbable. However, beginning in the 1950s, a wealth of new evidence emerged to revive the
debate about Wegener's provocative ideas and their implications. In particular, four major scientific developments spurred
the formulation of the plate-tectonics theory: (1) demonstration of the ruggedness and youth of the ocean floor; (2)
confirmation of repeated reversals of the Earth magnetic field in the geologic past; (3) emergence of the seafloor-spreading
hypothesis and associated recycling of oceanic crust; and (4) precise documentation that the world's earthquake and volcanic
activity is concentrated along oceanic trenches and submarine mountain ranges.
Ocean floor mapping
About two thirds of the Earth's surface lies beneath the oceans. Before the 19th century, the depths of the open ocean were
largely a matter of speculation, and most people thought that the ocean floor was relatively flat and featureless. However, as
early as the 16th century, a few intrepid navigators, by taking soundings with hand lines, found that the open ocean can differ
considerably in depth, showing that the ocean floor was not as flat as generally believed. Oceanic exploration during the next
centuries dramatically improved our knowledge of the ocean floor. We now know that most of the geologic processes
occurring on land are linked, directly or indirectly, to the dynamics of the ocean floor.
"Modern" measurements of ocean depths greatly increased in the 19th century, when deep-sea line soundings (bathymetric
surveys) were routinely made in the Atlantic and Caribbean. In 1855, a bathymetric chart published by U.S. Navy Lieutenant
Matthew Maury revealed the first evidence of underwater mountains in the central Atlantic (which he called "Middle
Ground"). This was later confirmed by survey ships laying the trans-Atlantic telegraph cable. Our picture of the ocean floor
greatly sharpened after World War I (1914-18), when echo-sounding devices -- primitive sonar systems -- began to measure
ocean depth by recording the time it took for a sound signal (commonly an electrically generated "ping") from the ship to
bounce off the ocean floor and return. Time graphs of the returned signals revealed that the ocean floor was much more
rugged than previously thought. Such echo-sounding measurements clearly demonstrated the continuity and roughness of the
submarine mountain chain in the central Atlantic (later called the Mid-Atlantic Ridge) suggested by the earlier bathymetric
measurements.
The Mid-Ocean Ridge In 1947, seismologists on the U.S. research ship Atlantis found that the sediment
layer on the floor of the Atlantic was much thinner than originally thought. Scientists had previously
believed that the oceans have existed for at least 4 billion years, so therefore the sediment layer should
have been very thick. Why then was there so little accumulation of sedimentary rock and debris on the
ocean floor? The answer to this question, which came after further exploration, would prove to be vital to
advancing the concept of plate tectonics.
Computer-generated topographic map of the Mid-Ocean Ridge
In the 1950s, oceanic exploration greatly expanded. Data gathered by oceanographic surveys conducted by
many nations led to the discovery that a great mountain range on the ocean floor virtually encircled the
Earth. Called the global mid-ocean ridge, this immense submarine mountain chain -- more than 50,000
kilometers (km) long and, in places, more than 800 km across -- zig-zags between the continents, winding
its way around the globe like the seam on a baseball. Rising an average of about 4,500 meters (m) above
the sea floor, the mid-ocean ridge overshadows all the mountains in the United States except for Mount
McKinley (Denali) in Alaska (6,194 m). Though hidden beneath the ocean surface, the global mid-ocean
ewq549ridge system is the most prominent topographic feature on the surface of our planet.
Ics1 sem1 block 2 Plate tectonics
A theoretical model of the formation of magnetic
striping
New oceanic crust forming continuously at the
crest of the mid-ocean ridge cools and becomes
increasingly older as it moves away from the ridge
crest with seafloor spreading (see text):
a. the spreading ridge about 5 million years ago;
b. about 2 to 3 million years ago; and
c. present-day.
Magnetic striping and polar reversals
Beginning in the 1950s, scientists, using magnetic instruments (magnetometers)
adapted from airborne devices developed during World War II to detect
submarines, began recognizing odd magnetic variations across the ocean floor.
This finding, though unexpected, was not entirely surprising because it was
known that basalt -- the iron-rich, volcanic rock making up the ocean floor-contains a strongly magnetic mineral (magnetite) and can locally distort compass
readings. This distortion was recognized by Icelandic mariners as early as the late
18th century. More important, because the presence of magnetite gives the basalt
measurable magnetic properties, these newly discovered magnetic variations
provided another means to study the deep ocean floor.
Early in the 20th century, paleomagnetists (those who study the Earth's ancient
magnetic field) -- such as Bernard Brunhes in France (in 1906) and Motonari
Matuyama in Japan (in the 1920s) -- recognized that rocks generally belong to
two groups according to their magnetic properties. One group has so-called
normal polarity, characterized by the magnetic minerals in the rock having the
same polarity as that of the Earth's present magnetic field. This would result in
the north end of the rock's "compass needle" pointing toward magnetic north.
The other group, however, has reversed polarity, indicated by a polarity
alignment opposite to that of the Earth's present magnetic field. In this case, the
north end of the rock's compass needle would point south. How could this be?
This answer lies in the magnetite in volcanic rock. Grains of magnetite -behaving like little magnets -- can align themselves with the orientation of the
Earth's magnetic field. When magma (molten rock containing minerals and
gases) cools to form solid volcanic rock, the alignment of the magnetite grains is
"locked in," recording the Earth's magnetic orientation or polarity (normal or
reversed) at the time of cooling.
Magnetic striping in the Pacific Northwest
As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to
be random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic
patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes
of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with
normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these
alternating bands of normally and reversely polarized rock, became known as magnetic striping.
Ics1 sem1 block 2 Plate tectonics
Seafloor spreading and recycling of oceanic crust
The discovery of magnetic striping naturally prompted more questions: How does the magnetic striping pattern form? And
why are the stripes symmetrical around the crests of the mid-ocean ridges? These questions could not be answered without
also knowing the significance of these ridges. In 1961, scientists began to theorize that mid-ocean ridges mark structurally
weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest. New magma from deep within the
Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust.
This process, later called seafloor spreading, operating over many millions of years has built the 50,000 km-long system of
mid-ocean ridges. This hypothesis was supported by several lines of evidence: (1) at or near the crest of the ridge, the rocks
are very young, and they become progressively older away from the ridge crest; (2) the youngest rocks at the ridge crest
always have present-day (normal) polarity; and (3) stripes of rock parallel to the ridge crest alternated in magnetic polarity
(normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has flip-flopped many times. By explaining both the
zebralike magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis quickly
gained converts and represented another major advance in the development of the plate-tectonics theory. Furthermore, the
oceanic crust now came to be appreciated as a natural "tape recording" of the history of the reversals in the Earth's magnetic
field.
Additional evidence of seafloor spreading came from an unexpected source: petroleum exploration. In the years following
World War II, continental oil reserves were being depleted rapidly and the search for offshore oil was on. To conduct offshore
exploration, oil companies built ships equipped with a special drilling rig and the capacity to carry many kilometers of drill
pipe. This basic idea later was adapted in constructing a research vessel, named the Glomar Challenger, designed specifically
for marine geology studies, including the collection of drill-core samples from the deep ocean floor. In 1968, the vessel
embarked on a year-long scientific expedition, criss-crossing the Mid-Atlantic Ridge between South America and Africa and
drilling core samples at specific locations. When the ages of the samples were determined by paleontologic and isotopic dating
studies, they provided the clinching evidence that proved the seafloor spreading hypothesis.
Glomar Challenger and JOIDES Resolution
A profound consequence of seafloor spreading is that new crust was, and is now, being continually created
along the oceanic ridges. This idea found great favor with some scientists who claimed that the shifting of
the continents can be simply explained by a large increase in size of the Earth since its formation. However,
this so-called "expanding Earth" hypothesis was unsatisfactory because its supporters could offer no
convincing geologic mechanism to produce such a huge, sudden expansion. Most geologists believe that the
Earth has changed little, if at all, in size since its formation 4.6 billion years ago, raising a key question:
how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth?
This question particularly intrigued Harry H. Hess, a Princeton University geologist and a Naval Reserve
Rear Admiral, and Robert S. Dietz, a scientist with the U.S. Coast and Geodetic Survey who first coined the
term seafloor spreading. Dietz and Hess were among the small handful who really understood the broad
implications of sea floor spreading. If the Earth's crust was expanding along the oceanic ridges, Hess
reasoned, it must be shrinking elsewhere. He suggested that new oceanic crust continuously spread away
from the ridges in a conveyor belt-like motion. Many millions of years later, the oceanic crust eventually
descends into the oceanic trenches -- very deep, narrow canyons along the rim of the Pacific Ocean basin.
According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old
oceanic crust was consumed in the trenches, new magma rose and erupted along the spreading ridges to
form new crust. In effect, the ocean basins were perpetually being "recycled," with the creation of new crust
and the destruction of old oceanic lithosphere occurring simultaneously. Thus, Hess' ideas neatly explained
why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on
the ocean floor, and why oceanic rocks are much younger than continental rocks.
Ics1 sem1 block 2 Plate tectonics
Concentration of earthquakes
During the 20th century, improvements in seismic instrumentation and greater use of earthquake-recording instruments
(seismographs) worldwide enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably
along the oceanic trenches and spreading ridges. By the late 1920s, seismologists were beginning to identify several
prominent earthquake zones parallel to the trenches that typically were inclined 40-60° from the horizontal and extended
several hundred kilometers into the Earth. These zones later became known as Wadati-Benioff zones, or simply Benioff zones,
in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States. The
study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph
Network (WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The
much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake
concentration worldwide.
Earthquake zones
But what was the significance of the connection between earthquakes and oceanic trenches
and ridges? The recognition of such a connection helped confirm the seafloor-spreading
hypothesis by pin-pointing the zones where Hess had predicted oceanic crust is being
generated (along the ridges) and the zones where oceanic lithosphere sinks back into the
mantle (beneath the trenches).
Ics1 sem1 block 2 Plate tectonics
Continent drift and Plate tectonics
Question
1. What four major scientific developments spurred the formulation of the plate-tectonics theory?
2. As early as when did a few intrepid navigators take soundings with hand lines? What did they find?
3. What did a bathymetric chart, in 1855 reveal about the ocean floor?
4. What was when echo-sounding devices used for after World War 1?
5. What was noticed about the ocean floor sediment layer?
6. How is the seam on a baseball similar to the Mid-Oceanic ridge system?
7. What rock that is iron-rich covers the ocean floor?
8. Why is there a strip pattern on the ocean floor?
9. In the twentieth century what did paleomagnetists notice about the rock?
10. Which way does the needle of a compass point for a north end of a normal polarity rock?
11. Which way does the needle of a compass point for a north end of a reverse polarity rock?
12. How do grains of magnetite in volcanic rock behave?
13. What is the “Zebra pattern on the ocean floor?
14. How does the magnetic striping pattern form?
15. Why are the stripes symmetrical around the crests of the mid-ocean ridges?
16. What are the three lines of evidence for sea floor spreading?
17. What mission did the Glomar Challenger take on in 1968?
18. How can new crust be continuously added along the oceanic ridges without increasing the size of the
Earth?
19. Why does the Earth not get bigger with sea floor spreading?
20. Why is so little sediment accumulation on the ocean floor?
21. Why are oceanic rocks much younger than continental rocks?
22. Where do Earthquakes tend to concentrate?
23. What does WWSSN stand for?
24. What is the significance of the connection between earthquakes and oceanic trenches and ridges?
Ics1 sem1 block 2 Plate tectonics
Before You Read Statement After You Read
A or D A or D
1. ______All minerals are solids, but not all solids are minerals.
2. ______The word crystalline means that atoms are arranged in a repeating pattern.
3. ______The two most abundant elements in Earth’s crust are silicon and carbon.
4. ______Like vitamins, minerals are organic substances, which means they contain carbon.
5. ______Color is always the best physical property to use when attempting to identify minerals.
6. ______A mineral’s hardness is a measure of how easily it can be scratched.
7. ______Most gems or gemstones are special varieties of particular minerals.
8. ______Synthetic, or human-made, diamonds are minerals.
9. ______A mineral or rock is called an ore only if it contains a substance that can be mined for a profit.
10. ______The three major types of rock are igneous, sedimentary, and metamorphic rocks.
11. ______During the rock cycle, any given rock can change into any of the three major rock types.
12. ______When magma reaches Earth’s surface and flows from volcanoes, it is called lava.
13. ______The pressure exerted by rocks produces all the heat used to form magma.
14. ______All igneous rock is formed from lava that cooled on Earth’s surface.
15. ______Before any rock is transformed into a metamorphic rock, some of the minerals must be melted.
16. ______Metamorphic rock can form only under intense heat and pressure.
17. ______Sandstone, limestone, chalk, rock salt, and coal are all examples of sedimentary rocks.
18. ______Sedimentary rocks can be made of just about any material found in nature.
19. ______Fossils of tropical plants are never found in Antarctica.
20. ______Because of all the evidence that Alfred Wegener collected, scientists initially accepted his
hypothesis of continental drift.
21. ______Wegener’s continental drift hypothesis explains how, when, and why the continents drifted apart.
22. ______Earthquakes and volcanic eruptions often occur underwater along mid-ocean ridges.
23. ______Seafloor spreading provided part of the explanation of how continents could move.
24. ______Earth’s broken crust rides on several large plates that move on a plastic-like layer of Earth’s
mantle.
25. ______The San Andreas Fault is part of a plate boundary.
26. ______When two continental plates move toward each other, one continent sinks beneath the other.
27. ______Scientists have proposed several explanations of how heat moves in Earth’s interior.
Ics1 sem1 block 2 Plate tectonics
Plate Tectonics
Ocean Floor
Magnetic
Field
Age
Sea Floor
Topography
Plate Boundaries
Convergent
Divergent
Transform
Plates collide
plates slide past
each other
three types
Zebra Pattern
Carbon dating
mid ocean ridge
Normal reverse
polarity
Radioactive
dating
subduction
zones
both sides of the
mid ocean ridge
Magnetic field
dating
trenches
continentcontinent
continentoceanic oceanicoceanic
mountains form
plates pull away
new crust forms
deep fault lines
known as
fracture zones
Mountain ranges
form
most under
water
subduction
zones form
San Andreas
fault excemption
deep trenches
form
Rocks
Physical
Conditions
Texture
Classification
Rocks
Chemical
Conditions
Crystalline
Structure
Elements