Ics1 sem1 block 2 Plate tectonicsName_________________________per___

ICS 1 Block 2 Plate TectonicsOcean Floor Characteristics, Types / Properties of Plate Boundaries, Physical / Chemical Conditions of Rocks

1. Cover sheet / Agree or Disagree2. WCW- your paper3. Standards/crossword4. Vocabulary5. Notes: Ocean floor6. Continental Drift Handout

Content ObjectiveTo 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 ObjectiveDescribe 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 tectonicsOn 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

Warm-up

The seven continents are… The 4 major oceans are… The reason for different Land

forms is …

Critical Thinking

Some coastal Mountain ranges are farther from the convergent boundaries that construct them because…

Wrap-up Most Earthquakes and Volcanic

Ics1 sem1 block 2 Plate tectonics

eruptions occur along or near ____Why?

Ics1 sem1 block 2 Plate tectonicsNotes: Ocean Floor

Harry Hess Pacific mtn rangeSonar-mid-atlantic ridge12,000 miles longOcean floor 4-5km deepMid-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 movedNow we know its like an escalator

Ics1 sem1 block 2 Plate tectonics

Standards Fill in the blankES 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 tectonicsCalifornia State Standards with FrameworkES 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 tectonicsVocabulary

1. Sea-floor topography the relief features or surface configuration of an area2. 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 pieces3. Continental drift the lateral movement of continents resulting from the motion of crustal plates4. Mid-ocean ridge extend through the Atlantic, Indian, and South Pacific oceans: each is hypothesized to

be the locus of seafloor spreading5. Rift valleys chasm extending along the crest of a mid-ocean ridge, locus of the magma upwelling that

accompany seafloor spreading6. Sediment mineral or organic matter deposited by water, air, or ice7. Fossils remains, impression, or trace of a living thing of a former geologic age, as a skeleton, footprint,

etc8. 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 particle9. Zebra stripes pattern created by the changing Earth’s magnetic field on the ocean crust10. Polarity the property or characteristic that produces unequal physical effects at different points in a

magnet or storage battery11. Lava the molten, fluid rock that issues from a volcano or volcanic vent12. 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 out13. Ocean Basin a large geologic areas that are below sea level14. 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 above16. 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 other18. Transform boundary type of tectonic boundary where two plates are moving past each other in opposite

directions19. Oceanic crust part of Earth's lithosphere that covers the ocean basins20. Continental Crust the layer of igneous, sedimentary, and metamorphic rocks which form the continents21. Mantle is a highly viscous layer between the crust and the outer core22. Subduction one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the

plates converge23. Volcanic island arcs generally they result from the subduction of an oceanic tectonic plate under another

tectonic plate, and often parallel an oceanic trench24. Volcanic mountain chain when oceanic crust subducts beneath continental crust on an adjacent plate,

creating an arc-shaped mountain belt25. 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 lava28. Crystalline texture the distribution of crystallographic orientations of a polycrystalline sample29. Sedimentary rock minerals and/or organic particles settle and accumulate to form a new material30. Metamorphic rock the transformation of an existing rock type to a new rock type through heat and/or

pressure

Ics1 sem1 block 2 Plate tectonicsEarth Science ocean floor, rock type, Plate tectonics

Ics1 sem1 block 2 Plate tectonics

Across3. the transformation of an existing rock type to a new rock type through heat and/or pressure5. a volume of circulating material, under gravity, that is heated from below and cooled from above7. formed through the cooling and solidification of magma or lava8. the molten, fluid rock that issues from a volcano or volcanic vent9. remains, impression, or trace of a living thing of a former geologic age, as a skeleton, footprint, etc10. pattern created by the changing Earth’s magnetic field on the ocean crust15. the lateral movement of continents resulting from the motion of crustal plates21. type of tectonic boundary where two plates are moving toward each other22. when oceanic crust subducts beneath continental crust on an adjacent plate, creating an arc-shaped mountain belt23. 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 out24. generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench25. the relief features or surface configuration of an area26. ridge extend through the Atlantic, Indian, and South Pacific oceans: each is hypothesized to be the locus of seafloor spreading27. mineral or organic matter deposited by water, air, or ice28. the property or characteristic that produces unequal physical effects at different points in a magnet or storage battery29. type of tectonic boundary where two plates are moving in the opposite direction.

Down1. theory in which the lithosphere is divided into pieces, each move on the plastic asthenosphere to collide with, slide under, or move past adjacent pieces2. part of Earth's lithosphere that covers the ocean basins3. is a highly viscous layer between the crust and the outer core4. 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 material11. one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge12. chasm extending along the crest of a mid-ocean ridge, locus of the magma upwelling that accompany seafloor spreading13. near a magnet, electric current, or moving charged particle in which a magnetic force acts on any other magnet, electric current, or moving charged particle14. 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 level17. the distribution of crystallographic orientations of a polycrystalline sample18. type of tectonic boundary where two plates are moving past each other in opposite directions19. the layer of igneous, sedimentary, and metamorphic rocks which form the continents20. continental transform fault that runs a length of roughly 810 miles (1,300 km) through California

Ics1 sem1 block 2 Plate tectonicsNotes: Ocean Floor CharacteristicsStandard(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 tectonicshttp://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 tectonicsSeafloor 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.

Ics1 sem1 block 2 Plate tectonics

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.

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 tectonicsQuestion

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 tectonicsBefore You Read Statement After You ReadA 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

Zebra PatternNormal reverse

polarityboth sides of the mid ocean ridge

Age

Carbon datingRadioactive

datingMagnetic field

dating

Sea Floor Topography

mid ocean ridgesubduction

zonestrenches

Plate Boundaries

Convergent

Plates collidethree typescontinent-continentcontinent-

oceanic oceanic-oceanic

Mountain ranges form

subduction zones form

deep trenches form

Divergent

plates pull awaynew crust forms

Transform

plates slide past each other

mountains formdeep fault lines

known as fracture zones

most under water

San Andreas fault excemption

Rocks Physical

Conditions

Texture Classification

Rocks Chemical

Conditions

Crystalline Structure Elements