Chapter 17-Plate Tectonics

which way do compasses usually point?

North

what did scientists question?

why there was no trace of older oceanic crust

oceanic crust is more ____ than cont

young

the three parts of plate tectonics

1. divided into pieces 2. move past, beside, and into each other 3. creates topo features

what 2 discoveries were the result of analysis of the rocks and sediments on the seafloor?

1. the ages of the rocks that make up the seafloor vary across the ocean floor and these variations are predictable 2. ocean-floor sediments are typically a few hundred meters thick

there are about ____ major plates

8

there are about ____ major plates and several smaller ones

8

deep-sea trench

A long, narrow, elongated, deep depression in the seafloor. it typically runs parallel to a plate boundary and makes a subduction zone.

oceanic-oceanic

Alaska or Aleutian islands

oceanic oceanic is found in...

Aleutian Islands in Japan, Mariana's Trench, Fiji

Where do plates interact?

Along their boundaries

oceanic-cont

Andes or Chile

oceanic-continental is found in...

Andes, Mt. Pinatubo, Cascades, Krakatau

transform

California

T or F- ridge push is thought to be at least twice as important as slab pull in moving oceanic plate away from an ocean ridge

F

T or F- this question has been answered

F

T or F- magnetic reversal has never before occurred on Earth.

F, it has occurred many times in earths history

T or F- polarity is almost always normal

F, periods of normal polarity alternate with periods of reversed polarity.

magnetic reversal

Happens when the flow in the outer core changes, and Earth's magnetic field changes direction, causes compasses to point South

divergent boundaries are found...

Iceland, Surtsey Island, Galapagos Island, Mid-Oceanic Ridges

seafloor spreading usually occurs under the sea, but an exception is made in...

Iceland, where a portion of the Mid-Atlantic Ridge rises above sea level and erupts

examples of ridges

Juan de Fuca, Central Indian, SW Indian, Pacific Antarctic

an example of a trench

Mariana Trench, deepest in the world

cont-cont

Nepal or Himalayas

subduction

One plate going under another plate

where is it located?

Pacific Ocean

a divergent boundary formed the ____ ____ Ocean

South Atlantic

Marine geologist

Studies geological processes of the ocean floor

T or F- The Mariana Trench is bigger than Mt. Everest

T

T or F- convection currents can be thousands of km across

T

seafloor spreading

The theory that explains how new ocean crust is formed at ocean ridges, slowly moved away from ocean ridges, and destroyed at deep-sea trenches

convection is...

The transfer of thermal energy (heat) by the movement of heated material from one place to another

why did these two topographic features puzzle scientists?

What could have formed an underwater mtn range that extended around earth? What is the source of volcanism associated with these mtns? What forces could depress earths crust enough to create trenches nearly 6 times as deep as the Grand Canyon?

one of the main questions about the theory of plate tectonics

What force/ forces cause tectonic plates to move?

it, however, does not produce an arc of islands, instead producing...

a chain of volcanoes along the edge of a cont. plate

basalt

a dark gray to black dense to fine-grained igneous rock

magnetometer

a device that can detect small changes in magnetic fields

they flow, however, at a rate of...

a few cm per year

a tectonic plate is...

a huge piece of earths crust and rigid upper mantle that fits with others at its edges to cover earths surface

mid-ocean ridge

a major feature along the ocean floor consisting of an elevated region with a central valley

it produces...

a mtn range w/ many volcanoes

most cause what to happen?

cause sections of ocean ridges to become offset

this process also...

causes mantle to melt

Plates move _______ per year

centimeters

the matching ____ of continents on opposite sides of the Atlantic ocean suggest the continents were once joined.

coastlines

divergent vs transform boundary

comes apart vs slides past forms mid-ocean ridge vs islands, trenches, and volcanoes crust is formed vs crust is fractured/deformed

data gathered from paleomagnetic studies of cont. lava flows allowed scientists to....

construct a magnetic polarity time scale

wergner's hypothesis

cont drift

which conv boundary has no subduction zone

cont/cont

some divergent boundaries form on...

continents

it appears as a...

continuous mtn chain on the ocean floor

the cooling of matter causes it to slightly ____ and ____ in density

contract, increase

this up-and-down flow produces a pattern of motion called a...

convection current

what causes an ice cube to melt when placed in a glass of water?

convection currents

what are thought to drive plate movements?

convection currents in the mantle

it is destroyed at...

convergent boundaries

in the convection currents of the mantle...

cooler mantle material is denser than hot mantle material

why?

cooling

divergent boundaries can ____ rift valleys

create

Earth's ____ and ____ ____ are broken into slabs called tectonic plates

crust, upper mantle

underwater mtn chains' counterparts-

deep-sea trenches

Oceanic plates are ___ than continental plates

denser

in oceanic-oceanic convergence, one of the plates is ___ than the other

denser

this method enabled scientists to quickly create ____ maps of the ocean floor

isochron

this part of the mantle moves as...

part of tectonic plates

MOST geologic activity occurs at...

plate boundaries

transform boundary

plates slide past each other; shear

plates move at different ____

rates

this process does what to oceanic crust formed at the ridge?

recycles

a magnetic field that is opposite to the present field has ____ polarity

reversed

continents are not pushing through ocean crust as he proposed, but are more like passengers that...

ride along while ocean crust slowly moves away from oncean ridges

in seafloor spreading, magma intrudes into the ocean floor through a...

ridge

plate movements result from ...

ridge push and slab pull

a divrgent boundary within a cont forms a....

rift valley

divergent boundaries are usually found in...

rift valleys

new oceanic crust forms as magma...

rises at ridges and solidifies

Wergner's three evidences

rocks, fossils, and glacial deposits

data from topographic, sedimentary, and paleomagnetic research led to the proposition of...

seafloor spreading

the process that creates new ocean crust by the up welling of magma at ocean ridges

seafloor spreading

what was the missing link?

seafloor spreading

why is Iceland growing?

seafloor spreading

the regions with normal and reverse polarity formed what across the floor?

series of stripes

the ____ and ____ of the continents suggests that they were once joined together

shape, geology

advances in technology in the 1940s and 50s did what?

showed that all of these widely accepted ideas ere incorrect

the cooled matter then ____

sinks

mantle that has cooled at the base of tectonic plates slowly does what?

sinks downward toward the center of the earth

the evidence for seafloor spreading suggested that cont and oceanic crust move as enormous ____

slabs

the evidence for seafloor spreading suggested that cont. and oceanic crust move as enormous ____

slabs

they move ____

slowly

this cycle of spreading and the intrusion of magma continues the formation of ocean floor,which...

slowly moves away from the ridge

earths mantle is composed of primarily ____ material

solid

radioactive decay heats up the mantle, causing

solid rock to slowly flow and enormous convection currents to move material throughout the mantle

sonar uses ____ ____ to measure ____

sound waves, distance

these convection currents are thought to be set in motion by....

subducting slabs

seep-sea trenches are formed by...

subduction

the process in which tectonic plates sink back into the mantle

subduction

the combination of mechanisms such as these are involved in plate motions at...

subduction zones

the pattern of thickness across the ocean floor was ____ across ocean ridges

symmetrical

these slabs are called ...

tectonic plates

these slabs are known as...

tectonic plates

Theory of Plate Tectonics

the "marrige" of cont drift and seafloor spreading

this showed that...

the age of oceanic crust increases with distance from a ridge

by matching the patterns on the seafloor with the known pattern of reversals on land, scientists were able to determine...

the age of the ocean floor from magnetic recording.

Where does it begin?

the central rift

Convection

the circulatory motion that occurs in a fluid at nonuniform temperature owing to the variation of its density and the action of gravity

this field causes...

the compass to point north

secondly, ....

the continuous spreading and intrusion of magma results in the addition of new oceanic crust. two halves of the oceanic crust spread apart slowly and move apart like a conveyor belt

so, what happens?

the edges of both cont.s collide and become crumpled, folded, and uplifted

seafloor spreading along a divergent boundary can cause...

the expansion of an ocean basin

lastly, ...

the far edges of the oceanic crust sink beneath the cont. crust. as it descends, water in the minerals is released, melting the overlying mantle, forming magma. the magma rises and forms part of the cont. crust.

Earth's magnetic field is generated by...

the flow of molten iron in the liquid outer core

the rifting might eventually lead to...

the formation of a new ocean basin

these stripes were ____ to ocean ridges

parallel

when scientists towed magnetometers behind ships to measure the magnetic orientation of the rocks of the ocean floor, what happened?

a surprising pattern emerged

it results in...

a transfer of energy between earths hot interior and its cooler exterior

continental-oceanic convergence, like oceanic-oceanic, produces...

a trench and volcanic arch

this forms...

a vast mtn range

this trend was symmetric ____ and ____ to ocean ridges

across, parallel

older crust is found...

along deep-sea trenches

the process of subduction here creates...

an ocean trench

why?

because the cont rocks are less dense and will not sink into the mantle

as they crystallize, these minerals do what?

behave like tiny compasses and align with earths magnetic field

the part of the mantle that is too cold and stiff to flow lies...

beneath the crust and is attached to it

a rift zone is like a...

big tear or a baby divergent boundary

the cont crust can or cannot descend?

cannot

what was another advancement that allowed scientists to study the ocean floor in great detail?

development of echo-sounding methods, consisting of sonar

hot mantle material is then ____

displaced, and it rises

warmed matter is then ____ and forced to ____

displaced, rise

new crust is formed at...

divergent boundaries

Rift valleys are formed by

divergent boundaries on cont crust

when an oceanic plate is completely subducted, it can...

drag an attached continent behind it toward the subduction zone

the San Andreas Fault system is responsible for many of the ____ in California every year

earthquakes

why were they named this way?

earths crust changes its relative direction and velocity from one side of the boundary to the other

convection currents in the mantle result in energy transfer between...

earths hot interior and cooler exterior

long term changes in earth's magnetic field are called...

epochs

short term changes are called...

events

they...

fit together at their edges to cover earth's surfacce

seafloor spreading provided evidence that the ocean floor is not ____ and ____

flat, unchanging

on transform boundaries, it is...

fractured or deformed

magnetometers' data did what for scientists?

furthered their understanding of rocks underlying the ocean floor

each boundary has certain ____ ____ and ____ associated with it

geologic features, processes

why?

gravity

divergent boundaries are associated with...

high heat flow, volcanism, and earthquakes

at transform boundaries, plates move ____

horizontally

dense

how compacted is it?

2nd problem:

how were the conts moving (through rock)

tectonic plates

huge pieces of crust and rigid upper mantle

tectonic plates

huge pieces of crust and rigid upper mantle that fit together at their edges to cover Earth's surface

isochron

imaginary line on a map that shows points of the same age; formed at the same time

plates move...

in different directions and at different rates relative to one another

thickness of the sediments ____ with distance from an ocean ridge

increases

basaltic rock contains large amts of...

iron-bearing minerals of volcanic origin

When lava solidifies...

iron-bearing minerals such as magnetite crystallize.

How is crust recycled?

it subducts and melts into magma to repeat the process

subduction zone

juan de fuca

many scientists think that what drives this tectonic movement?

large-scale motion in the mantle (earths interior between the crust and the core)

cont. crust is usually...

less dense and lighter colored

cont-cont boundaries form when?

long after an oceanic plate has converged w/ an oceanic plate.

transform boundaries are characterized by...

long faults and shallow earthquakes

rift valley

long, narrow depression that forms when continental crust begins to separate at a divergent boundary

the first part of seafloor spreading-

magma intrudes into the ocean floor along a ridge and fills the gap created

what causes mid-oceanic ridges?

magma rising through the rift's faults

scientists hypothesized that the rocks on the ocean floor would show a record of....

magnetic reversals

one technological advance used to study the ocean floor was the ____

magnetometer

the radioactive decay is from both the ____ and the ____ beneath it

mantle, core

the ages and widths of the stripes ____ from one side to another. (pg 477)

matched

what two things did sonar tech. allow scientists do?

measure water depth and map the topography of the ocean floor

it does this by...

measuring the time it takes for sound waves sent from the ship to bounce off the seafloor and return to the ship

where in the world is divergent

mid-atlantic

Where does seafloor spreading happen?

mid-ocean ridges

Oceanic crust is mostly made of....

minerals high in iron and magnesium, or basaltic rocks

plate tectonics describes how tectonic plates....

move and shape earth's surface

continental drift was the idea that continents do what?

move around on Earths surface

the evidence for seafloor spreading suggests that cont. and oceanic crust do what?

move as enormous slabs

as new oceanic crust forms, the old crust...

moves away from the ridges

even though the mantle is solid, much of it...

moves like a soft, pliable plastic

cont/cont forms

mtn ranges

____ ____ and ____ form at the boundaries between plates

mtns, volcanoes, trenches

why is this?

new ocean crust forms here (usually)

a magnetic field that has the same orientation as earths present field is said to have ____ polarity

normal

until the mid-1900s, most people thought that...

ocean floors were essentially flat

in the isochron map in our book, the relatively young ocean-floor crust is near...

ocean ridges

oceanic crust forms at...

ocean ridges

sonar and magnetometers' data revealed vast, underwater mountain chains called ____ _____ that run along the ocean floors

ocean ridges

oceanic crust forms at... and becomes a part of....

ocean ridges, the seafloor

continents are often carried along attached to...

oceanic crust

many people also had misconceptions that...

oceanic crust was unchanging and much older than cont. crust

which convergence has a better chance of high explosivity?

oceanic-oceanic

signs of a transform fault:

offset train tracks, roads, fences, and creeks

cont drift vs plate tectonics

once been joined vs puzzle pieces

how does one use this device?

one can tow it behind a ship and use it to record the magnetic field generated by ocean floor rocks

the arc of volcanic islands formed by conv. boundaries ____ the trench

parallel

what forms the longest continuous mtn range on earth?

the ocean ridges

seafloor spreading

the process by which new oceanic crust forms at ocean ridges and slowly moves away from the spreading center until it is subducted and recycled at deep-sea trenches

ridge push

the process that occurs as the older portion of the seafloor sinks, and the weight of the uplifted ridge is thought to push the oceanic plate toward the trench formed at the subduction zone

slab pull

the pulling of a tectonic plate as the relatively cool, dense subducting plate subducts deep into the mantle

how are convergent and divergent movements of tectonic plates related to mantle convection?

the rising material in the convection current spreads out as it reaches the upper mantle and causes both upward and sideways forces. these forces lift and split the lithosphere at divergent plate boundaries. as the plates separate, material rising from the mantle supplies the magma that hardens to form new ocean crust. the downward part of a convection current occurs where a sinking force pulls tectonic plates downward at convergent boundaries.

reversals in the polarity of earths magnetic field are recorded in...

the rocks that make up the ocean floor

what causes a convection current to flow?

the sinking of a cold material

paleomagnetism

the study of the history of earths magnetic field, studies solidified lava

convection

the transfer of energy via the movent of heated matter

convection currents aid in...

the transfer of thermal energy from warmer regions of matter to cooler ones

plate tectonics is a...

theory

the second discovery puzzled scientists because...

they knew erosion and deposition occur in earths oceans but did not understand why seafloor sediments were not as thick as their cont. counterparts

convection currents in the mantle are sustained by...

this rise and fall of material

puzzle pieces move

together, apart, and by each other

convection currents does what with thermal energy?

transfers it from the earths interior to its exterior

what boundary separates southwestern California from the rest of the state?

transform, San Andreas Fault system is part of it

sonar data revealed what two things?

trenches and ridges in the ocean

oceanic-cont forms

trenches and volcanic mountain ranges

oceanic/cont forms

trenches, volcanic mtn range, volcanoes

it is heated ____ by radioactive decay

unevenly

oceanic-oceanic convergence forms

volcanic island arch, islands, trenches, volcanos

the magma then rises and forms...

volcanic island arcs

____ and ____ are common along ridges and trenches

volcanism, earthquakes

why?

water lowers the melting point of the overlying mantle when it is carried into Earth by the subducting plate

Wegener could not explain

what caused the landmasses to move or how they moved

1 problem:

what force pushes it

subduction vs convergent boundary

when one plate forces another under it vs when plates move together three types vs only one type

can transform boundaries occur on continents?

yes

oceanic crust of geologically ____

young

they also discovered that even the old est parts of the seafloor are geologically ____

young

rack samples from near ocean ridges were found to be ____ than ones from near the deep-sea trenches

younger