Tectonic Plates

Question 1

Why does melting take place at divergent boundaries?

Answer 1

• Decompression melting
• Pressure raises the melting point of objects, so when the plastic asthenosphere is decompressed, it melts due to the relief of pressure
• Mantle rises underneath ridge, melts when at coolest point (100 km), decompression melting – taking pressure off, pressure low enough for mantle to melt

Question 2

Why does melting take place at convergent boundaries? (ex. of volcanoes)

Answer 2

• As the oceanic lithosphere sinks into the mantle, it heats up
• At a depth of 100 km, the plate dehydrates and the water enters the mantle above the plate
• The water lowers the melting point of the mantle, causing it to melt
• The magma rises, causing volcanoes
• Product of this melting of the mantle: a line of volcanoes (an “arc”) over the subducted lithosphere
• Examples: Mt. St. Helens, Mt. Pinatubo, Mt. Jujiyama, Krakatoa
• Volcanoes often more explosive + dangerous - contain more gases such as water vapor and carbon dioxide

Question 3

Where is the oldest and newest oceanic crust found?

Answer 3

• Newest oceanic crust: divergent boundaries

- Oldest: convergent boundaries

Question 4

Be able to calculate how old an ocean basin is knowing its width and the rate of spreading at the ridge

1) Width, spreading rate— find age
2) Age, plate velocity— find width
3) Width, age— find plate velocity

Answer 4

1) width in cm / spreading rate in cm
2) plate velocity * 2 = spreading rate
(age * spreading rate in cm) / 100,000
3) (width in cm / age) /2

Question 5

What is the difference between plate velocity and the spreading rate of an ocean basin?

Answer 5

• Plate velocity is how far a plate moves

- Spreading rate is double that, or the added velocity of both plates on each side of the oceanic trench

Question 6

Know how scientists used the history of Earth’s magnetic field reversals to support the idea of plate tectonics.

Answer 6

• Scientists know that when rock cools, the iron in it lines up with the current magnetic field
• The polarity of the rock on both sides of the divergent boundary was mirrored, so they knew that new rock was coming up from the mantle, and cooling off

Question 7

What process produces the hot water springs at oceanic ridges, and why are they important to the origin of life on Earth?

Answer 7

• Water seeps into the ground and gets heated by hot magma below
• This water then rises into a spring
• These can contain minerals from the rock below
• The first life on Earth may have originated in these springs

Question 8

Where and why do Earthquakes happen at convergent boundaries? (ex)

Answer 8

• Shallow, intermediate, and deep earthquakes can occur
• Earthquakes will happen where the subducting crust is located, until a depth of about 700 kilometers, where the subducting crust moves as a plastic
• The subducting crust will fracture on its way down, so earthquakes will happen

Question 9

How are hotspots created?

Answer 9

• New oceanic crust is made by the melting of the mantle at divergent boundaries (decompression melting)
• The melting process concentrates radioactive elements (Uranium, Thorium, Potassium) because they want to be in liquid
• Plate is denser, it will sink all the way down to the core-mantle boundary with help from the radioactivity
• Stay there for billions of years of radioactive decay, hotter because of radioactive elements, density low enough, starts to rise, decompression melting : volcano

Question 10

Axial Valley (ex)

Answer 10

At mid ocean ridges, two plates crack, a valley forms where lava is erupted and hot water springs exist
Axial valley above sea level: Iceland

Question 11

Black Smoker

Answer 11

1) Cold sea water seeps into cracks on the sea floor
2) Water heats up and becomes acidic as it approaches the magma chamber (300-200 C) - acidic (magma chamber shooting out acidic material)
3) Water heats up – less dense, rises – dissolves oceanic crust its passing through on its way to the surface (loaded with materials), reemerges in the vents and deposits minerals that build up the vents
4) Dissolved materials come out (precipitate)

Question 12

Chimney

Answer 12

A vent structure composed of limestone in which minerals from the rocks below come out of.

Question 13

Continental Arc

Answer 13

A volcanic arc that forms on a preexisting continent

Question 14

East African Rift Valley

Answer 14

Where the African plate is starting to split apart due to changes of the orientation of convection cells in the mantle

Question 15

Hot Spot (ex)

Answer 15

A place on Earth that has volcanic activity that is nowhere near plate activity (age progression as the plate moves)
Ex: Hawaii

Question 16

Lake Baikal

Answer 16

• The deepest lake in the world
• Marks a spot where the Eurasian plate will separate
• In addition it is in Russia

Question 17

Lithospheric Pressure

Answer 17

The pressure over rock by the weight of overlying material.

Question 18

Magnetic Polarity

Answer 18

The direction in which Earth’s magnetic pole is facing, either normal or reverse polarity.

Question 19

Magnetic Reversal

Answer 19

When the Earth’s poles undergo a flip, and change their polarity.

Question 20

Mariana Trench

Answer 20

• The deepest part of the world’s oceans

- Marks the convergent boundary between the Pacific and Philippine plates

Question 21

Oceanic Arc

Answer 21

An island arc that forms over an ocean

Question 22

Radioactivity

Answer 22

The emission of ionizing radiation or particles caused by the spontaneous disintegration of atomic nuclei.

Question 23

Spreading Rate

Answer 23

The combined rate at which the ocean floor spreads on both sides of the trench.

Question 24

Wadati-Benioff Zone

Answer 24

An inclined zone of earthquakes that indicates the passage of a subducted plate into the mantle, up to a depth of 700 kilometers

Question 25

Importance of convergent boundary

Answer 25

Continents grow and evolve by two ways:

1) The addition of brand new material at continental arcs
2) The accretion of oceanic arcs onto the continents

Question 26

What determines convergent angle of subduction? (ex)

Answer 26

-Density (steeper angle = older = denser) old in world: Mariana trench – farther from ridge
Steep dip = arc closer

Question 27

Subduction zone

Answer 27

Where an oceanic plate descends into the asthenosphere at a convergent margin

Question 28

Trench

Answer 28

An elongated depression on the seafloor created as an oceanic plate turns down into the interior of the Earth at a convergent margin

Question 29

Arc

Answer 29

A chain of volcanoes that occurs over a subjected area at a convergent margin

Question 30

What is the difference between the asthenosphere and the lithosphere in terms of temperature, pressure, and rigidity?

Answer 30

• The lithosphere consists of the crust and uppermost mantle and is rigid
• It is colder than the asthenosphere and is less dense
• The asthenosphere consists of the rest of the mantle and behaves as a plastic
• It is warmer than the lithosphere and denser

Question 31

What are the differences between oceanic and continental plates?

Answer 31

• Oceanic plates are denser and thinner than continental plates
• They also tend to be younger, because they subduct at convergent boundaries

Question 32

In what hemisphere are the majority of earthquakes and volcanoes located? Why?

Answer 32

• In the northern hemisphere, there are more plate boundaries, which is where volcanoes form
• They especially form at convergent boundaries, which the northern hemisphere has a lot of

Question 33

Where are the majority of earthquakes and volcanoes located?

Answer 33

At plate boundaries (usually convergent)

Question 34

Divergent boundary (ex)

Answer 34

• Ex: mid-Atlantic ridge
• Where two plates are pulled apart from each other
• Shallow earthquakes can occur (100 km)
• Volcanic activity occurs due to decompression melting
• Oceanic ridges are created, with a rift valley in the center

Question 35

Convergent boundary

Answer 35

• Where two plates move toward each other
• Shallow, intermediate, and deep earthquakes can occur
• Volcanic activity occurs - melting of the mantle
• Structures: trench, subduction zone, arc

Question 36

Transform boundary (ex)

Answer 36

• Where two plates slide past each other
• There can be shallow earthquakes (0-100 km)
• No volcanic activity
• Most occur in the middle of the oceans and connect segments of oceanic ridges (divergent margins)
• Ex: San Andreas fault (California)

Question 37

Ocean-ocean convergent plate boundary (ex)

Answer 37

• Where two oceanic plates meet and one is subducted beneath the other
• The older plate (which is colder and denser) subducts beneath the younger one - denser
• Ex: Philippines, Aleutian Islands, Lesser Antilles

Question 38

Ocean-continent convergent plate boundary (ex)

Answer 38

• Where an oceanic plate meets a continental plate and the denser oceanic plate sinks beneath the less dense continental plate
• Ex: Andes Mountains, Cascade Mountains (continental arcs)

Question 39

Continent-continent convergent plate boundary (ex)

Answer 39

• Where two continental plates collide with each other
• Neither plate can sink beneath the other (can’t be subjected) because they both have low densities
• Instead, the crust folds and enormous mountain chains are produced
• Ex: Himalayas, Alps, Appalachians