Chapter 2

Question 1

What was Wegener’s continental drift hypothesis? What was his evidence? Why didn’t other geologists agree?

Answer 1

continents once formed a single landmass (Pangaea) and drifted apart. His evidence included matching coastlines, fossil distribution, and similar rock formations across continents. Other geologists rejected it because he couldn’t explain the mechanism driving the movement.

Question 2

How do apparent polar-wander paths show that the continents, rather than the poles, have moved?

Answer 2

Apparent polar-wander paths from different continents don’t align, implying that the continents have moved relative to the poles and to each other over time, rather than the poles themselves shifting.

Question 3

Describe the hypothesis of sea-floor spreading.

Answer 3

Sea-floor spreading suggests that new oceanic crust forms at mid-ocean ridges and spreads outwards. This process drives the movement of oceanic plates and contributes to continental drift.

Question 4

Describe the pattern of marine magnetic anomalies across a mid-ocean ridge. How is this pattern explained?

Answer 4

Marine magnetic anomalies form symmetric stripes of normal and reversed polarity on either side of a mid-ocean ridge. This pattern is explained by Earth’s magnetic field reversing over time as new crust forms.

Question 5

How did drilling into the sea floor contribute further proof of sea-floor spreading

Answer 5

Drilling revealed that the age of oceanic crust increases with distance from the ridge, supporting sea-floor spreading

Question 6

How did the sea-floor-spreading hypothesis explain variations in ocean floor heat flow?

Answer 6

Drilling revealed that the age of oceanic crust increases with distance from the ridge, supporting sea-floor spreading. The hypothesis explains that heat flow is highest near ridges where new crust is forming.

Question 7

What are the characteristics of a lithosphere plate? Can a single plate include both continental and oceanic lithosphere?

Answer 7

Lithosphere plates are rigid and composed of both crust and the upper mantle. A single plate can include both continental and oceanic lithosphere.

Question 8

How does oceanic lithosphere differ from continental lithosphere in thickness, composition, and density?

Answer 8

Oceanic lithosphere is thinner, denser, and primarily basaltic, while continental lithosphere is thicker, less dense, and primarily granitic.

Question 9

How do we identify a plate boundary?

Answer 9

Plate boundaries are identified by the presence of seismic activity, volcanic activity, and distinct geological features like mid-ocean ridges, subduction zones, and transform faults.

Question 10

Describe Divergent Plate Boundary

Answer 10

Plates move apart (mid-ocean ridges).

Question 11

Describe Convergent Plate Boundary

Answer 11

Plates move towards each other (subduction zones, mountain formation).

Question 12

Describe Transform Plate Boundary

Answer 12

Plates slide past each other (fault lines).

Question 13

How does crust form along a mid-ocean ridge?

Answer 13

New oceanic crust forms as magma rises from the mantle at divergent boundaries, cools, and solidifies as it moves away from the ridge.

Question 14

Why is the oldest oceanic lithosphere less than 200 Ma?

Answer 14

Oceanic lithosphere is continually recycled into the mantle at subduction zones, preventing it from becoming older than 200 million years.

Question 15

Describe the major features of a convergent boundary.

Answer 15

Features include subduction zones, deep ocean trenches, volcanic arcs, and mountain ranges, where one plate is forced beneath another.

Question 16

Why are transform plate boundaries required on an Earth with spreading and subducting plate boundaries?

Answer 16

Transform boundaries accommodate the horizontal motion between divergent and convergent boundaries, allowing plates to slide past each other.

Question 17

What is a triple junction?

Answer 17

A triple junction is a point where three plate boundaries meet. Depending on the types of boundaries involved, it can be stable or unstable.

Question 18

How is a hot-spot track produced, and how can hot-spot tracks be used to track the past motions of a plate?

Answer 18

A hot-spot track forms as a tectonic plate moves over a stationary mantle plume. The track records the direction and speed of the plate’s movement.

Question 19

Describe the characteristics of a continental rift and give examples of where this process is occurring today.

Answer 19

A continental rift is where a continent is stretching and breaking apart, forming a rift valley. Examples include the East African Rift and the Basin and Range Province in the western U.S.

Question 20

Describe the process of continental collision and give examples of where this process has occurred.

Answer 20

Continental collision occurs when two continental plates converge, causing the crust to thicken and form mountain ranges. An example is the collision of the Indian and Eurasian plates, forming the Himalayas.

Question 21

Discuss the major forces that move lithosphere plates.

Answer 21

Major forces include slab pull, ridge push, and mantle convection. Slab pull occurs as a dense plate sinks into the mantle, while ridge push results from gravity acting on the elevated mid-ocean ridges.

Question 22

Explain the difference between relative plate velocity and absolute plate velocity.

Answer 22

Relative plate velocity measures the movement of one plate relative to another, while absolute plate velocity measures a plate’s movement relative to a fixed point, like the mantle or a hotspot.

Question 23

Why are the marine magnetic anomalies bordering the East Pacific Rise in the Pacific Ocean wider than those bordering the Mid-Atlantic Ridge?

Answer 23

The East Pacific Rise has a faster spreading rate, which creates wider magnetic anomaly stripes compared to the slower spreading Mid-Atlantic Ridge.

Question 24

The North Atlantic Ocean is 3,600 km wide. Sea-floor spreading along the Mid-Atlantic Ridge occurs at 2 cm per year. When did rifting start to open the Atlantic?

Answer 24

The Atlantic began opening approximately 180 million years ago (3,600,000 meters ÷ 0.02 meters/year).

Question 25

Mid-Ocean Ridges:

Answer 25

decompression of the mantle leads to partial melting, generating basaltic magmas and underneath that there’s sheeted dikes which are feeders to the volcanic eruptions

Question 26

Plutonic Rocks vs Volcanic Rocks

Answer 26

Volcanic rocks erupt and crystallize on the surface of the Earth and plutonic rocks crystallize more slowly below the surface (magmas cooled below surface)

Question 27

Full Spreading Rate

Answer 27

How fast two points on either side are moving away from each other;; hot, high melt supply

Question 28

Half Spreading rate

Answer 28

rate of the new crust formation on one side of the ridge, cooler, lower melt supply

Question 29

Convergent:

Answer 29

two plates, one of which is oceanic, move toward one another except the oceanic plate sinks down (subduction)

Question 30

Wadati-Benioff Zone

Answer 30

band of earthquakes in a downgoing plate

Question 31

Oceanic-Oceanic Convergent

Answer 31

Leads to the formation of island arcs (the pacific plate)
The Aleutians, Western Pacific

Question 32

Oceanic Continental Convergent

Answer 32

always subducted at ocean-continent convergent boundaries; Cascade Volcanoes

Question 33

Continental Continental Convergent

Answer 33

Lead to very thick crust, generate large mountain ranges
Formation of the Himalayas

Question 34

Continental Crust

Answer 34

thicker crust, thicker lithosphere, less dense, older on average, more complex

Question 35

Oceanic Crust

Answer 35

thinner crust, thinner lithosphere, more dense, younger, less complex

Question 36

What are the general features of Subduction Zones?

Answer 36

Rising magma, partial melting near this in the asthenosphere. The lithosphere has a downgoing plate and an overriding plate. Then rising magma hits the crust and volcanic arcs form. From the oceanic side, trenches form.

Question 37

What are the geologic features of Convergent Plate Boundary

Answer 37

Deep Earthquakes, Seismic Tomography of Subducting Slabs, Volcanic Arc

Question 38

Volcanic Arc Formation

Answer 38

forms where oceanic plate subducts beneath the continental lithosphere then the volcanoes grow on the continent and form a continental volcanic arc

Question 39

Why do Slabs Subduct?

Answer 39

they’re denser than the mantle because Mantle Lithosphere is colder and denser than mantle asthenosphere AND Slabs become denser with depth due to the growth of metamorphic minerals

Question 40

what is an accretionary prism

Answer 40

a geological structure that forms at a convergent plate boundary. It is created when sediments and rocks from the ocean floor are scraped off the subducting plate and accumulate in a wedge-shaped mass at the edge of the overriding plate.

Question 41

Mantle Melting Subduction Zones:

Answer 41

Mantle Melting due to the release of water from the slab; Magmas rise up to form overlying volcanoes. The Ring of Fire: Subduction zones around the Pacific lead to earthquakes and volcanism

Question 42

Transform Plate Boundaries

Answer 42

ocean ridges exist in short segments that are offset laterally from each other called fracture zones

Question 43

Fracture Zone

Answer 43

linear feature on the ocean floor that represents a fault or series of faults where tectonic plates have moved past each other. These zones typically occur perpendicular to mid-ocean ridges accomodating the horizontal movement of tectonic plates.

Question 44

Transform Boundary (transform fault)

Answer 44

actively slipping segment of a fracture zone between two ridge segments. one plate slides sideways past another but no new plate forms and no old plate is consumed

Question 45

Hot Spot Volcanoes

Answer 45

isolated points that are not a consequence of movement at a plate boundary

Question 46

How are hotspot volcanoes formed?

Answer 46

active ones occur at the end of a chain of dead volcanic islands and seamounts. Heat sources for hot spots are mantle plumes, a column of very hot rock rising up through the mantle to the base of the lithosphere.

Question 47

What happens during Continental Rifting?

Answer 47

linear belt in which continental lithosphere pulls apart. Lithosphere stretches horizontally and thins vertically and as continental lithosphere thins, the hot asthenosphere rises beneath the rift and starts to melt. Eruption of the molten rock produces volcanoes along the rift

Question 48

Collision

Answer 48

process which two buoyant pieces of lithosphere converge and squeeze together

Question 49

What Drives Plate Motion

Answer 49

model in which convection, ridge push, and slab pull all contribute to driving plates; it isn’t just a conveyor belt like system; Convection involvement

Question 50

What is Convection?

Answer 50

Process of heat transfer in fluids: warmer, less dense regions of a fluid rise while cooler, denser regions sink, creating a circular motion known as a convection current.

Question 51

Ridge-push Force

Answer 51

develops simply because the lithosphere of mid-ocean ridges lies at a higher elevation than that of the adjacent abyssal plains

Question 52

Slab-pull force

Answer 52

force that subducting down-going plates apply to oceanic lithosphere at a convergent margin, arises simply because the lithosphere that was formed around 10 Ma is denser than the asthenosphere so it can sink into the asthenosphere

Question 53

Relative Plate Velocity

Answer 53

refers to describing the movement of late A with respect to plate B

Question 54

Absolute Plate Velocity

Answer 54

describing the movement of both plates relative to a fixed location in the mantle below the plates

Question 55

Mantle Plume

Answer 55

an upwelling of abnormally hot rock within the Earth’s mantle. These are believed to originate deep within the mantle, potentially from the core-mantle boundary, and rise towards the Earth’s crust. They can lead to volcanic activity and the formation of large igneous provinces.