Chapter 17- Science

The Mid-Atlantic Ridge is an example of a

divergent boundary

True or False: The oldest ocean floor rocks are about *3.8 billion* years old.

180 million

A field with the same orientation as today's field is said to have ____.

Normal polarity

True or False: *Deep-sea trenches* are vast, underwater mountain chains.

Ocean ridges

Number these steps in the order they occur- seafloor spreading: ____Magma fills the gap that is created ____Magma hardens to form new ocean crust ____Magma is forced upward toward the crust

2.Magma fills the gap that is created 3.Magma hardens to form new ocean crust 1.Magma is forced upward toward the crust

Explain how the parts of a convection current in the mantle are related to plate motions.

The rising part of a convection current spreads out as it reaches the upper mantle and causes lateral and upward forces. These forces lift and split the lithosphere at a divergent boundary. The downward part of the current occurs where a sinking forces pulls plates downward at convergent boundaries.

Where are most divergent boundaries formed?

on the seafloor

Coal beds in Antarctica indicated to Wegener that this continent was

once located closer to the equator

To support his hypothesis of continental drift, Wegener did NOT use A. Ancient climatic evidence B. Magnetic field data C. Data on ancient reptiles and ferns D. Evidence from rock formations

B. Magnetic field data

Compare and contrast ridge push and slab pull.

Both are processes associated with plate movements. Ridge push is a process in which the uplifted ridge is thought to push the oceanic plate toward the subduction zone. Slab pull is a process whereby the weight of a subducting plate helps to pull the trailing lithosphere into the subduction zone.

Describe the formation of convection currents in the mantle.

Even though it is a solid, the upper mantle can flow like a soft plastic. Convection currents are set in motion by the transfer of energy between Earth's hot interior and cooler exterior. Regions of hot mantle are less dense than areas of cooler mantle and are slowly forced toward the crust. Cooler parts of the mantle sink back toward the core. The convection currents that result are probably set in motion by subducting slabs.

What is one hypothesis regarding the formation of a divergent boundary on a continent?

Large continental masses may act as insulating blankets. The underlying mantle becomes warmer and causes the upward leg of a convection current to develop. Over time, the continent splits.

A field that is opposite the present field has ____.

Reversed polarity

True or False: Earthquake activity and volcanism are common along *ocean ridges.*

True

When the ocean floor's magnetic readings match the present field, the 2 fields _____. This produces a _______ than normal reading.

combine, stronger

Based on observations of fossils of Glossopteris, Wegener concluded that

continental rocks containing these fossils had once been joined

Places where tectonic plates move apart are called

divergent boundaries

Was constructed from data gathered from continental basalt flows

geomagnetic time scale

Fossil evidence that supported Wegener's idea of continental drift included

land-dwelling animals

Based on the glacial deposits he observed, Wegener argued that

landmasses drifted away from the south pole

Each cycle of spreading and magma intrusion along an ocean ridge results in the formation of this

new ocean crust

Convergent boundaries are classified according to the

type of crust involved

Which of the following best describes what happens when an oceanic plate converges with a continental plate?

A trench and a mountain range with many volcanoes form

Explain the process of convection.

Convection is the transfer of thermal energy by the movement of heated matter. Heating matter causes it to expand and decrease in density. This warmed matter is forced upward as a result of buoyancy. The cooler matter is pulled downward as a result of gravity. This up and down flow creates a convection current.

Which of the following is NOT associated with transform boundaries? A. deformed and fractured crust B. shallow earthquakes C. long faults D. volcanoes

D. volcanoes

Earth's ____ has changed over time.

Magnetic field

True or False: An *isocron* is a change in Earth's magnetic field.

Magnetic reversal

True or False: *Sonar* uses sound waves to measure water depth

True

True or False: The magnetic patterns on either side of a *deep-sea trench* are mirror images of each other.

True

What feature is associated with a continental-continental plate boundary?

a mountain range

What landform result from divergence of continental crust?

a rift valley

Pangaea was an ancient supercontinent made up of

all of Earth's continents

Oceanic crust is made mostly of

basalt

When the magnetic readings of the ocean floor are reversed compared to today's field, the 2 fields partially _____ to produce a ____ than normal reading.

cancel, lower

True or False: The thickness of ocean floor sediments *decreases* with distance from an ocean ridge.

increases

Minerals containing this act like small compass needles and record the orientation of Earth's magnetic field at the time of their formation

iron

This type of line connects points on a map that have the same age

isochron

Magnetic data of the ocean floor has been used to generate _____ maps, which have shown the ocean floor is ______ near ocean ridges and _____ near deep-sea trenches.

isochron, younger, older

Device that can detect small changes in magnetic fields

magnetometer

Early mapmakers thought continents might have moved based on their observations of

matching coastlines

What can happen when 2 oceanic plates converge and one is subducted into the mantle?

melted magma erupts and forms an arc of islands

Which theory states that Earth's crust and rigid upper mantle move in different directions and at different rates over Earth's surface?

plate tectonics

Fossils of aquatic reptiles found in freshwater rocks suggested to Wegener that these reptiles

probably did not cross the ocean

True or False: The theory of *continental drift* states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches.

seafloor spreading

Which of the following features forms when oceanic plates converge?

subduction zones

True or False: Maps made from sonar and magnetometer data led to the discovery of *ocean ridges and deep-sea trenches.*

True

True or False: The study of the magnetic record preserved in Earth's rocks is called *paleomagnetism*

True

True or False: The theory of seafloor spreading explains that Earth's continents move because they *ride atop ocena crust as it moves away from ocean ridges.*

True

Tectonic plates interact at places called plate

boundaries

Places where tectonic plates come together are called

convergent boundaries

True or False: As new seafloor is carried away from an ocean ridge, it *heats up, expands, and becomes less dense* than the material beneath it.

cools, contracts, and becomes more dense

Most scientists at the time rejected Wegener's hypothesis of continental drift because he

couldn't explain how or why the continents moved

At which tectonic plate boundary do plates slide horizontally past each other?

transform boundary

True or False: Rock samples taken near ocean ridges are *older* than rock samples taken near deep-sea trenches.

younger