Earth Science Chapter 7 Notes
Terms in this set (41)
Briefly describe the view held by most geologists regarding the ocean basins and continents prior to the 1960s.
had fixed geographic positions and were very old.
What group of geologists were the least receptive to the continental drift hypothesis?
North American geologists,because most of the evidence was from Africa, South America, and Australia; these continents were unfamiliar to these geologists.
What was the first line of evidence that led early investigators to suspect that the continents were
Similarity of continental coastlines and jigsaw-like fit of the continents.
Explain why the discovery of the fossil remains of Mesosaurus in both South America and Africa,
but nowhere else, supports the continental drift hypothesis.
It was a freshwater reptile and could not have migrated across the ocean. In addition, there is
no evidence of any bridge or other connector between these two continents.
. Early in the twentieth century, what was the prevailing view of how land animals migrated across
vast expanses of open ocean?
Lower sea level and land bridges, rafting, and island stepping stones.
How did Wegener account for the existence of glaciers in the southern landmasses at a time when
areas in North America, Europe, and Asia supported lush tropical swamps?
Pangaea; southern continents were one landmass situated about the South Pole.
What two aspects of Wegener's continental drift hypothesis were objectionable to most Earth
Gravitational force of the Sun and Moon moved the continents the way they moved the tides and the idea that continents plowed through the oceanic crust.
What major ocean floor feature did oceanographers discover after World War II?
Mid ocean ridge
Compare and contrast the lithosphere and the asthenosphere.
_______is the hard outer portion of the Earth's crust. ______lies beneath and is more
ductile, allowing the lithospheric plates to "float" and be mobile.
List the seven largest lithospheric plates
North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic.
plates move apart
plates move towards each other
plates slide laterally past each other.
Describe how two plates move in relation to each other along divergent plate boundaries.
They move apart, with new rock being formed between them due to magmatic upwelling
What is the average rate of seafloor spreading in modern oceans?
5 cm (2 in) per year
List four facts that characterize the oceanic ridge system.
Long, continuous underwater mountain chain, divergent plate boundary, marked by formation of new
basaltic rock, rift valleys along the crest.
Briefly describe the process of continental rifting.
Mantle plumes beneath continental crust generate magmatic upwelling and forces that pull the crust apart,
generating a new rift valley. The rift valley eventually widens to create a new ocean basin.
Where is continental rifting occurring?
Explain why the rate of lithosphere production roughly balances with the rate of lithosphere
Subduction at convergent boundaries, which destroys old lithosphere, occurs at the same rate as seafloor
spreading and new rock generation at divergent plate boundaries.
A continental volcanic arc occurs on the edge of a continental land mass near an ocean-continent convergent plate boundary. These volcanoes tend to produce more felsic to intermediate magma
volcanic island arc
A volcanic island arc occurs at ocean-ocean plate convergent plate boundaries and produces an island chain
in the ocean; these volcanic islands are basaltic.
Describe the process that leads to the formation of deep-ocean trenches.
When two oceanic plates collide, one is subducted. As one plate descends beneath the other, a trench is
formed; deeper trenches tend to form where older, denser basaltic ocean floor can descend very deeply
beneath the other plate, often at very steep angles
Why does oceanic lithosphere subduct, while continental lithosphere does not?
Oceanic lithosphere is denser than continental lithosphere. Continental lithosphere is too buoyant to allow
Briefly describe how mountain belts such as the Himalayas form.
As two continental plates collide, the crust buckles, thickens vertically, and fractures. This leads to the
uplift of major mountain ranges such as the Himalayas
Describe how two plates move in relation to each other along a transform plate boundary.
The plates slide laterally past each other.
Differentiate between transform faults and the other types of plate boundaries
They do not generate new lithosphere or destroy old lithosphere. The movement involves two plates sliding past each other rather than moving apart or coming together.
What two plates are growing in size
The African and Antarctic plates
Which plate is shrinking?
Pacific plate is shrinking.
What new ocean basin was created by the breakup of Pangaea?
Briefly describe some major changes to the globe when we extrapolate present-day plate movements 50 million years into the future.
North America's Baja Peninsula and parts of southern California will be closer to Alaska. Africa will
collide with Eurasia, closing the Mediterranean Sea. Australia will migrate towards the equator, North
and South America will separate, and the Atlantic Ocean basin will be larger.
What is the age of the oldest sediments recovered using deep-ocean drilling? How do the ages of
these sediments compare to the ages of the oldest continental rocks?
The oldest ocean sediments are 180 million years old, while the oldest continental rocks are more than 4
billion years old.
Assuming that hot spots remain fixed, in what direction was the Pacific plate moving while the
Hawaiian islands were forming? When Suiko Seamount was forming?
The Pacific plate was moving northwest when the Hawaiian islands were forming and moving north when
the Suiko Seamount was forming.
How do sediment cores from the ocean floor support the concept of seafloor spreading?
can be radiometrically dated, and their distance from a spreading center recorded. Data
have shown that older cores are further from the mid-ocean ridges while the youngest rocks and
sediments are near the ridges.
Describe how Fred Vine and D.H. Mathews related the seafloor-spreading hypothesis to magnetic
They found that new rock made from cooling magma magnetizes itself in the current direction of Earth's
magnetic polarity. They found lateral symmetry in magnetic patterns on either side of mid-ocean ridges,
showing that these separated stripes were formed at the same time at the mid-ocean ridges.
What do transform faults that connect spreading centers indicate about plate motion?
are aligned parallel to the direction of spreading. Measuring their alignments
carefully will reveal the direction of plate movement
which three plates appear to exhibit the highest rates of motion
The Pacific plate, the Australian-Indian plate, and the Nazca plate
Describe slab pull and ridge push
As old, dense lithosphere subducts, the slab is pulled deep into the mantle by gravity and destroyed. The
elevation of mid-ocean ridges allows new lithosphere to slide down; this is ridge push.
Which of these forces appears to contribute more to plate
Slab pull seems to
be the dominant force.
What role are mantle plumes thought to play in the convective flow in the mantle?
are thought to originate deep within the mantle, close to the core. The heat from the bottom of these plumes rises through the mantle, generating convective flow.
also called the plume model. Subducted lithosphere descends to the coremantle
boundary and this is balanced by buoyantly rising mantle plumes that move hot material to the
Layer cake model
has two zones of convection; one is a thin, dynamic upper mantle layer and one is
slower and thicker in the lower mantle. Slow convection in the lower level carries heat upward but the
upper layer is what generates surface volcanism.