Geol 104 - HW 2, Ch 5a

What caused the formation of the Himalayas?

the collision of India with Asia

What was Pangaea?

a supercontinent that began to break apart about 200 million years ago

Which tectonic boundary results when two tectonic plates pull apart from each other?

divergent boundaries

What are the three types of plate boundaries?

divergent, convergent, and transform

Divergent boundaries are not one, smooth ridge of separation. Instead, they are broken into a series of segments, with different segments of fault offset from others. What features allow for the offset of divergent boundaries?

transform faults

Divergent and convergent boundaries produce both volcanic and seismic activity within continents. However, the presence of certain geologic structures can distinguish a divergent boundary from a convergent boundary. Compare the diagrams below. The one on the left shows a divergent plate boundary, whereas the one on the right illustrates a convergent plate boundary. Notice that at the divergent boundary, the crust is experiencing tensional stress, which is causing a normal fault that results in seismic activity, and that the volcanic activity is caused by upwelling of mafic lavas from the mantle. On the other hand, convergent boundaries are the result of compression stresses leading to plate collisions, which create numerous thrust and reverse faults that are responsible for seismic activity. Volcanic activity at convergent boundaries is created by andesitic lavas produced by the partial melting of a subducting plate. You will determine which features and characteristics are associated with divergent boundaries and which are not.

would be found at a divergent boundary: Volcanoes producing mafic lavas Normal faults would NOT be found at a divergent boundary: Volcanoes producing andesitic lavas Thrust faults

The rocks that make up Iceland have a very specific age pattern related to the presence of the Mid-Atlantic Ridge. What is this age progression?

youngest rocks near the center of Iceland, oldest rocks near the East and West Shores

Where are tectonic plates located?

at Earth's surface

Which plate boundary is NOT associated with volcanic eruptions?

transform

Structures such as columnar joints are found near Gullfoss in the Reykjanes Peninsula. How were these structures created?

Cooling lava contracts, creating fractures.

Which supercontinent contained Africa and South America?

Gondwanaland

Iceland is unique along the Mid-Atlantic Ridge because it is the only part of the ridge that is elevated above sea level. Why might this be the case?

Iceland is receiving magma from decompression melting along the Mid-Atlantic Ridge. Iceland is receiving magma from a mantle plume.

Why are volcanic chains associated with convergent boundaries?

Partial melting occurs due to subduction.

What evidence supports that the glaciers on the southern continents were once part of a single, massive ice sheet?

Striations

Which type of plate boundary is most closely associated with uplifting continental regions and mountain building?

convergent boundaries

Which type of tectonic boundary is the result of two tectonic plates colliding with each other?

convergent boundary

What was Panthalassa?

the single ocean that existed after the Pangaean supercontinent formed

Identify each type of convergent plate boundary.

1. oceanic-continental plate convergence 2. oceanic-oceanic plate convergence 3. continental-continental plate convergence

Depending on the type of plate convergence, different features could result. Examples of these features include volcanic island arcs, continental volcanic arcs, and a collisional mountain chain.

1. volcanic island arcs 2. continental volcanic arcs 3. collisional mountain chain

The seafloor rocks (oceanic lithosphere) formed at centers of seafloor spreading are transported away from the mid-ocean ridges. As time passes, oceanic lithosphere moves slowly away from the spreading center at which it formed as plates continue to move and new lithosphere forms at the spreading center. In the map below, the age of oceanic lithosphere is denoted with colors. Red indicates the youngest seafloor, which is less than two million years old. Dark blue indicates the oldest seafloor, which is over 144 million years old. Sedimentary rocks of the continental shelves are colored in light blue and are not considered part of the oceanic lithosphere.

C E A D B

In a collision between two continental plates, can a continental plate subduct? Why or why not?

Continental plates are too buoyant to subduct.

Which of the following statements best describes Wegener's idea of continental drift?

Continents were formerly in different positions on the Earth and have shifted to their present locations over time.

Which of the following statements about earthquakes with magnitude greater than 4.0 is most accurate?

Earthquakes with magnitude greater than 4.0 occur in a pattern than correlates closely with plate boundaries.

How was the fossil record of ancient life used to support Wegener's hypothesis about Pangaea?

Fossils of identical organisms were found on several continents, indicating the continents were once joined.

What were the names of the two smaller super continents that formed when Pangaea broke apart?

Laurasia and Gondwanaland

Which of the following scenarios best describes the activity present along the Mid-Atlantic Ridge?

Magma wells up at the center of the ridge, which pushes the old seafloor apart as new seafloor is created.

Which of the following statements about tectonic plates is true?

Most tectonic plates contain both oceanic and continental lithosphere.

Plate A has oceanic lithosphere on its leading edge that is 10 million years old. Plate B has oceanic lithosphere on its leading edge that is 200 million years old. Which plate will be subducted if they collide with each other, and why?

Plate B will subduct because it is older—and therefore colder—and more dense.

Although all areas of seafloor spreading have tectonic plates diverging and molten magma upwelling from below, not all areas of seafloor spreading are the same. The rate of spreading can affect the shape of the seafloor and the location of molten rock. Spreading centers with high spreading rates, such as those where plates are diverging rapidly, are characterized by a long, gently sloping ridge. The ridge forms as a well-developed magma chamber beneath causes the rock to swell upward. In contrast, spreading centers with slow spreading rates have poorly developed magma chambers and a well-defined rift valley developed at the center of the ridge.

Slow spreading rate Top middle- rift valley Top Right- poorly- developed magma chamber Rapid Speading rate Top left- Swell Top Right- Well- Developed magma chamber Bottom middle- Diverging plates

When matching up once-joined continents such as South America and Africa, the coastlines do not fit together as nicely as the continental shelves. Why might this be the case?

The continental shelves are the edges of the continent. The shoreline simply shows where the sea level is in relation to topography.

At a subduction zone where oceanic lithosphere meets continental lithosphere, which would subduct and why?

The oceanic lithosphere would subduct because it has a higher density at 3.0 grams per cubic centimeter.

The Mid-Atlantic Ridge crosses through Iceland. Physical features such as tensional fractures and cinder cone volcanoes are all aligned NE-SW. What does this indicate about the directions the tectonic plates are moving?

The tectonic plates are separating to the NW and SE.

Divergent boundaries can create continental rift zones. The continent will break apart as a consequence of this rifting, with each new landmass being separated by a newly formed seafloor: As shown in the figure below, continental rifting is caused by the tensional forces generated by diverging plates. The tensional forces cause continental crust to stretch and thin out, which, in turn, creates upwarping of the crust. Stretching and upwarping of the crust generate numerous normal faults and seismic activity within the rift zone. Thinning of the crust allows magma to upwell, which generates significant volcanic activity within the rift zone. With time, the tensional forces cause the continental crust to spread out laterally from the rift, and the upwelling magma will generate numerous basaltic lavas that harden to form a new seafloor, which will be filled with ocean water. Eventually, the continental landmasses will be separated from each other by extensive oceanic crust. Label the diagram below, which shows the last stage of continental rifting.

Top left - oceanic crust Top right - mid-ocean ridge Far right - continental crust Bottom right - upwelling Bottom left - rift valley

What is the relationship between continent edges and plate boundaries?

Very few plate boundaries follow the edges of continents.

Some of Iceland's volcanoes are covered by large glaciers. What are some of the hazards that may occur if one of those glacier-covered volcanoes erupted?

Violent eruptions send ash and cinders high into the air. Meltwater can result in massive jökulhlaups.

Late Paleozoic sedimentary rocks often contain extensive coal seams that were used to support the existence of Pangaea. What would the climate have been at that time and at the location where the coal deposits were formed, and what would it indicate about the continent's past latitude?

Warm, humid climate near the Equator

What was the Tethys Sea?

a piece of the Panthalassa Ocean that existed after the Pangaean supercontinent formed

Crustal rifting is currently occurring in East Africa. Which of the following features would you expect to find there?

all of the above are correct (normal faults; long, wide valley and lava flows)

Which phenomenon can explain the presence of volcanoes in the middle of the Pacific Ocean?

hot spots

Consider the events in Alfred Wegener's life that led to his development of the continental drift hypothesis. Review the items below and determine whether each item is (1) an observation that led Alfred Wegener to develop his continental drift hypothesis, (2) evidence Wegener gathered to support his proposal, or (3) evidence that is inconsistent with the hypothesis, which led to the rejection of this hypothesis by most of the scientific community.

observations: Wegner observed the breakup and movement of sea ice, how pieces of sea ice move apart and together, joining and breaking apart. supporting evidence: Mesoaurus fossils found in eastern south america and southwestern africa. Glossopteris fossils found in australia, africa, south america, antartica, and india. The fit of south america and africa suggest that these two continents once fit together. inconsistent evidence: Suggestion that gravitational forces of moon and sun are capable of gradually moving the continents. Suggestion that larger, sturdier continents broke through thinner oceanic crust much like a raft floats on water.