Rock On #2
The above diagram is from one of the Geomations in the unit. It shows three possible fault styles. A and B are cross-sections, with a collapsed building on top to show you which way is up—the yellow band is a distinctive layer of rock that was broken by the earthquake that also knocked down the building. C is viewed from a helicopter, looking down on a road with a dashed yellow line down the middle; the road was broken by an earthquake along the green fault, and the earthquake knocked down a building to make the funky-looking brown pile in the upper right.What is accurate about the different earthquake styles?
B is pull-apart, C is slide-past, and A is push-together. Feedback: Imagine putting the image on paper, cutting out the blocks (one block on each side of the fault), and then sliding them back together to make the original, unbroken features. A and B stand up from the table, C lies down on the table. Now, slide them to make the picture as seen here. In A, you'll be moving the right-hand block up and toward the other block, so it is push-together. In B, you'll be moving the right-hand block down and away from the other block, so it is pull-apart. And in C, you'll slide one past the other (geologists distinguish right-lateral and left-lateral motion for C, but you don't have to worry about that much detail).
Heat is moved around by convection, conduction and radiation (and by lemmings carrying space heaters, if lemmings ever carry space heaters). Which statement is more nearly correct?
Convection moves heat efficiently through the soft, hot rocks of the Earth's mantle, but is not efficient at moving heat through the space between the Sun and the Earth. Feedback: Heat from deep in the Earth is moved up through the soft bulk of the planet primarily by convection, but convection of rocks certainly does not continue beyond the planet, where radiation becomes dominant. In the shallowest, uppermost layers of the Earth, most of the heat transfer is by conduction. And the poor lemmings deserve a rest and a snack.
You hear an astronomer on the evening news, pointing out a coming alignment of planets and predicting that the extra gravitational attraction is sure to trigger a huge earthquake in California during the few hours of alignment. Based on what has been covered in this class, a reasonable approach is to:
Ignore it; although gravitational forces such as tides and planetary pulls might possibly exert a very small effect on earthquakes, no one has successfully predicted the where-and-when of earthquakes. Feedback: By keeping track of where earthquakes happen, combing written and oral histories of past earthquakes, looking at geological deposits to see where shaking has occurred and broken rocks or tree roots or caused sand boils, and measuring where rocks are moving and where they aren't, good estimates can be made of earthquake hazards; but, we can't figure out exactly when the next quake will hit. Planetary-alignment predictions have been made, and have failed miserably. The tiny effect of gravity of the planets on the Earth has not been shown to affect earthquakes at all, although it remains possible that some very small influence exists.
Bigger earthquakes occur less frequently, but a bigger quake releases more energy and does more damage. An interesting question to ask about earthquakes (and about almost anything else!) is whether the increase in energy release and damage done is larger or smaller than the decrease in frequency as one looks at bigger earthquakes. Asked a different way, is most of the damage done by the many little earthquakes or by the few big earthquakes?
Most of the damage is done by the few, big earthquakes Feedback: An increase of 1 in magnitude increases ground shaking about 10-fold, increases energy release about 30-fold, and decreases frequency about 10-fold; the 30-fold increase in energy more than offsets the 10-fold decrease in frequency of occurrence. We wish earthquakes did no damage, but the millions of people who have been killed in earthquakes over the centuries would, if they could, testify to the damage done by earthquakes. And earthquakes are actually very poor at distinguishing the brands of soda dispensed by or advertised on machines.
The processes that made Death Valley have been operating for millions of years, and continue to operate today. For this question, ignore the sand and gravel moved by water and wind, and think about the big motions of the rocks beneath. If you had visited Death Valley 1 million years ago, you would have found the valley then to have been (choose the best answer):
Narrower and shallower than it is today Feedback: The pull-apart action that is spreading Death Valley and surroundings also involves uplift of mountains or downdrop of valleys, and Death Valley has dropped as its flanking mountains have moved apart. Thus, in the past the valley was narrower and shallower than it is now, and the motions have deepened and widened the valley.
The picture above shows a fault in a place where mountains come down near the coast. What likely happened to form the ramp (also called a scarp) behind the person?
Pull-apart forces pulled the rocks apart, making the break, and allowing one side to drop relative to the other. Feedback: The down-side dropped along the ramp compared to the up-side. (This is actually an interesting one; it formed in Alaska during the 1964 earthquake. That was a push-together quake, but it was so huge and moved so much rock in different directions that some of the rock ended up having pull-apart motions, such as this one.)
Geophysical evidence indicates that convection is occurring in the Earth's mantle. What is the most likely physical explanation for why convection can occur in the mantle?
Rocks deep in the Earth expand and so become lower in density and tend to rise as they are heated, and the deep rocks are warm enough to flow slowly even though they are mostly solid. Feedback: Convection seems so easy, but describing it in words is not. For "ordinary" convection, one needs something capable of flowing (gas, liquid, or soft solid), heat below and cold above with expansion reducing density on heating and contraction increasing density on cooling, and then a bit of time and a perturbation of some sort to get the motion started. If you had something that expanded on cooling and contracted on heating, and you had cooling below and warming above, you could also make convection work. The mantle is mostly solid, the outer core can't directly stir the mantle or cause convection, and the magnetic field doesn't do much to move rocks.
You drill through the muds at the bottom of the sea floor and sample the rocks beneath, and you then determine the ages of those rocks, using standard scientific techniques. As described in the course materials, you will find that:
Rocks farthest from spreading ridges are oldest, with ages decreasing as you move toward a ridge. Feedback: At sea-floor spreading ridges, hot magma rises up, cools and solidifies. These rocks then split and move apart as yet more magma rises, cools and solidifies. Over time, the rocks are moved great distances (tens or hundreds of miles) from the spreading ridges. The rocks close to the ridges were deposited recently (they are "young"), but the rocks far from the ridge were deposited long ago and then moved away slowly (they are "old").
When discussing earthquakes that happen in the upper part of the Earth's crust, geologists believe that most are caused by elastic rebound. This means:
Rocks on opposite sides of a break, or fault, move in opposite directions, get stuck against each other for a while, bend, then "snap back" when something breaks or gives along the fault. Feedback: Try sliding a boulder over the ground, and you'll find the boulder gets stuck for a while. Lean harder, the boulder jerks forward suddenly, and you just had a tiny earthquake. Implosion earthquakes probably exist, but the rocks don't bounce back to their original size, and such quakes only can happen deep. We have no information on Graham Spanier's choice in socks, but his choice is unlikely to shake much beyond the immediate vicinity of University Park.
What is accurate about seismic waves moving through the Earth?
S-waves (also called shear-waves) move through solids but not liquids Feedback: S-waves are a bit like waves on a rope—grab an end and move it sideways, which moves the neighboring part sideways... This works with solids, but not liquids, which cannot "grab" and move the neighboring part
Most of the material moved by volcanoes is from the few, big ones rather then from the many, little ones. Most of the material moved downhill in landslides is in the many, little ones rather than the few, big ones. In comparing the importance of the few, big earthquakes to the many, little earthquakes, are earthquakes more like volcanoes (the few big ones matter most) or like landslides (the many little ones matter most)?
The few, big earthquakes matter most (like volcanoes) Feedback: An increase of 1 in earthquake magnitude increases ground shaking about 10-fold, increases energy release about 30-fold, and decreases frequency about 10-fold; the 30-fold increase in energy more than offsets the 10-fold decrease in frequency of occurrence. We wish earthquakes did no damage, but the millions of people who have been killed in earthquakes over the centuries would, if they could, testify to the damage done by earthquakes. And historical records of earthquakes clearly preceded the Simpsons.
On the Richter scale of earthquake intensity:
The ground is shaken 10 times less by a magnitude-7 quake than by a magnitude-8 quake. Feedback: One problem in describing earthquakes is that the ground shaking in the smallest one you can feel is 1,000,000,000 times smaller than the ground shaking in the largest quakes. We usually dislike having a scale that requires us to talk about an event of, say, size 100,000,000; instead, if a magnitude-1 quake moves the ground 10 units (say, 10 nanometers at some specified distance from the quake), than we say that a magnitude-2 quake moves the ground 100 units, and a magnitude-3 quake moves the ground 1000 units, and so on. You'll notice that the magnitude is just the number of zeros after the 1; this is a logarithmic scale.
On the Richter scale of earthquake intensity:
The ground is shaken 10 times more by a magnitude-8 quake than by a magnitude-7 quake Feedback: One problem in describing earthquakes is that the ground shaking in the smallest one you can feel is 1,000,000,000 times smaller than the ground shaking in the largest quakes. We usually dislike having a scale that requires us to talk about an event of, say, size 100,000,000; instead, if a magnitude-1 quake moves the ground 10 units (say, 10 nanometers at some specified distance from the quake), than we say that a magnitude-2 quake moves the ground 100 units, and a magnitude-3 quake moves the ground 1000 units, and so on. You'll notice that the magnitude is just the number of zeros after the 1; this is a logarithmic scale.
In the picture above, Dave Janesko holds two rocks next to each other.The black one (to the upper left in the picture) is from a lava flow, and is much younger than the red one (to the lower right in the picture), which is a lake sediment. In nature, these rocks are found the way Dave is showing, with the younger black one next to the older red one rather than being on top of the older red one. As described by Dave Janesko in the online video, what happened here?
The lake sediments were deposited, then the lava flowed on top, and then a pull-apart Death-Valley-type fault formed, breaking the rocks and dropping the lava flow to be next to the lake sediments. Feedback: The spreading that opened Death Valley affected a lot of the west, all the way over to Bryce Canyon in Utah. The Sevier Fault, just west of Bryce, formed as pull-apart action broke the rocks, allowing younger rocks including the black lava flow to drop down next to older rocks including the red lake sediments. There really are cases where lava hardens in cracks, or where lava flows fill valleys, but a careful examination of the rocks here shows that the lake sediments have not been heated by nearby lava, so these lake sediments and the lava must have been placed together after the lava cooled. Folding does occur, but not here.
Which is accurate about the Earth?
The lithosphere is a layer containing both the uppermost part of the mantle and the crust, where breaking is more common than flowing. Feedback: The uppermost mantle is cold enough that it doesn't flow easily, so it rides along with the crust in lithospheric plates rafting on deeper, softer asthenospheric mantle. The deepest crust is a bit soft, because at a given temperature the high-silica crust is softer than the low silica mantle, but let's not get too complex here.
In the bottom of Death Valley, you will find layers of gravel deposited by rivers. Based on materials presented in class, what is a likely explanation for this occurrence of river gravels in the valley bottom?
The valley was dropped relative to the mountains by faulting, and rivers now are carrying gravels down from the mountains into the valley and depositing the gravels at the valley bottom.
People visit Death Valley for all sorts of reasons. Some people even go there to study volcanoes. What is accurate about those Death Valley volcanoes?
The volcanoes near the edges of Death Valley produce rocks that are similar in composition to the rocks made by volcanoes at undersea spreading ridges, because Death Valley is in many ways geologically linked to undersea spreading ridges. Feedback: Death Valley has spreading-ridge-type volcanoes, and if you go south from the Valley, you find the spreading ridge in the Gulf of California; Death Valley and the Gulf of California are geologically related. There have been recent eruptions in Death Valley (within the last centuries), but as of this writing, no volcanoes are currently erupting in Death Valley, nor have any erupted for over a century.
