Midterm Study
27. Composition and temperature have important effects on a lava's viscosity. Based on what you learned here, consider how less viscous magmas will erupt differently than more viscous magmas. Due to differences in viscosity, it is not surprising that lavas form different landforms depending on composition and temperature. - Basalts flow in sheets that form smooth, ropy surfaces known as pahoehoe flows. - As basalts cool, the surface can become rugged and broken up by the continued movement of the flow's interior. - The sharp, spiny lava flow that results is called an aa flow. Cooler andesitic lavas are more viscous than the coolest basalt that forms aa flows. A rapidly cooling lava results, leading to a rounded blocky surface called block lava flows, characterized by blocks with rough surfaces and smooth sides. - Pillow lavas form when lavas erupt underwater quickly, cooling the surface of lava flows into round blobs of glass. - Lava tubes form when an insulating outer shell forms and the lava inside can continue to flow as a liquid. - The surface texture of a lava flow when it finally solidifies reflects the timing of solidification relative to its movement. Eruption underwater freezes lava flow almost instantly, while eruption on the surface can lead to long lava flows
- Aa flow has a rough, broken surface. It is caused by rapidly moving basalt that cools quickly. - Pahoehoe flow has a smooth, billowy surface. It is caused by slowly moving basalt that is still hot. - Lava tube results from a relatively fluid lava cooling and forming an outer shell. The lava continues to flow inside the outer shell. - Block lava has a rough surface composed of smooth-sided fragments. It is formed by viscous lava cooling quickly. - Pillow lava is composed of solid, bubble-like masses. It is caused by extremely quick lava quenching. Correct Lavas can form a variety of structures depending on the initial viscosity and temperature. Aa flow forms a rough, broken surface. Pahoehoe flow forms a smooth, billowy surface. Lava tubes form an outer shell, underneath which lava continues to flow. Block lava forms a rough surface of smooth-sided fragments. Pillow lava forms a solid, bubble-like mass.
13. Based on your comparison of the two maps above, which three of the following statements are probably true?
- The 7.9-magnitude earthquake that occurred just northeast of the Himalayas on May 12, 2008, killing about 70,000 people, was probably caused by the northward movement of the Indo-Australian Plate in relation to the southward movement of the Eurasian Plate. - Earthquakes are more likely to occur along the edges of tectonic plates than elsewhere. - Volcanoes are more likely to be located along the edges of tectonic plates than elsewhere. Correct Earthquakes, identified with a yellow dot, are more likely to occur along the edges of tectonic plates than elsewhere. This can be seen by noting the many yellow dots near the edges of tectonic plates. Volcanoes, identified with a red triangle, are more likely to be located along the edges of tectonic plates than elsewhere. This can be seen by noting the many red triangles near the edges of tectonic plates. The Indo-Australian Plate is one of the faster-moving plates, and lies south of the Sichuan region of China. Its northward movement in relation to the Eurasian Plate is credited with building the Himalaya Mountains, as well as with causing the 2008 earthquake that devastated the Sichuan region of China. These exercises are intended to provide you with an understanding of plate tectonics, from their origin and the dynamics that cause them to the geologic hazards that result. Underneath Earth's surface, heat at the core causes convection cells to form, which in turn help provide the energy that moves tectonic plates across Earth's surface. As these tectonic plates move, they cause volcanoes and earthquakes, which can cause massive loss of life and property. The majority of geologic hazards (volcanoes and earthquakes) occur along the edges of the tectonic plates.
37. Pyroclastic material is ejected from a volcano during an explosive eruption. These fragments are defined by their size, which ranges from less than 2 mm to greater than 64 mm. - Ash is less than 2 mm in diameter, with an appearance of dust or sand. - Lapilli, or cinders, range from 2 mm to 64 mm in diameter. These pyroclasts are commonly pea to walnut sized. - Blocks and bombs represent the largest fragments, which are greater than 64 mm. Blocks are erupted as hardened lava with a mostly angular shape, while bombs are semi-molten and develop a streamlined-shape while flying through the air. The photo below shows two distinct examples of pyroclasts. Notice the rock hammer for scale, which is 265 mm long. What are the specific names of the pyroclasts outlined by A and B, respectively?
A) Lapilli, B) Bomb Correct Label A outlines lapilli, which are particles ranging from 2 to 64 mm in diameter. Label B shows a volcanic bomb, which is greater than 64 mm. This fragment is characterized by its streamlined appearance, shown by the small ridges or lines along the surface. The size of the hammer provides assistance to answering this question. The hammer, which is 265 m in length, is clearly larger than the lapilli and smaller than the volcanic bomb.
30. A sedimentary rock made up of particles of other rocks or minerals, especially feldspar and quartz
Arkose
41. Look at the figure below showing a slice through Earth's interior again. There is one layer in Earth that will transmit only one type of wave based on its composition. Select the layer from A, B, and C that doesn't transmit all waves.
B Correct With this information on the transmission of P and S waves through Earth's interior, we can use earthquakes as a tool to record the density of materials S and P waves travel through. It gives us insight into Earth's interior and a way to monitor change.
17. ________ fine-grained,common volcanic rock, dark in color
Basalt
26. Molten material that erupts from the subsurface during volcanic activity (lava) cools and solidifies on Earth's surface, which creates volcanic rocks. Characteristics of the molten material before it reaches the Earth's surface (magma) have an important control upon several factors related to flow and eruption of lava, as well as the resultant rocks. - Styles of volcanic activity range from slow-moving lava flows to explosive eruptions that rain ash on the surrounding area. - The eruptive style depends on the magma's viscosity, or resistance to flow, which is controlled by composition, temperature, and volatile content. - The concentration of dissolved volatiles (gases, such as water and carbon dioxide) in lava also affects its mobility. When water dissolves in magmas, it breaks the silicon-oxygen bonds in silica chains and reduces viscosity. The composition of lava is the most important characteristic in determining its behavior. The major compositional groups are: - Basaltic lava has low silica contents (~50%) and high eruption temperatures, causing them to flow rapidly and have a low viscosity. Basalts flow far from volcanic vents and form broad, sheet-like lava flows. - Rhyolitic lava has high silica contents (~70%) and low eruption temperatures, giving them high viscosities. Rhyolitic lavas mound up and stay near the volcanic vent because of their resistance to flow. - Intermediate lavas, such as andesitic lava, have compositions, temperatures, and volatile contents between those of basalt and rhyolite. The mineral composition of these groups determines each lava's prevalence. Some minerals melt more readily, leading to larger volumes of lava. Basalts and rhyolites are thought to form early and late in the melting process, respectively. More than 90% of the total volume of lava on Earth is estimated to be basaltic in composition, while rhyolites make up as little as 1% of the total. Andesites and other lavas account for most of the rest. By considering the compositional group and temperature of lava, we can evaluate the lava's viscosity and eruptive style.
Basaltic composition - 90% of lava on Earth - fast lava flow - flows in sheets Andesitic composition - intermediate lava flow Rhyolitic composition - slow lava flow - silica-rich lava - 1% of lava on Earth Correct The composition of lavas determine its and flow rate. Basaltic lavas are more fluid due to hotter eruption temperatures and lower silica contents. They produce fast-moving lava flows, flow in sheets, and make up 90% of the lava on Earth. Rhyolitic lavas are less fluid due to cooler temperatures and higher silica contents. The produce slow-moving lava flows and make up 1% of the lava on Earth. Andesitic lavas are a type of intermediate lava flow and have properties between those of basaltic and rhyolitic lavas.
3. ________ a dark-colored mica abundant in schist and granite.
Biotite
2. ________ a carbonate mineral which is a component of limestone.
Calcite
10. This part of the exercise changes the scale of your analysis of plate tectonics. The diagram above shows in detail the dynamics that power plate tectonics. The following map shows the location of tectonic plates across Earth and the relative direction of plate movements. Review the map below, and then answer the questions that follow. Puerto Rico is the site of many earthquakes, including some recent ones. Which tectonic plate listed below is nearest to Puerto Rico (marked with a star on the map)?
Caribbean Plate Correct The Caribbean Plate is nearest to Haiti. Haiti is located on the island of Hispaniola, which it shares with the Dominican Republic, and is located in the Caribbean.
19. ___________ sedimentary rock made of pebbles or other larger sediment and held together with finer material
Conglomerate
9. Which structure represented in the above diagram moves the tectonic plates?
Convection cells
14. At convergent plate boundaries, two plates are moving toward each other. Depending on the relative density of the rock that makes up each plate, one plate (the more dense one) will slide beneath the other at a subduction zone. - Because the rocks of oceanic crust are more dense than those of continental crust, it is the plate with oceanic crust that is subducted below the continent. Volcanic activity is common at oceanic-continental subduction zones. - If the two plates moving toward each other both have oceanic crust, then the denser of the two (typically the older of the two) will be subducted. Volcanic activity is common at oceanic-oceanic subduction zones. - If both plates have light continental crust, they will collide, moving rock upward into mountain chains, and neither plate is subducted so there is no volcanism. Refer to the images in the three scenarios below that show different types of convergent plate boundaries and label items accordingly.
Correct
23. Earthquakes are vibrations in the ground caused by a sudden release of energy. For example, an explosion can cause the atoms in the ground to move in a particular direction. Since the atoms are held in place within minerals by chemical bonds, they will elastically rebound in the opposite direction and then vibrate back and forth around their crystal lattice position. The systematic vibration of countless numbers of atoms is responsible for the large-scale vibrations people call earthquakes. Explosions are actually a very rare cause of earthquakes. Most earthquakes are caused by the release of elastic energy in rocks sitting along fault planes. Most earthquakes are caused by the release of elastic energy along fault planes. The following diagram shows how elastic energy is stored and released in a wood stick: Bending the stick stretches the chemical bonds holding the stick's atoms together, which provides a source of elastic energy. The elastic energy is suddenly released when the stick breaks and the two ends straighten. The rocks near a fault plane behave similarly. Compressional, shear, and tensional stresses cause bodies of rock to move along fault planes. However, friction between the rocks prevents smooth motion along the fault plane. Instead, the rocks near the fault plane bend and store elastic energy. The elastic energy is released when the rock bodies suddenly move along the fault and the bending rocks straighten, which causes an earthquake. Arrange the diagrams below in chronological order.
Correct
7. In this exercise, you will explore the dynamics that power plate tectonics. You will also explore the geographic distribution of tectonic plates—the pieces of lithosphere that cover Earth's surface—and the relationship between tectonic plates and geologic hazards, such as the location of earthquakes and volcanic eruptions around the Pacific Ocean known as the Ring of Fire. The theory of plate tectonics describes the large-scale motion of Earth's lithosphere—the crust and upper mantle—which slides upon the asthenosphere, a fluid layer within the mantle. Below the asthenosphere, hot mantle rocks rise from the core and then drop back downward after they cool slightly. This motion is called convection. Each area of rotation within the mantle is called a convection cell. Identify the following features on the diagram:
Correct
40. There are two types of body waves: P waves and S waves. They both travel through Earth's interior in different ways. - P and S waves are refracted as they travel deep through Earth, their paths bending due to increased pressure. - P and S waves are able to reflect off the boundaries between layers as well. - P and S waves can both move through solid rock layers. - One notable and important difference is that P waves are able to move through liquid rock layers, whereas S waves are not transmitted through liquid rock at all. Look at the figure below that shows a slice through Earth's interior and note the star at the top indicating an earthquake epicenter. From the star, various paths are shown to represent seismic waves stemming from that epicenter. The paths of waves curve (refract) in the solid mantle. Here, pressure increases with depth and correlates with seismic velocity of rocks increasing with depth. Some waves will reflect off the liquid outer core, whereas others can pass through the outer core. Determine which types of waves would be able to pass through the various solid and liquid layers of Earth. Label each path with either P, S, or P and S (if either type of wave is possible) to indicate which type(s) of body waves could be represented by that path.
Correct It makes sense that the composition of Earth's layers would affect the path of seismic waves just as other types of waves, such as sound waves, are affected by the density of materials.
38. Pahoehoe is a type of lava exhibiting a ropy smooth texture on the surface. It is created from the differences in the cooling rate between the surface and the interior of the lava flow. As the hot fluid interior of the flow moves, the slightly cooled exterior bunches up and pahoehoe is formed. Look at the pahoehoe below. In which direction was the lava moving? Choose the correct arrow to indicate flow direction.
Correct The curved ridges, or ropes of the flow are clearly observable. They bend toward the bottom right of the photo, which indicates flow in that direction.
36. Cinder cones and lava flows are built by explosive and effusive eruptions, respectively. Due to their unique evolution, these features are composed of distinct rock types. The diagram below shows a volcanic landscape with features outlined by A, B, C, and D. These labels correspond to photos 1, 2, 3, and 4 below. Match the rock type or volcanic landform shown in the photos with the appropriate label on the diagram.
Correct The diagram shows a cinder cone, which corresponds to Photo 4. The conduit and flanks are highlighted and defined by specific rock types. Once volcanism ceased, magma in the conduit crystallized to form dense columnar basalt. Shown by Photo 1, the columnar shape formed when the magma cooled and contracted in the conduit. The flanks of a cinder cone are composed of loose pyroclasts, which are the lapilli shown in Photo 2. The lava flow is displayed in Photo 3 and is characterized by large blocks of broken lava with ropy pahoehoe on the surface.
28. Violent volcanic eruptions occur because gases cannot escape easily from viscous magmas, causing pressure to build to the point where gases burst the magma to escape. - Pyroclastic rocks come in a range of sizes, the largest bombs (smoother particles erupted as liquids) and blocks (rougher particles erupted as solids) are greater than 64 millimeters (2.5 inches) in size. - Smaller pyroclastic particles range from walnut-sized lapilli (particles with sizes of 2-64 millimeters) down to ash, which looks like a fine dust or powder. Some pyroclastic rocks retain evidence of the presence of gases in its foamy, sponge-like texture. - Vesicular rocks are characterized by their color, which is derived from composition. Darker vesicular rocks are known as scoria, while lighter vesicular rocks are pumice.
Correct The different sizes of pyroclastic material provide insight into the eruptive style of the volcano that ejected the material from the subsurface. Many factors affect the eruptive style of a volcano, such as a lava's composition, temperature, and volatile content.
42. Geologists determine the geologic history of an area by observing rock units exposed in outcrops and analyzing their spatial relationships. Today, you will be the geologist by visiting a field site virtually. You will explore the site by zooming in and out on a Gigapan image of an outcrop to observe rock samples and rock layers, much like you would in the field. In this exercise, you will use Gigapan technology to: - identify rock samples, and - sketch spatial relationships of rock units in geologic outcrops. To begin our analysis of these rocks, we need to consider some fundamental aspects of geology. * The Law of Superposition: a surface-deposited rock bed on the bottom must be older than the rock bed on the top. * The Principle of Original Horizontality: beds of sedimentary rock were originally deposited as flat-lying, horizontal layers. * The Principle of Lateral Continuity: layers of sedimentary rock, when formed, extended horizontally in all directions. Gigapan technology mosaics thousands of photos together into a single image, allowing you to zoom in and see the tiniest of details. Imagine zooming in on a grain of sand on a photo of a beach! Choose the cross-section sketch that best represents the rock units you observe at this scale
Correct You have come to see that there are two main rock units stacked on one another. Next you will interpret the type of environment where this rock formed. Understanding the rock unit features in an outcrop can be important in order to develop some interpretation for how it formed. Next you will interpret the type of environment where this rock formed.
8. Where is the heat that activates the convection cells generated?
Earth's core
5. __________ a group of mineral compounds that is the most abundant constituent of the earth's crust.
Feldspar
34. Foliated rock which may result from the metamorphism of granite
Gneiss
16. ________ coarse-grained rock, which cooled slowly
Granite
32. Coarse-grained rock containing quartz, feldspar, and mica
Granite
4. ________ a common mineral of the amphibole group.
Hornblende
11. The Himalayan Mountains (marked with a circle on the map) were created by the northward movement of which tectonic plate?
Indo-Australian Plate Correct The Himalayan Mountains were created by the movement of the Indo-Australian Plate. It is one of the faster-moving plates and is moving nearly opposite to the Eurasian Plate. Because of this, the Himalayan Mountains continue to grow.
35. In the video, you learned about the evolution of cinder cones and basaltic lava flows. These features are formed by volcanic eruptions when magma is erupted at the surface of the Earth. The photo in Part A shows Red Hill, located in central New Mexico. This site shows a distinct red cinder cone and adjacent lava flow. The basaltic lava flow has been broken up by processes of weathering and erosion.
Lava flow - formed by effusive eruptions - pahoehoe is common Cinder cone - formed by explosive eruptions - pyroclastic material is more common Both features - basaltic material is more common - composed of extrusive igneous rocks
33. Rock of variable color and texture consisting mainly of calcite
Limestone
29. Representative of recrystallized limestone
Marble
6. ___________ light-colored mica which is a common component of igneous and metamorphic rocks.
Muscovite
15. _______ glassy textured volcanic rock which cooled rapidly
Obsidian
25. Earthquakes occur in many places in the U.S. Most occur due to natural causes, but some are induced by human activity. Identify which statements are relevant to earthquakes in each of the states.
Oklahoma - wastewater injection - earthquakes induced by human activity - not near a plate boundary California - earthquakes due to plate tectonics - transform fault boundary
46. Geologists collect observations from field sites and then summarize their interpretations. One of the ways they do this is to consider the order of events chronologically. They take information from their observations and then apply the fundamental aspects of Geology to tell the story. If you can, first imagine the sequence of events that must have taken place to form this outcrop even before you look at the question. Always ask yourself, what must happen before something else can happen? For example, sediments need to be weathered, eroded, and transported from a source area before they can be deposited somewhere else. Recall, from the previous section, that you already know that the lower unit formed before the upper one. It's your turn to take everything you learned while exploring the rocks along the coast of Lake Superior in Minnesota at this site and synthesize it into a coherent story. Arrange the following geologic events in the order that they occurred.
Order is from TOP (OLDEST) to BOTTOM (YOUNGEST) Oldest - Deposition of sediments on a floodplain - Compaction and cementation of sediments (lithification) - A nearby volcano erupts, creating a lava flow - Lava flow erodes sedimentary layers and then cools on top of them Youngest Correct The geologic history of this site is sediments are deposited and lithified, then a nearby eruption causes lava to flow into the area, erode and cool on top of the sediments. Next steps: If this were your research field site, what research questions would you ask next? What data would you collect to further support your conclusions? What data could you collect to determine the age(s) of the rock layers?
39. Cabezon Peak, shown in the photo below, is a classic example of a volcanic neck. This particular eye-catching landform was once a cinder cone. The only remnant left is the harder, more resistant rock of the conduit. Think about the processes (like volcanism and erosion) responsible for the transition of a cinder cone into a volcanic neck. For this question, focus on the series of geologic events that led to the current expression that we see today.
Order is from Top (Oldest) to Bottom (Youngest) Oldest Event - Magma erupts explosively unto Earth surface. - Volcanism ceases and magma crystallizes into rock. - The flanks (sides) of the volcano and surrounding country rock start to erode away. - The conduit is exposed at Earth's surface to form a volcanic neck. Youngest Event Correct Recall from the video that cinder cones form by explosive eruptions, which eject pyroclastic material. The pyroclastics build up the flanks of the volcano through time. Once volcanism ceases, this material is no longer building the volcano and magma in the chamber and conduit begins to crystallize. The flanks and surrounding country rock erode easily relative to the dense basalt in the conduit. Eventually the conduit, or volcanic neck, is exposed at the surface as a prominent feature.
31. Rock named after the groundmass containing definite phenocrysts
Porphyry
1. ______ a common rock-forming mineral composed only of silicon and oxygen.
Quartz
21. _________ sandstone fused under great pressure
Quartzite
43. As a brief review on some of the most typical features of each major rock type: a. Igneous rocks are solidified magma that possess interlocking crystals of varying sizes (from visible to too small to see) that possess a range of textures. b. Sedimentary rocks tend to be layered deposits of transported material fragments (from boulder-sized to clay sizes) with various degrees of sphericity (rounding) and sorting. They may also include some structure such as cross-beds. c. Metamorphic rocks often have elongated or differential stress-changed mineral foliations (their preferred orientation). Metamorphic rocks also possess the full range of grain sizes from very fine to very coarse. After exploring the Gigapan image, arrange the following observations and inferences by their respective rock unit. These observations and inferences describe the material, appearance, and weathering pattern of the respective rock units.
Rock unit #1 - Crystals too small to see - Weathers into blocky pieces - Appears to be massive (few thick, layers) - Black and dark gray in color Rock unit #2 - Sediments too small to see - Weathers into fine-grained sediment at the base of the outcrop - Appears to have many thin layers - Red and white in color
44. Geologists consider rock type, mineral composition, and type of environment in which rock units are formed to develop an understanding of what happened to form each outcrop. Some key ideas to keep in mind: - Sedimentary rocks are often well layered, and those formed on land are often oxidized (e.g., red in color) due to weathering in an oxygen-rich environment. - Igneous rocks crystalize from magma at fast rates on the surface and slower rates in the subsurface producing tiny and large mineral crystals, respectively. The compositional spectrum of igneous rocks is fundamentally determined by the silica content. Basalts and gabbros are low silica and dark in color, compared to rhyolite and granite that are high in silica content and lighter in color. Look at these types of characteristics listed below, and then assign them to their respective rock unit. Some characteristics will fall within one rock unit or the other. All characteristics will fall within one of the two rock unit categories.
Rock unit #1 - Formed by lava cooling and crystallizing on the surface - Basalt - Rock with a low-silica content - Igneous rock Rock unit #2 - Mudstone - Sedimentary rock - Deposited as sediment in a terrestrial (land) environment Correct Rock unit #1 is a dark, low-silica content igneous basalt that cooled rapidly on the surface. Rock unit #2 is red, thinly layered, sedimentary mudstone deposited on land. Did you notice this feature when you were exploring? The red color in the rock layer on the left indicates that those rocks were once soils. Soils preserved in the rock record like this are called paleosols. Finding them indicates that these rocks were formed on land. The squiggly vertical lines are where plant roots once were. They appear white in color because of the different chemistry between the original soil and the microbes on the plant roots. If you look closely at the contact between the rock units, you can see the rock above appears to flow over and tear up part of the red rocks below.
45. Now that you've identified the rock types present (basalt and mudstone) and started to interpret how they formed in the past (cooled lava flow and old soils and sediments), let's reflect on the timing of the formation of each rock unit. Based on what you know about the law of superposition (discussed at the beginning of this exercise), which rock unit is older and formed first?
Rock unit #2 Correct The lower rock unit formed first as explained by the law of superposition: the mudstone was deposited first, and then the basalt was deposited on it more recently. Now, let's bring your findings together to briefly explain the geologic history of this area.
20. ________ fine-grained rock often deposited as mud
Shale
22. ________ formed when magnesium-rich rocks are altered
Talc
12. Take a look at this map, and then answer the questions below. The Pacific Ocean "Ring of Fire" refers to the widespread, often destructive volcanoes common on the coastlines bordering the Pacific Ocean. Which of the following locations is affected by the Ring of Fire?
The east coast of Eurasia Correct The east coast of Eurasia is on the western side of the Pacific, and is subject to violent volcanoes.
24. Which type of plate boundary is most likely associated with the image from Part A?
Transform fault boundary
18. ______________ lightweight volcanic rock with a vesicular texture
Volcanic Cinder
