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Geology 101 Lab 1 (27)
Laboratory 1 - Plate Tectonics and the Origin of Magma By Joann Deakin
Objectives:
a. Analyze Earth forces, faults, and plate boundaries to determine if Earth's volume is increasing, decreasing or staying the same.
b. Measure and calculate rates of plate tectonic processes.
c. Use physical and graphical models of rock melting to infer how magma forms in relation to pressure, temperature, water and plate tectonics.
Introduction:
Ever since the creation of the first world maps, people have attempted to create models that explain the features and origin found on the Earth's surface. Some people believed that a series of catastrophic events were the reasons for the mountains, valleys and even ocean basins. We now know that the global relief we see is the result of tectonism, large scale movements of Earth's crust and lithosphere. In lab one, you learned the reason as to why the continents are vertically higher than the ocean basins. This is mainly due to isostasy, because the granitic crust of the continents has a lower density than the crust of the ocean basins. You also know from lecture 2 that many geologists proposed that the continents "drifted" due to forces from within the Earth. For a long time, many scientists could not accept the theory of continental drift because the mechanism to make the plates move was not understood. Even though Alfred Wegener has presented evidence for a previous supercontinent, Pangaea, such as the jigsaw like fit of the continents, fossil evidence and similar rock types the continental drift idea was
not readily accepted. The most convincing evidence finally came from the deep ocean mapping projects which revealed paleo magnetic data that could not be ignored. This gave rise to the idea of seafloor spreading as proposed by Harry Hess
and the plate tectonic model was born.
Plate Boundaries
In this plate tectonic model, we know the lithosphere is composed of plates, much like the cracked shell of a soft-boiled egg. Plates are formed and moved away from divergent boundaries, called ocean ridges. Plates are subducted or destroyed along convergent boundaries where one plate may be taken back into the Earth's mantle or both plates may crumple together and form mountains. Plates can also slide past each other to form transform fault boundaries where they are neither created nor destroyed.
Below is an illustration of the tectonism experienced by the North West United States and Canada.
From: http://web.viu.ca/geoscape/images/1700_quake.jpg
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Geology 101 Lab 1 (27)
From: http://www.chm.bris.ac.uk/webprojects1997/TomS/fig13.gif
The figure below is a map of the Earth showing the plate boundaries. 2
Geology 101 Lab 1 (27)
From: http://www.indiana.edu/~geol116/week7/plates.jpg
Questions:
1. Look carefully, which plate is almost nearly fully surrounded by a spreading or divergent plate boundary ____________Antarctic boundary
_______________________________________________________
2. Which plate is nearly fully surrounded by a convergent boundary? ____________Caribbean Plate
_________________________________________________
3. Which plate does not contain significant areas of continental land masses? ________________cocos plate
__________________________________________________
4. Name the plates that are bounded in part by the Mid Atlantic Ridge.
___south american and african plate
________________________________________________________________
5. Name the plates bounded in part by the East Pacific Rise. _____nazca and cocos plate
______________________________________________________________
6. What Island mass lies on the axis of the Mid Atlantic Ridge? ___lceland
________________________________________________________________
The picture below shows the earthquake epicenters, for earthquakes greater than magnitude 4.0 plotted as dots in the ten-year period between the years 1990 and 2000.
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Geology 101 Lab 1 (27)
From: http://comp.uark.edu/~sboss/tectonic02.jpg
7. Find the Nazca Plate on the figure above and draw in its approximate boundary using the earthquake epicenters as a guide.
8. The epicenter of an earthquake is the approximate location on the surface of the earth where the earthquake happens below the surface. Propose a hypothesis as to why the epicenters do not fall along a single line. (Hint, think about the type of boundary where most earthquakes happen and think about the geometry of those plate boundaries). ____because the plate boundaries are where most earthquakes happen, they are not linear but complex and irregular as far as shape goes. I’m guessing that because the plate boundaries are not straightlines, the epicenters of the earthquakes wont be either.
________________________________________________________________________________
_____________________________________________________________________________________
________________________________
9. As you look at the map, along what type of boundaries do most earthquakes occur? ___convergent boundaries
_________________________________________________________
10. What topographic submarine feature exists along the eastern margin of the Nazca Plate that indicates it is a convergent plate boundary. ____peru-chile trench
_______________________________________________________________
Origin of Magma
Seismic studies indicate that Earth's outer core contains a significant portion of melted rock called magma. However, this magma is so deep inside Earth that it cannot erupt to the surface. Studies indicate that nearly all of Earth's mantle and crust are solid rock, not magma. Therefore except for some specific locations where active volcanoes occur, there is no reservoir or layer of magma beneath the surface. On a global scale the volume of magma that feeds active volcanoes is very small. Magma generally forms in three plate boundary settings (divergent, convergent, and hot spots). Its origin 4
Geology 101 Lab 1 (27)
(rock melting) is also influenced by underground temperature, pressure and the kind of minerals that comprise the underground rocks.
Temperature and Pressure
As you go down into the Earth the temperature rises. This is called the geothermal gradient. All miners know this because they experience the heat in deep mines. The temperature increases about 77
o
F for every kilometer in depth downward. Since rocks are composed of many minerals and each mineral has a different melting point, parts of rocks will
melt at certain depths.
As you go deeper in the Earth the pressure increases. If you have ever been in an airplane or at the bottom of a swimming pool you have experienced changes in atmospheric pressure and pressure from the amount of water above, you. Rocks are about 3 times denser than water, so it does not take much rock (3.3 meters) to exert a pressure equal to 10 meters of water. The confining pressure under the Earth is so great that a mineral crystal cannot melt at its normal melting point observed at the surface. The pressure prevents the rock from melting. It is a combination of these three things; temperature, depth and pressure that determine whether a rock will melt or not. The diagram below is a called a PT (pressure versus temperature) diagram and shows how the conditions affect certain types of rocks.
From: http://tasaclips.com/illustrations/Geothermal_gradient.jpg
Magma is a complex mixture of liquid, solid, and gas. The main elements in magma are oxygen (O), silicon (Si), aluminum (Al), calcium (Ca), sodium (Na), potassium (K), iron (Fe), and magnesium (Mg). However, it is two major molecules found in magma that controls the properties of the magma. These two molecules are silica (SiO
2
) and water (H
2
O). Silica can comprise as much as 75 percent of the magma. When rock melts deep underground, the magma rises through the earth's crust because the molten rock is less dense than solid rock. In many cases, the magma is unable to reach the surface, and it will cool in place many miles under the ground. This underground solidified magma is called intrusive and often in the form of large plutons. This underground cooling produces the largest crystal sizes because it cools more slowly. Sometimes the magma extrudes onto the surface, either on land or underwater. Magma which comes to the surface is called lava and is classified as extrusive.
Much of the magma that rises from the mantle comes from subduction zones, where oceanic crust dives beneath continental crust. As the ocean crust goes beneath the continent, ocean water goes down with it. Magma also rises to the surface under what is known as "hot spots." These are areas of volcanic activity that are not related to subduction zones. Current well-known hotspots include the Hawaiian Island chain and Yellowstone National Park. Hot spots can be long lived and persist in one location for tens of millions of years. Geologists believe that they are either mantle plumes
of hot rock rising rapidly from the mantle and forming magma by decompression melting like a stream of lava rising in a lava lamp or the slow decompression melting of a large mass of hot mantle rock in the upper mantle that persists for a long interval of geologic time. Lava can also be formed at divergent zones where it flows out forming new ocean crust and is often in the form of Pillow Lava, lava that cools quickly and looks like big fluffy pillows. Along divergent zones the crust is thin, there is less pressure and magma can melt due to the reduced pressure.
Types of Magma
There are two basic types of magma which form in distinct tectonic environments. Basaltic magma (also known as "mafic") is created from the partial melting of the mantle, and it extrudes along rift valleys associated with ocean spreading centers. Popular belief says that magma originates deep within the earth's core, but this is not the case. The 5
Geology 101 Lab 1 (27)
magma originates from the partial melting of material at a depth of no greater than 125 miles (200 kilometers). This type
of melting is called decompression melting.
The other type of magma is granitic magma, which is commonly referred to as felsic magmas. They are generated by subduction zones by the partial melting of oceanic crust as it dives underneath continental crust. Not all magma is a complete liquid. As the temperature rises, the minerals which comprise a body of rock each have their own melting point. Thus, the body of magma moving toward the surface could contain molten rock and crystallized minerals. This principle is known as partial melting.
Another key term to understand is magmatic differentiation. When a magma cools, some of the crystallized minerals may
be left behind as the rest of the magma continues on its journey to the surface. In other words, the magma sorts itself out
according to the melting point of its constituent minerals. 11. Examine the cross section of a plate boundary in the figure below.
a. What kind of boundary is this? ___divergent plate boundary
______________________
b. Name the specific process that led to the formation of magma in this cross section. ____pressure that causes the magma to rise to the mantle and crystalize
_____________________________________________________________
c. What type of magma will form and rise out of these volcanoes? _____basaltic magma
_____________
From Tarbuck and Lutgens Earth, 10
th
Edition
12. Examine the cross section of a plate boundary below. a. What kind of boundary is this? __convergent plate boundary
_______________________
b. Name the specific process that led to the formation of magma in this cross section. __continental to ocean plate down under& eventually they began to melt and rise__________________________________________________________________________________
__________________________________________________
c. What type of magma will form and rise out of these volcanoes? ____granitic magma
_______________________________________________________________
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Geology 101 Lab 1 (27)
From Tarbuck and Lutgens Earth, 10th Edition
Hot Spots and Plate Movement: Hawaii, Yellowstone, and the Juan de Fuca Ridge
The Hawaiian Islands as well as the Emperor Seamount chain are the result of the pacific plate moving across a stationary hotspot. As the lithospheric plate moves across the hotspot a volcano develops directly above it. Refer to the following page and answer the following questions.
The figure below shows the distribution of the Hawaiian Island and Emperor Seamount chain. The insert in the figure the numbers indicate the age of each island in millions of years (Ma.), obtained from the basaltic igneous rock of which each island is composed.
From: http://quakeinfo.ucsd.edu/~gabi/erth15-06/lecture08/hawaii-diag.jpeg
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Geology 101 Lab 1 (27)
13 a. Use the km scale on the figure to determine how many kilometers apart the volcanic peak of Kauai (5.1) and the volcanic peak of Molokai (1.9) are on the map. ___270 cm
__________
b. How many millions of years did it take for the plate to move from Kauai to Molokai? ____2million years ______________________________________________________________
c. What is the rate of movement in centimeters per year and the direction of plate movement. (Show Work – distance in centimenters divided by #of years in millions)
__8.4375 cm ÷millions of years?
________________________________________________________________
d. What was the rate of movement in centimeters per year from 1.8 Ma. ago to the present time (Loihi)? (Show Work!) ____16.388cm/year
_______________________________________________________________
e. How does the rate and direction of the Pacific plate movement during the last 1.6 m. y. differ from the older rate and direction (4.7 - 1.6 Ma.) of plate motion? __now it is moving to the northwest between 7& 11 cm per year and beforeit was moving north.
_________________________________________________________________
___________________________________________________________________
f. Locate the Hawaiian Island and Emperor Seamount Chains of islands in the top part of figure A. How are these chains related? ___by a feature moving over a hotspot
________________________________________________________________
___________________________________________________________________
From: https://www.google.com/search?q=yellowstone+hotspot+track+map&source
Yellow stone National Park is believed to be a hot spot which is underneath the North American Crust. Because this mantle plume is under continental crust, as magma rises some of the “roots” of the continents are incorporated into the magma. This changes the chemical composition of the magma and leads to magmas that are very explosive. These types of
volcanoes erupt violently spewing life threatening ash, pumice and Rhyolite. showing the distribution of circular areas that were centers of crustal faulting and buckling when they were located over the Yellowstone hotspot. The numbers 8
Geology 101 Lab 1 (27)
indicate the ages of deformation, as determined by geologist Mark Anders. Notice how the plume and volcanic sites become progressively younger to the North East.
14 a. Think! It the Yellowstone hot spot is “moving” North East, what direction is the North American plate moving, according to Anders data? Explain your reasoning. __southwest because the oldest deformation is above the newest hotspot that has moved southwest.
_____________________________________________________________________________
_______________________________________________________
b. What was the average rate in centimeters per year that the North American plate has moved over the past 11 million years?( Show Work!) __moving to the southwest at 1inch per year
_________________________________________________________________
___________________________________________________________________
Divergence and Convergence: Age of Sea Floor
The Juan de Fuca plate is being subducted off the coast of
North America. To the west of it is a divergent zone at which new ocean crust is being created. The ages of the seafloor volcanic rocks in this area are given by the scale. The modern seafloor rocks of this region are forming along a divergent boundary called the Juan de Fuca ridge. The ridge is red in the picture below. The farther one moves away from the plate boundary, the older the seafloor rocks. The oldest rocks are dark in color greens to violets.
From: http://quake06.stanford.edu/centennial/tour/stop11.html
a. Notice that seafloor rocks older than 10 million years are present west of the Juan de Fuca Ridge but not east of the ridge. What could have happened to these rocks along the eastern side of the ridge that explains their absence? ____rocks are moving along the line segment and it is moving further awayfrom the continental crust
_____________________________________
________________________
b. What kind of plate boundary is represented by the line running through location of the heavy black line? __transform plate boundary
____________________________________________________
c. If the North American Continent continues to move to the Southwest, what will eventually happen to the Juan de Fuca Ridge? ____Juan Fuca Ridge will eventually divide itself
_____________________________________________________________
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