EAS 205
Environmental Geology, 6th Edition, Chapter 3
Praise the LORD, I
tell myself; O LORD my God, how great you are!
You are robed with honor and with majesty; you are dressed in a robe of light.
You stretch out the starry curtain of the heavens …
You placed the
world on its foundation so it would never be moved.
You clothed the earth with floods of water, water that covered even the
mountains.
At the sound of your rebuke, the water fled; at the sound of your thunder, it
fled away.
Mountains rose and valleys sank to the levels you decreed.
Note … I’ve organized this material a bit differently from your text … I find her organization confusing! Read my notes first, then the text … it will all make sense!
The earth appears to be made up of several distinct layers. The three major layers are:
The uppermost mantle and crust are solid, stick together, and are very brittle. This area (50 - 70 km below the surface), on which we live, is called the “lithosphere” (the “rock layer”).
· the lithosphere is thickest under the continents (see Figure 3.4, 6th ed., p. 47)
· the lithosphere is thinnest under the ocean basins (see Figure 3.4, 6th ed., p. 47)
Between the lithosphere and the Earth’s interior is a soft layer where rock is close to the boiling point (like the tip of a red hot iron rod): the asthenosphere (70 - 250 km below the surface). Because of this the uppermost mantle and crust (the lithosphere) "float" on the asthenosphere. The asthenosphere appears to slowly move, thus causing the lithosphere (the uppermost mantle + the crust) – floating upon it – to move as well. This is believed to be mechanism behind the theory of plate tectonics (below).
NOTE: this is a theory (not a law)
The theory is very helpful to help us understand the present and think about future possibilities.
Do not get “worried” about the dates associated with this theory in some texts. The theory is important because – whichever time-scale you prefer – it helps describe current global phenomena and may help predict future possibilities (earthquakes, volcanoes, etc.).
The theory’s historical explanations are extrapolations (guesses) based on present observations. The are only guesses. The dates are not particularly important … the processes and implications of the theory for predicting potential natural disasters are very important!
The value of this theory is in its ability to help us understand and account for present day phenomena, and think through potential future events.
1. Background
As early as the 16th century, Abraham Ortelius noticed that the continental coastlines of Africa and South America appeared to "fit” together. He suggested there may have been a "super-continent" at one time. Francis Bacon (17th c.) and Benjamin Franklin (18th c.) noted the same similarities. In 1668, Blaise Pascal that this apparent fit was evidence of a global flood.
The theory of “continental drift” was ridiculed until the early 1900s when two scientists independently proposed the theory in a formal way and provided scientific observations to support it.
- F. B. Taylor (USA)
- Alfred Wegener (Germany)
In particular, they noted
·the apparent continental fit, (Figure 3.1, 6th ed., p. 44),
·the presence of identical, rare fossils in both Europe and North America, and in South America and Africa (Figure 3.13, 6th ed., p. 56).
·the similarity of geological structures on both sides of the Atlantic. (Figure 3.12, 6th ed., p.55).
They suggested the continents had moved over time, starting from a hypothetical super-continent (Figure 3.14, p.57). Their theory seemed to fit their observations of global geologic phenomena, but there were problems with the theory!. In particular, they could not explain HOW this was happening. The mechanism driving plate movement was missing. Other scientists found their observations intriguing, but with no description of the how things could have changed so dramatically, the theory was largely dismissed.
For more on the history and development of the theory of plate tectonics, visit here!
The U.S. Geological Survey also has a good site on the development of the theory.
2. Evidence
Evidence and a mechanism for HOW this may have happened was developed in the 1950s and 1960s as scientists began to study the floor of ocean basins in detail for the first time (see Figure 3.7, 6th ed., pp. 50-51).
Measurements over a period of several decades shows that the ocean floors are spreading along "mid-oceanic ridges" (about 2.5 cm/yr). For more detail, visit the US Geological Survey
- the ages of rocks are identical on opposite sides (Figure 3.10, 6th ed., p. 53)
- magnetic orientation reversals are identical on opposite sides (Figure 3.8, 3.9, 6th ed., p. 52)
- sediments are thickest away from the ridges, implying the sea floor is older further from the ridges, and youngest right along the ridge.
- there are many young mid-oceanic volcanoes along these ridges (this is a wonderful hyperlink with lots of info & graphics) (Figure 3.5, 6th ed., p. 48).
- the youngest rock on earth is found along these mid-ocean ridges
- evidence that continents have moved differently in relation to the north pole (polar-wander curve) Figure 3.11, 6th ed., p. 54.
The theory of plate tectonics suggests the oceans are getting bigger. They are spreading apart in the middle (See Figure 3.20, 6th ed., p. 63 for the apparent pattern of plate movements).
As they do so, volcanoes form along these mid-ocean ridges, often as arcs of volcanic islands.
This information has encouraged the idea that the lithosphere (the crust and uppermost mantle) is composed of huge plates which "float" on the asthenosphere. The lithosphere is solid. It parts were to move, it would make sense that it would crack into large pieces or plates. The asthenosphere, the layer of superheated, near-molten rock just below the lithosphere, could act like a plastic substance upon which plates could slowly move. The asthenosphere appears to have slow convectional movements – which may either drive plate movement or be driven by plate movement (Figure 3.21, 6th ed., p.63). In either case, the plates do appear to move!
As the plates move, the various pieces spread apart, slide beside each other, or collide along plate boundaries … this is where the action is!
At the continental margins, collisions are occurring:
§ the denser plate (normally the oceanic plate – basalt) is being "subducted" (pushed) under the lighter plate (normally the continental plate – granite), causing deep ocean trenches at the continental margins. Note that the deepest parts of the oceans are in fact along the edges of continents – off Japan and Peru.
§ along these "subduction zones" rock is forced downward, causing it to melt. The magma often wells back up to the surface through weaknesses in the crust, to form volcanoes.
§ also, the friction often causes earthquakes. Plates of rock do not slide under one another smoothly … they tend to go in jerky motion.
§ rocks may be bent or warped by the pressure
See Figure 3.22 (6th ed., p, 64) for a complete picture of this cycle …
1. Divergent Boundaries
Divergent plate boundaries occur where plates appear to be moving apart - for example along mid- oceanic ridges or along the Great Rift Valley in East Africa. These are characterized by many volcanoes and some earthquakes (Mt. Kilimanjaro).
See Figure 3.15 and 3.16, 6th ed., p. 58.
2. Transform Boundaries
Transform plate boundaries occur where plates appear to be sliding sideways past each other. These are typically characterized by earthquakes and sometimes volcanoes (San Andreas Fault).
3. Convergent Boundaries
Convergent plate boundaries occur where plates appear to be coming together. Typically these occur along oceanic/continental margins as the ocean crust is subducted under continental margins. These are characterized by many earthquakes and some volcanoes (Japan, Peru).
See Figure 3.17 and 3.18, 6th ed., p. 59.
** Note the close relationship between proposed plate boundaries and global patterns of volcanic and earthquake activity - they match almost perfectly. (Figure 3.5 and 3.6, 6th ed., p. 48) Volcanoes and earthquakes virtually ONLY occur where predicted by this theory. This is strong evidence in support of the theory. And it is very influential in predicting where future earthquake and volcanic risk is highest.
A great description of the different types of plate boundaries (with descriptions, maps, pictures, and more), is on the U.S. Geological Survey site.
4. Hot Spots
"Hot spots" are isolated points of weakness in the middle of plates. In these points magma wells to the surface as a volcano. As the plate moves, a trail of volcanoes would be left behind. It is hypothesised that the Hawaiian Islands are the result of such a hot spot (See Figure 3.19, 6th ed., p.62). Iceland, one of the most volcanically active places in the world, is supposed to be located on a hot spot located on a mid-ocean ridge.
With all this movement going on, rocks have to respond to the Stress and Strain.
Stress refers to the fact that a force – movement – acts upon the rock.
Strain is how rocks react to stress.
Some rocks are more ductile or elastic than others – they have a certain amount of flexibility and will bend in response to stress (don’t worry about your text’s discussion of elastic and plastic deformation and elastic limit).
Other rocks are more brittle – they will rupture – fracture or break – in response to stress.
There are three types of stress:
1. Tension or tensile stress (side-to-side stretching)
2. Shear (side-to-side twisting/tearing)
3. Compression (side-to-side shortening)
1. Descriptions …
The Theory of Plate Tectonics helps describe or account for the present features and processes on the Earth’s surface. The pattern of mid-ocean ridges, ocean trenches, volcanoes, and earthquakes is well-described by the theory. It makes sense. The theory allows for a greater understanding of HOW our world – God’s creation – may work!
2. Predictions …
Understanding the theory allows scientists the possibility to predict future events. For instance, we can hypothesise which locations on the Earth are most susceptible to earthquakes and volcanic activity. We can then plan around that! Thus, many regions which are vulnerable to earthquakes (plate boundaries) have instituted building codes that are much more stringent than those in places believed to be less at risk.
3. Plate Tectonics and Christian Faith …
Is there anything in this theory that would run contrary to Christian faith? Depending on your interpretative theory of how old the Earth is, the proposed timeline (465 million years?) may be problematic. However the theory could be substantially explained in terms of a global flood in the short-term (by the way, a global flood could also fit into an old-Earth model, no problem). Indeed, a flood and its ancillary effects could increase plate movement and associated earthquake and volcanic activity. Otherwise, there is no incongruity between this theory and Christian faith.
Thinking about these big issues -- how the earth, as a whole, works -- can be overwhelming! Just as overwhelming can be thinking about how you and I can actually make a difference on the earth! I find these reflections from Loren and Mary Ruth Wilkinson (from Regent College, Vancouver), helpful:
"As Christians we believe we are concerned about the whole planet. But it's discouraging business, caring for the whole planet. So although we need to know about our planet, our care for it begins at home, in our own backyard. That can be the literal backyard where we raise our garden, or the extended backyard where our food and fuel come from and our garbage goes.
"It is a cosmic task. But we approach it a day at the time: with love [for creation is rich and varied, a waterfall of gifts to us from the Creator] and with humility [for it is God who made us, and not we ourselves, and most of our attempts to mend or improve on God's creation only mar it].
"The words Jeremiah conveyed to exiles in Babylon are good for us to hear when we faced such a task: "This is what the Lord Almighty, the God of Israel, says to all those I carried into exile from Jerusalem to Babylon: 'Build houses and settle down; plant gardens and eat what they produce ... seek the peace and prosperity of the city to which I've carried you into exile.'" (Jer 29:4-5, 7)
"The Jewish people were called to a double awareness: first to make the place where they lived "home." We too are called to the daily, weekly, yearly task of tending families, gardens, communities, and the created world with our whole heart. But secondly, they [and we] are reminded that, lovely as the earth is, it is not the only reality. The cycles of creation and daily life are intersected and centered by the cross. The cross is the symbol of our greatest failing: our rebellion against the Creator. But it also points to our ultimate fulfillment: a restored relationship to the Creator, through his own suffering.
"Thus as we think about the tasks of caring for creation, we can think also about the new life we have in Christ: the one story of salvation unfolding through history and ultimately restoring us to Creator and creation alike." (Loren and Mary Ruth Wilkinson, Caring for Creation, Vancouver: Regent College Publishing, 1992. Pages 16-17.)
Do you agree with the Wilkinsons? Why? Why not? Feel free to discuss this quote on the course discussion site, www.nicenet.org ...
To Review:
Terms to know …
In Questions to Review
This page is the intellectual property of the author, Bruce Martin, and is copyrighted © 2005 by Bruce Martin. This page may be copied or printed only for educational purposes by students registered in EAS 205 (Taylor University College). Any other use constitutes a criminal offence.
Scripture quotations marked (NLT) are taken from the Holy Bible, New Living Translation, copyright © 1996. Used by permission of Tyndale House Publishers, Inc., Wheaton, Illinois 60189. All rights reserved