CE 212 GEOLOGY FOR CIVIL ENGINEERS - REVIEWER

PLATE TECTONICS AND CLIMATE CHANGE

Discussion issues on plate tectonics and climate change

Plate Tectonics and its relevance to civil engineering
“If one tectonic plate suddenly slips concerning another plate, the release of energy can cause earthquakes that impact
structures.”

 Most earthquakes are associated with tectonic plate boundaries.

Tectonic plates - are huge slabs of the Earth’s crust, which fit together like pieces of a puzzle.
-The plates are not fixed but are constantly moving atop a layer of solid and molten rock called the
mantle.
- Sometimes these plates collide, move apart, or slide next to each other.
 The San Andreas Fault - a continental transform fault that extends roughly 1,200 kilometers.
- It forms the tectonic boundary between the Pacific Plate and the North American Plate.
- Its motion is right-lateral strike-slip.
- Basically, since it's a big fault and that is close to some big cities, a magnitude 7+ earthquake
could cause major damage to the nearby cities like the San Francisco Bay Area and would kill or
injure thousands of people. In relation to civil engineering, structures built especially along
these identified faults should always be earthquake resistant, meaning it can sustain earthquake
loads thus reducing any possible damage to the structure and to the occupants as well.

IS CLIMATE CHANGE ANTHROPOGENIC OR IS IT PART OF EARTH’S NATURAL PHENOMENA AS

CLAIMED BY SOME SCIENTIST?
“The present climatic changes we are witnessing cannot be attributed to only one factor but the combination of
the two (nature and human)."
In my personal analysis, climate change is both anthropogenic and a natural phenomena. We agree that Human-
produced greenhouse gases such as carbon dioxide play an important factor that influence the earth's climate. But we
should also take into account the effect of the Earth's oceans absorbing huge amounts of solar energy. The oceans covering
more than 70% of the Earth's surface are the heart of our planet's weather and climate system and we know that water in
its various forms is the most important substance in a climate system. Scientists are studying changes that will in turn
influence our future weather and climate. Water vapor is considered a greenhouse gas, even more effective at absorbing
thermal radiation from the earth's surface than carbon dioxide. This tremendous ability to store and release heat over long
periods of time gives the oceans a central role in changing or stabilizing the earth's climate system. Heat stored will
eventually be released adding to surface warming in the future.

PLATE TECTONICS
 - theory dealing with the dynamics of Earth’s outer shell—the lithosphere
- revolutionized Earth sciences by providing a uniform context for understanding mountain-
building processes, volcanoes, and earthquakes as well as the evolution of Earth’s surface and
reconstructing its past continents and oceans.
- The concept of plate tectonics was formulated in the 1960s.
- According to the theory, Earth has a rigid outer layer, known as the lithosphere, which is
typically about 100 km (60 miles) thick and overlies a plastic (moldable, partially molten) layer
called the asthenosphere.

 The lithosphere is broken up into seven very large continental- and ocean-sized plates,
six or seven medium-sized regional plates, and several small ones.
 These plates move relative to each other, typically at rates of 5 to 10 cm (2 to 4 inches)
per year, and interact along their boundaries, where they converge, diverge, or slip past
one another. Such interactions are thought to be responsible for most of Earth’s
seismic and volcanic activity, although earthquakes and volcanoes can occur in plate
interiors.
- Plate motions cause mountains to rise where plates push together, or converge, and continents to
fracture and oceans to form where plates pull apart, or diverge. The continents are embedded in
the plates and drift passively with them, which over millions of years results in significant
changes in Earth’s geography.
- The theory of plate tectonics is based on a broad synthesis of geologic and geophysical data. It is
now almost universally accepted, and its adoption represents a true scientific revolution,
analogous in its consequences to quantum mechanics in physics or the discovery of the genetic
code in biology.

Incorporating the much older idea of continental drift, as well as the concept of seafloor spreading, the theory
of plate tectonics has provided an overarching framework in which to describe the past geography of continents and
oceans, the processes controlling creation and destruction of landforms, and the evolution of Earth’s crust,
atmosphere, biosphere, hydrosphere, and climates. During the late 20th and early 21st centuries, it became apparent
that plate-tectonic processes profoundly influence the composition of Earth’s atmosphere and oceans, serve as a
prime cause of long-term climate change, and make significant contributions to the chemical and physical
environment.

PRINCIPLES OF PLATE TECTONICS

Earth Surface Layer

- Consist of layers, 50 to 100 km (30 to 60 miles) thick, is rigid and is composed of a set of large
and small plates.
- Together, these plates constitute the lithosphere.
 Lithosphere - from the Greek lithos, meaning “rock.”
 The lithosphere rests on and slides over an underlying partially molten
(and thus weaker but generally denser) layer of plastic partially molten
rock known as the asthenosphere.
 Asthenosphere - from the Greek asthenos, meaning “weak.”
- Plate movement is possible because the lithosphere-asthenosphere boundary is a zone of
detachment. As the lithospheric plates move across Earth’s surface, driven by forces as yet not
fully understood, they interact along their boundaries, diverging, converging, or slipping past
each other. While the interiors of the plates are presumed to remain essentially undeformed, plate
 boundaries are the sites of many of the principal processes that shape the terrestrial surface,
including earthquakes, volcanism, and orogeny (that is, formation of mountain ranges).

The process of plate tectonics may be driven by convection in Earth’s mantle, the pull of heavy old pieces of crust
into the mantle, or some combination of both. For a deeper discussion of plate-driving mechanisms, see Plate-driving
mechanisms and the role of the mantle.

EARTH’S LAYERS

Knowledge of Earth’s interior is derived primarily from analysis of the seismic waves that propagate through
Earth as a result of earthquakes. Depending on the material they travel through, the waves may either speed up, slow
down, bend, or even stop if they cannot penetrate the material they encounter.

Collectively, these studies show that Earth can be internally divided into layers on the basis of either gradual or
abrupt variations in chemical and physical properties. Chemically, Earth can be divided into three layers.

1. A relatively thin CRUST

- typically varies from a few kilometres to 40 km (about 25 miles) in thickness

 - sits on top of the mantle.

- (In some places, Earth’s crust may be up to 70 km [40 miles] thick.)

TWO TYPES OF CRUST - differ in their composition and thickness

a) Continental Crust - The distribution of these crustal types broadly coincides with the
division into continents and ocean basins, although continental shelves, which are
submerged, are underlain by continental crust.

- is typically 40 km (25 miles) thick

- The continents have a crust that is broadly granitic in composition and,

with a density of about 2.7 grams per cubic cm (0.098 pound per cubic
inch), is somewhat lighter than oceanic crust,

b) Oceanic Crust – basaltic (i.e., richer in iron and magnesium than granite) in composition
and has a density of about 2.9 to 3 grams per cubic cm (0.1 to 0.11 pound per cubic
inch).

- much thinner than continental crust

- averaging about 6 km (4 miles) in thickness.

These crustal rocks both sit on top of the mantle, which is ultramafic in composition (i.e., very rich in magnesium
and iron-bearing silicate minerals). The boundary between the crust (continental or oceanic) and the underlying mantle is
known as the Mohorovičić discontinuity (also called Moho), which is named for its discoverer, Croatian seismologist
Andrija Mohorovičić. The Moho is clearly defined by seismic studies, which detect an acceleration in seismic waves as
they pass from the crust into the denser mantle. The boundary between the mantle and the core is also clearly defined by
seismic studies, which suggest that the outer part of the core is a liquid.

2. The MANTLE

- is much thicker than the crust

- it contains 83 percent of Earth’s volume and continues to a depth of 2,900 km (1,800 miles).

3. CORE

- extends to the center of Earth

- some 6,370 km (nearly 4,000 miles) below the surface.

- Geologists maintain that the core is made up primarily of metallic iron accompanied by smaller
amounts of nickel, cobalt, and lighter elements, such as carbon and sulfur.

 The effect of the different densities of lithospheric rock can be seen in the different average elevations of
continental and oceanic crust. The less-dense continental crust has greater buoyancy, causing it to float much
higher in the mantle. Its average elevation above sea level is 840 metres (2,750 feet), while the average depth
of oceanic crust is 3,790 metres (12,400 feet). This density difference creates two principal levels of Earth’s
surface.
 The lithosphere itself includes all the crust as well as the upper part of the mantle (i.e., the region directly
beneath the Moho), which is also rigid. However, as temperatures increase with depth, the heat causes mantle
 rocks to lose their rigidity. This process begins at about 100 km (60 miles) below the surface. This change
occurs within the mantle and defines the base of the lithosphere and the top of the asthenosphere. This upper
portion of the mantle, which is known as the lithospheric mantle, has an average density of about 3.3 grams
per cubic cm (0.12 pound per cubic inch). The asthenosphere, which sits directly below the lithospheric
mantle, is thought to be slightly denser at 3.4–4.4 grams per cubic cm (0.12–0.16 pound per cubic inch).
 In contrast, the rocks in the asthenosphere are weaker, because they are close to their melting temperatures.
As a result, seismic waves slow as they enter the asthenosphere. With increasing depth, however, the greater
pressure from the weight of the rocks above causes the mantle to become gradually stronger, and seismic
waves increase in velocity, a defining characteristic of the lower mantle. The lower mantle is more or less
solid, but the region is also very hot, and thus the rocks can flow very slowly (a process known as creep).
 During the late 20th and early 21st centuries, scientific understanding of the deep mantle was greatly
enhanced by high-resolution seismological studies combined with numerical modeling and laboratory
experiments that mimicked conditions near the core-mantle boundary. Collectively, these studies revealed
that the deep mantle is highly heterogeneous and that the layer may play a fundamental role in driving
Earth’s plates.
 At a depth of about 2,900 km (1,800 miles), the lower mantle gives way to Earth’s outer core, which is made
up of a liquid rich in iron and nickel. At a depth of about 5,100 km (3,200 miles), the outer core transitions to
the inner core. Although it has a higher temperature than the outer core, the inner core is solid because of the
tremendous pressures that exist near Earth’s centre. Earth’s inner core is divided into the outer-inner core
(OIC) and the inner-inner core (IIC), which differ from one another with respect to the polarity of their iron
crystals. The polarity of the iron crystals of the OIC is oriented in a north-south direction, whereas that of the
IIC is oriented east-west.