The Lithosphere

 Part-I: Thermodynamic Laws, Structure of solid Earth, and Plate Tectonics
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Key terms and concepts

Thermodynamic laws

Sections of solid Earth

Plate tectonics

Rock weathering


1. Understand the three laws of thermodynamics and their implications

2. Understand why do we say the law of conservation of mass and energy is the main foundation for global material cycles

3. What are commonly referred sections of solid Earth?

3. Capable of explaining the theory of plate tectonics

4. What are the major processes involved in global plate tectonic activities?

5. What is rock weathering? What factors are involved in rock weathering?

Key terms and concepts

Thermodynamic laws

Seismic waves (P: compressional; S: shear)

Crusts of Earth

Plate tectonics

Seafloor spreading

Polarity reversal

Rock weathering

Thermodynamic laws

Thermodynamics is the study of the inter-relation between heat, work and internal energy of a system.


First Law of thermodynamics:

Mass-energy is neither created nor destroyed, but can be changed to a different form.

In any chemical reaction, matter is neither created nor destroyed but merely changes from one form into another (Antoine Lavoisier, 1785)

The law of conservation of energy: in any chemical reaction, energy is neither created nor destroyed, but only transformed (Julius Robert von Mayer, 1842).

Due to the discovery of this equation (by Albert Einstein1905):

    E = mc2

It is now called the law of conservation of mass and energy.

This law implies: except for radioactive decay or very high-energy reactions, the amount of each element in a closed system remains the same. Any process on Earth that consistently moves elements from one place to another must be part of a CYCLE that moves elements back. Matter has to come from somewhere and go somewhere--Matter cycles

Based on this law, we infer that:

1. The total mass of planet Earth does not change to any significant degree through time (What are the potential sources of mass loss or addition to Earth?);

2. Any lasting movement of matter in the earth system has to take the form of cycling;

3. All elements participating in the living world have to go through biogeochemical cycles (What will happen if they do not cycle?).

The Second Law of Thermodynamiocs: The entropy of a closed system increases with time (or no process of energy transformation can be 100% efficient). This implies that energy sources have different qualities in terms of usability.

For more reading about the 2nd law, click these links: or

The Third Law of Thermodynamics: No substance can be cooled to a temperature of absolute zero.

Absolute Zero = 0 Kelvins = -273.15° Celsius

For more reading about the 3rd law, click these links:

Sections of solid Earth

The core of Earth
The mantle of Earth
The crust of Earth

The lithosphere of Earth
The asthenosphere of Earth

What is the primary basis for dividing Earth into sections?

Plate Tectonics

The theory of global tectonics--The lithosphere is divided into a number of crustal plates that move on the underlying plastic asthenosphere. These plates may collide with, slide under, or move pass adjacent plates in a nearly horizontal direction.

Let's see those plates

Please watch the movie (The Birth of a Theory) with the following questions in your mind:

1. What evidence was used to support Wegener's hypothesis of Pangaea (Pan-jee'-ah) and continental drift during early 1900?  Why wasn't the hypothesis widely accepted then?

2. What evidence has been used to support the theory of plate tectonics?

3. What is the relationship between the process of CONVECTION (real convection) in the mantle and plate tectonics?

The crust of Earth

Plate Margins

Rift zones (mid-ocean ridges)--spreading of the ocean floor

Live around a hydrothermal vent?
(very high pressure and temperature)

How do scientist know that the sea floor is spreading at the rift zone?

1. magnetic anomalies/polarity reversals
(Dr. Robert S. Coe, Earth Siences of UCSC  is an expert on palomagnetism)

(The Earth's magnetic field)
(Magnetization of magnetite)
(Frequencies of reversals)

2. radiometric dating of those basalts of magnetic anomalies

Subduction zones--submerging plates

The boundary line of an oceanic plate moves under a continental plate
(Feature: active earthquate zone; The zone near us)

Hot spots (Volcanoes)


A summary

Do you know about several important processes that affect the lithosphere and the surface of Earth?
Major processes:

Weathering and erosion
metamorphism and mountain building
seafloor spreading

Rock Weathering

All exposed rocks undergo weathering.

Weathering is a general term encompassing many processes by which parent rocks are broken down.

Mechanical weathering is the fragmentation of rocks without chemical changes. Wind abrasion
Rock splitting by the freezing of water
Growth of roots in rock crevices

Chemical weathering occurs when parent rock materials react with acidic and oxidizing substances. Usually chemical weathering involves water , and mineral constituents are released as dissolved ions.

In general the following equation is part of the sedimentary rock cycle:

Igneous rocks + acid volatiles = sedimentary rocks + salty oceans

The net effect of silicate mineral weathering is to convert soil carbon CO 2 , derived ultimately from photosynthesis, into dissolved HCO 3 - . A representative reaction is:

 2CO 2   + 3H 2 O + CaAl 2 Si 2 O 8 (Anorthite)---> Ca 2+ + 2HCO 3 - + Al 2 Si 2 O 5 (OH) 4 (Kaolinite)

The dissolved HCO 3 - is carried to the ocean by rivers and, if accompanied by dissolved Ca 2+ or Mg 2+ , the carbon is removed from the oceans as Ca-Mg carbonate minerals. In this way, if Ca and Mg silicates are involved in weathering , the overall process results in the removal of CO 2 from the atmosphere .

Rock weathering diagram (Schlesinger 97, Fig. 1.4, page 9)

Important factors involved in rock weathering

1. Parent rock types (e.i., igneous: basal, granite; or sedimentary: sandstone, limestone) As a general rule of thumb, the higher is aluminum-silicates content of the rock type, the less susceptible to weathering. 2. Climate (e.i., precipitation, temperature, wind, etc.)

3. Organisms (Plant roots, microorganisms, etc.) Because the primary chemical weathering mechanism is carbonic acid:

H 2 O + CO 2 <=> H + + HCO 3 - <=> H 2 CO 3

4. Topography More water at the bottom of a hill  than on the slopes, and the slopes that directly face the sun will be drier than slopes that do not.

5. Time All of the above factors assert themselves over time, often hundreds or thousands of years (e.i., Hawaii islands).

Factors influencing rock weathering