Discussion unit 3

John Aubert  American River College Introduction to the Lithosphere Physical Geography Lecture Notes Page 1 of 4 [email protected]

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Introduction to the Lithosphere Earth Structure and Composition

I. Geologic Time

A. Age of the Earth: 4.6 billion years (4,600,000,000 years)

B. Geologic Time Scale

•Two EONS comprise all of Geologic Time

Precambrian (4600 - 570mya – mya stands for million years ago) Phanerozoic (570 - present)

•Three ERAS comprise the Phanerozoic Eon

Paleozoic (570 - 245mya) Mesozoic (245 - 66mya) Cenozoic (66 - present)

•Two PERIODS comprise the Cenozoic Era

Tertiary (66 - 2mya) Quaternary (2 - present)

•Two EPOCHS comprise the Quaternary Period

Pleistocene (2mya - 10,000ya) Holocene (10, 000ya - present)

C. Uniformitarianism (the present is the key to the past): This is the assumption

that the natural laws and processes which we observe today have been operating similarly throughout geologic time.

II. Earth’s Internal Structure

A. General

1. Radius: Approx. 6400km from center of core to surface.

2. Density: Density increases from surface downward to the core.

3. Temperature: Temperature is estimated to be 3000°C to 6650°C at core.

B. Cross-Sectional View (Three Main Divisions)

1. Core: The core (thickness 3500km) has a solid inner core and a molten, outer core.

2. Mantle: A solid dark and dense layer of rock approx. 2900km thick.

John Aubert  American River College Introduction to the Lithosphere Physical Geography Lecture Notes Page 2 of 4 [email protected]

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3. Crust: The crust surrounds the mantle. Its thickness varies between 5 - 50km. There are two types of crust.

a) Oceanic: Averages 5km thick, dark, higher density (mostly basalt).

b) Continental: Averages 30-50km thick, light color, lower density. Much thicker where there are huge mountains.

c) Mohorovicic Discontinuity or "Moho": This is the boundary

between the crust and the mantle; there is a significant increase in density as you move from the crust into the mantle.

C. Other 1. Lithosphere (includes Crust and Upper Mantle): The lithosphere is the

brittle, outer “shell” of Earth; it is the layer which is broken into moving "lithospheric plates".

2. Asthenosphere: This is the hot, plastic layer of the mantle just below the

lithosphere. The brittle lithosphere is able to “slide” over the “soft” asthenosphere.

III. Composition of Crust – What’s it made of?

A. Elements Oxygen (O) 50% •*Calcium (Ca) 2-3% Silicon (Si) 25% •*Sodium (Na) 2-3% •Aluminum (Al) 8% •*Potassium (K) 2-3% •Iron (Fe) 5% •*Magnesium (Mg) 2-3%

•metals *important plant nutrients

B. Minerals: Minerals are inorganic (non-living) chemical compounds; their formation is affected by available elements, heat, pressure, time. An assemblage of minerals in a solid state is a rock. Quartz (SiO2) is the most common mineral in the crust…

IV. Rock Formation and Classification: Rocks are classified by the processes which form them. Main categories include igneous, sedimentary, and metamorphic.

A. Igneous Rocks: Formed from molten material in Earth's core. They are classified in two different ways, mineral type and cooling environment.

Mineral Group  Felsic Mafic Ultramafic

Mineral Qualities (color and density)

Light Color Dark Color Low Density High Density

Intrusive Examples -cooled beneath surface -cooled slowly (millions of years) -develops large crystals

GRANITE DIORITE GABBRO PERIDOTITE

Extrusive Examples -cooled at surface -cooled rapidly (hours / days) -develops tiny crystals (not visible to the eye)

RHYOLITE ANDESITE BASALT none

John Aubert  American River College Introduction to the Lithosphere Physical Geography Lecture Notes Page 3 of 4 [email protected]

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B. Sedimentary Rocks: Formed when rock fragments and/or mineral particles (called

sediment) are deposited in layers. They then sit for millions of years, during which time they undergo compaction and cementation (a process where the fragments are packed down and cemented together). Collectively, this is process is called lithification.

1. Clastic Sedimentary Rock Examples: Formed when fragments of pre-

existing rock collect in one place. The rock types below are categorized based on particle size.

a) Conglomerate: Formed from cobbles and pebbles mixed with finer

sediments. Similar to concrete.

b) Sandstone: Formed from sand grains (like at the beach).

c) Siltstone: Formed from silt particles.

d) Shale: Formed from clay particles.

2. Chemical Sedimentary Rock Examples: Formed when dissolved minerals precipitate (become solid) from a solution and collect in layers. Process usually happens in warm oceans.

a) Limestone: Calcium Carbonate - CaCO3

b) Dolomite: Calcium and Magnesium Carbonates - CaMg(CO3) 2

3. Organic Sedimentary Rocks (coal and peat): Formed when dead plant

material collects in wet, vegetated environments and becomes covered with other sediments (includes peat and coal; natural gas is a byproduct)

C. Metamorphic Rocks: Any rock which has been exposed to extreme heat and pressure

beneath the surface of the Earth. This occurs at convergent plate boundaries and/or near rising bodies of magma.

1. Examples

a) Slate: Starts as shale; when exposed to extreme heat and pressure it becomes slate.

b) Marble: Starts as limestone; when exposed to extreme heat and pressure

it becomes marble. c) Gneiss: Most coarse grained rocks (granite, conglomerate) which become

exposed to extreme heat and pressure become gneiss. V. The Rock Cycle (see text)

(see next page for sample questions)

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Sample Questions: Questions similar to these will be on your exam. As you study you should anticipate how I might use these questions to create new questions on the same concepts. 1 According to most geologists, planetary physicists, widely respected Earth scientists, and your textbook, the age of the Earth is:

2 In which eon (or era, or period, or epoch) do humans live?

3 The outermost layer of the Earth is the: A. core B. crust C. asthenosphere D. mantle E. moho

4 Which list of Earth’s internal layers is placed in order from highest to lowest density?

A. core, mantle, crust B. crust, mantle, core C. mantle, crust, core D. mantle, core, crust E. core, crust, mantle

5

Starting at Earth’s surface and moving downward, what is evident when passing through the Mohorovicic Discontinuity (Moho for short)?

A. a significant decrease in temperature B. a significant increase in the percentage of silica C. a significant increase in density D. a significant decrease in bacteria

6

What is the important function of the asthenosphere? A. It separates the oceanic and continental crust. B. It protects the Earth from dangerous radiation. C. It holds magma in, preventing many dangerous eruptions. D. It produces Moho constantly. E. It is plastic and allows the lithosphere to move over it.

7 Which of the following is NOT one of the eight most common elements in Earth’s crust?

A. Copper B. Oxygen C. Silica D. Aluminium E. Iron

8 Felsic minerals tend to be:

A. dark color, higher density B. dark color, lower density C. light color, higher density D. light color, lower density

9

Which list of intrusive, igneous rocks is placed in order from highest to lowest density? A. Rhyoite, Granite, Slate, Obsidian, Marble B. Basalt, Limestone, Gabbro, Shale C. Peridotite, Gabbro, Diorite, Granite D. Marble, Quartz, Gneiss, Mafic, Peridotite

10 What do large, visible crystals in an igneous rock indicate?

A. Rock is likely extrusive igneous. B. Rock cooled rapidly. C. The minerals are felsic. D. Rock cooled slowly.

11 In classifying clastic sedimentary rocks, what important quality did we focus on? A. Mineral composition B. size of sediment C. color D. cooling time E. resistance to weathering

12 Gneiss, a metamorphic rock, likely originated as what type of rock? A. conglomerate B. coal C. sandstone D. shale E. marble

13 Which of the following sediments are listed in order from largest to smallest? (You will be given the following terms in different orders - sand, pebble, clay, and silt)

14 What are the two main components of lithosphere?

GOOD NEWS!!! The practice questions above - plus many, many more – can be found in the online practice quizzes discussed in your syllabus and in class. You can take each quiz multiple times, and each time you will get some new questions. Once submitted, the quizzes are graded automatically, with the correct answers provided immediately. This is a great way to prepare for the exams!!! SERIOUSLY, TAKE THE ONLINE PRACTICE QUIZZES!!!