Geology 100 Telecourse Introductory lecture Part 2

	Geology 100 Introduction to Geology Telecourse
INTRODUCTORY LECTURE, Part 2
Important Concepts in Physical Geology Continued What Happens When Land Builds Up or the Mountains Erode? Glacial Rebound is Another Example of Isostatic Adjustment Earth Structure Surface Wave Types Earth's Interior Relative vs Absolute Time Atoms, Elements and Minerals The Rock Cycle

Important Concepts in Physical Geology Continued

Isostasy

The balance or equilibrium of adjacent blocks of brittle crust floating on the upper mantle. All crustal rock is less dense than mantle rock so it floats on the denser mantle like a block of wood floats on water. Crustal rocks will also sink or rise gradually until they balance the weight of the displaced mantle rock.

Isostatic Balance Diagram:
This diagram shows how blocks of wood floating on water can be compared to blocks of crust floating on the mantle. Like an iceberg floating in the water, only a small part of the wooden block shows above the surface.
The parts of the crust do the same when floating on the mantle. Tall land masses like mountains have huge roots pushing down into the mantle to stabilize them. The taller the mountains the deeper the roots push into the mantle. Valleys on land and oceanic trenches at sea have the most shallow roots.

What Happens When Land Builds Up or the Mountains Erode?

This is called Isostatic Adjustment and involves the pressing down into the mantle of crust burdened by sediment deposits, ice etc. and the rising up out of the mantle of crust when erosion takes place.

diagram illustrating isostatic adjustment

Isostatic Adjustment from Erosion and Deposition Diagram:
This diagram shows a piece of oceanic and continental crust. The continental crust is pushing further into the mantle because it is thicker.
The second part of the diagram shows what happens when erosion takes place. The continental crust is eroded into sediments which wash out to sea. This places more burden on the oceanic crust pushing it deeper into the mantle. Meanwhile the continental crust has lost some of its thickness due to erosion, so it will rise up, not pushing as deeply into the mantle as it did before erosion.

Glacial Rebound is Another Example of Isostatic Adjustment

The heavy ice of a glacier pushes the crustal rock into the mantle rock causing a depression. When the glacier melts and there is no longer a heavy burden on the crustal rock, the land will slowly rise. This is called a crustal rebound; when it is caused by a glacier, it is called glacial rebound.

Glacier Effects on Earth's Crust Diagrams:
The diagrams above show the effects of a glacier on the Earth's crust. A glacier forms adding weight to the Earth's crust and the crust subsides (sinks further into the mantle). When the ice melts, the weight is removed from the crust and the crust rebounds (rises towards its original position where it is not pushing as far into the mantle.

Earth Structure

Earth Structure is studied via gravity, heat, magnetism and seismic waves.

Gravity

The force of gravity between two objects varies with the mass of the objects and the distance between them.
Dense rocks such as mantle rock (Ultramafic rock) and ores will have a greater force of gravity than other less dense rocks that have less mass.
Gravity meters can be used to discover ore deposits and areas of the earth that are out of isostatic balance.

Isostatic Balance Gives Uniform Gravity Reading Diagram:
The diagram shows that when everything is in isostatic balance the gravity readings are uniform. But as we will see in the next two diagrams, if something is pushing up from below, or pushing down from above, the gravity readings will be abnormal.

illustration of positive gravity anomaly

illustration of negative gravity anomaly

Positive and Negative Gravity Anomaly Diagrams:
The first diagram above shows a positive gravity anomaly. This means that something like magma is pushing the crustal mass upward. The second diagram shows a negative gravity anomaly. This means that something is pulling or pushing the crust downward. Negative gravity anomalies are found in areas of subduction where one piece of crust is being pulled under another. Or in the case of a glacier pushing the crustal mass down under its weight. Negative gravity anomalies are also found when there is a pocket of dense rock within the crustal rock. This denser rock will have a stronger gravitational attraction.

Geothermal Gradient & Heat Flow

The temperature increase with depth into the earth is called the geothermal gradient.

The Earth's Internal Temperature Diagram:
This diagram shows how the Earth's temperature increases with depth to the mantle, outer and inner core. There are two lines indicating different temperatures, this represents the area of uncertainty of the estimate.
New estimates of the Earth's internal temperatures estimate that the Earth's center is 6,400 degrees Centigrade + 600 degrees Centigrade. That is hotter than the surface of the sun!

Heat Flow

Heat flow is the gradual loss of heat through the earth's surface. The origin of this heat is a mystery because we don't know if the earth formed hot and is cooling off or if it formed cold and radioactive decay is warming it up.

Heat Flow from Oceans and Continents Diagram:
This diagram illustrates that the heat flow from the continental and oceanic crust is the same. The mechanism for generating the heat flow from the continental and oceanic crusts differs, however. The heat flow continental crust is generated by radioactive materials within the crust. There are few radioactive materials within the oceanic crust, so its heat flow is probably generated by the hot mantle being so close to the surface.

Magnetic Field

A region of magnetic force surrounds the earth. The field has north and south magnetic poles, near the geographical north and south poles. This field is thought to be generated by convection currents in the liquid metal of the outer core.
The field usually travels in one direction, but occasionally this direction is reversed.

Earth's Magnetic Field Diagram and Geographic Poles Diagram:
The first diagram above shows the Earth's magnetic field as we know it today with a north-south axis. It demonstrates that the magnetic poles are in slightly different places than the geographic poles. The second diagram shows reversed polarity. The following diagrams and explanations will discuss how we know there have been magnetic reversals in the past.

The strength and direction of the magnetic field is recorded at the time rocks form. Most of the evidence of magnetic reversals comes from lava flows on the continents.
By conducting sea floor studies and correlating the information with that we have from stacked lava flows on land we now know the age of the sea floor. The sea floor is younger than any of the continents and much younger than the ocean water they contain- about 200 million years old.

Orientation of Earth's Magnetic Field Diagram and Cross Section of Stacked Lava Diagram:
The first diagram above shows how the direction of the Earth's magnetic field is preserved in magnetite (a magnetic rock) which is formed from lava and cools with particles aligning toward the magnetic poles. The second diagram shows how as lava beds/flows stack one-upon-another millennium after millennium they can show changes in the orientation of the Earth's magnetic field.

Oceanic Crust Rock Ages Diagram:
This diagram shows how the tracing of magnetic reversals and comparing them to stacked lava flows on the continent helped us date the sea floor. New sea floor originates at the mid-oceanic ridge and spreads outward. The youngest sea floor is near the mid-oceanic ridge.
All the sea floor is comparatively young, a maximum of 200 million years, because old floor is always subducting and thereby being melted by the heat of the mantle and incorporated into the continents or island arcs.

Seismic Waves

Waves of energy caused by a disturbance of the earth's surface and/or interior.
There are three types of seismic waves: surface waves that only travel at the earth's surface; primary or P waves that travel through solids, liquids and gases; and secondary or S waves that travel through solids only. By studying the speed and refraction of these waves we have developed the following picture of the earth's internal structure.

Surface Wave Types

Love Wave Diagram and Rayleigh Wave Diagrams:
The diagrams show the two different types of surface waves. Love Surface Waves move from side-to-side. Rayleigh Surface Waves move up and down. Surface Waves are the slowest of all the wave types and because they stay on the surface of the Earth they do the most damage in an earthquake.

Body Waves -- P Waves

Primary Wave Diagram and P-Wave Shadow Zone Diagrams:
P or Primary Waves are the first waves to arrive at a seismic station. As the first diagram above shows, they move in a motion of expansion and compression, like a long spring when you pull it out and then let go. The second diagram shows that P Waves travel through liquids, solids and gases. When P Waves are sent through the Earth there is a slight shadow on the opposite side of the Earth where no P Waves directly hit. We believe the P Waves are being refracted by the difference in density between the inner (solid) core and outer (liquid) core.

Body Waves -- S Waves

Secondary Wave Diagram and S-Wave Shadow Zone Diagrams:
S or Secondary Waves are the second waves to reach a seismic station (surface waves are last). As the first diagram above shows, they move in an up and down motion, like shaking a rope. The second diagram shows that S Waves travel through solids only. When S Waves are sent through the Earth there is a large shadow on the opposite side of the Earth where no S Waves hit at all. We believe that since S Waves travel only through solids that this is proof that the outer core is in liquid form.

Earth's Interior

Cross Section of Earth Diagram:
This diagram shows a cross-section of the Earth. It includes from the outer surface inward: the crust, mantle, outer core and inner core.

Core: the central zone of the earth has two parts, a solid inner core where high pressure prevents any liquification and a molten, liquid outer core.
- The presence of the magnetic field is one form of proof that the core is metallic because it would need to be a good conductor to generate the electricity necessary to create the magnetism and that the outer core is liquid.
Crust is a thin layer on the earth's surface. The crust is thinner beneath the oceans than it is on the continents. The oceanic crust is also more dense.
Mantle contains most of the earth's mass and lies between the crust and the core. There is an upper and lower mantle. The mantle is solid except for some plastic, partially molten spots and is composed of ultramafic rock. A heavy igneous rock made up of ferromagnesian minerals that lack feldspar
- Lithosphere -- Is rigid and is composed of the Crust + Uppermost Mantle
- Asthenosphere
  - Beneath lithosphere
  - Part of the Upper Mantle
  - Soft-- near the melting point
- Tectonic forces
  - Are due to movement within the mantle

Earth's Crust Diagram:
This diagram shows the various parts of the upper mantle and crust. The crust can be continental or oceanic. The lithosphere is solid and consists of the crust and uppermost mantle. The asthenosphere is below the lithosphere and is a plastic, moving part of the upper mantle. The lithosphere slides upon the moving asthenosphere in the theory of plate tectonics.

Relative vs Absolute Time

Geologists are generally more interested in the sequence of events (relative time) than a chronological age of a rock.
Relative Time is found by applying three principles:
- Original Horizontality: sediments are laid down in horizontal layers
- Lateral Continuity: sediments thin at their edges as they get further from their source
- Superposition: the sediments at the bottom are older than those on top
- Cross-Cutting Relationships: Anything intruding into the layers of sediments is younger than the layers of sediments

Atoms, Elements and Minerals

An element is a substance that cannot be broken down into other substances by ordinary chemical methods. An atom is the smallest possible particle of an element that retains the properties of that element. All atoms within an element are essentially identical. Examples of elements include hydrogen, oxygen, nitrogen.
Minerals are crystalline, that is the atoms are arranged in a 3-dimensional, regularly repeating, orderly pattern.
A definition of a mineral includes three things:
- It must be a crystalline solid
- It must occur naturally
- It must have a specific chemical composition
To identify unknown minerals one looks to their physical properties such as color, streak, luster, hardness, external crystal form, cleavage, fracture, specific gravity and other properties.

The Rock Cycle

Magma forms igneous rock which, along with all other rock types, erodes to form sediments
Sediments lithify (turn into rock) becoming sedimentary rock
More sediments pile up increasing heat & pressure
Additional heat & pressure causes either recrystallization without melting which lithifies into metamorphic rock
Or the rock melts and the magma lithifies becoming igneous rock once again -- completing the cycle

The Rock Cycle Diagram:
This diagram shows in pictures what I have just described above. Erosion creates sediments from all types of rock, which are lithified becoming sedimentary rock. Heat and pressure can either recrystallize the rock which lithifies creating metamorphic rock or melt the rock into magma which will become igneous rock. All erode when they reach the Earth's surface, thereby restarting the cycle.

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INTRODUCTORY LECTURE, Part 2