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Natural Disasters Topic 2 (Geological Systems)

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Overview of the tectonic and hydrologic systems for a GE-level course in natural disasters.

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Page 1: Natural Disasters Topic 2 (Geological Systems)

Photo by W. W. Little

Geological Systems

Page 2: Natural Disasters Topic 2 (Geological Systems)

Virtually everything we see on the earth's surface is the result of the interaction between two dynamic systems - tectonic and hydrologic.

Dynamic Systems

From Tarbuck and Lutgens

Page 3: Natural Disasters Topic 2 (Geological Systems)

transform

Plate Tectonics

Plate tectonics is the theory that the earth’s surface is broken into a number of plates that move with respect to one another.

divergent convergent

Boundary Types

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• Driven by earth's internal heat (generated by the decay of radioactive elements, friction as material moves through the earth, and residual heat related to planetary accretion)

• Builds things up (earthquakes, volcanic activity, mountain formation)• Primary processes include mantle convection and isostacy

Tectonic System

Unknown source

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Internally, the earth consists of concentric spheres that vary according to composition and mechanical properties. The compositional and mechanical layers don't necessarily coincide.

Earth's Internal Structure

Crust

Mantle

Core

Inner Core

Outer Core

Mesosphere

Asthenosphere

Lithosphere

Chemical Properties Mechanical Properties

From Hamblin & Christiansen

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Compositional layering occurs as low-density materials rise toward the crust and high-density materials sink into the lower mantle and core.

There are three major compositional layers, the core, mantle, and crust, that increase in density and thickness with depth

Compositional Layering

Crust

• Thin (up to 75 km for continents and 10 km for ocean basins)

• Low density (surface materials average 3g/cm3; whereas, the earth as a whole averages 5.5g/cm3)

• Fe/Mg-deficient silicate minerals

Mantle

• Thick (2900 km)

Bulk of earth’s mass and volume

• Moderate density

• Fe/Mg-rich silicate minerals

Core

• Thick (3500 km radius)

• Very high density

• Mostly Fe

Crust

Mantle

Core

From Hamblin & Christiansen

Page 7: Natural Disasters Topic 2 (Geological Systems)

Mechanical LayeringMechanical layering occurs as compositional layers are subjected to different levels of heat and pressure associated with depth, which affects material flow characteristics (rigid, fluid, plastic).

There are five major mechanical layers: inner core, outer core, mesosphere, asthenosphere, and lithosphere.

Lithosphere

• Rigid (brittle)

• Includes crust and uppermost mantle

Asthenosphere

• Plastic (flows as a solid)

• At or near melting T/P of many minerals.

• Part of the upper mantle

Mesosphere

• Rigid

• Includes most of the mantle

Outer core

• Liquid

• Rotation-and convection-induced flow might be the cause of earth’s magnetic field.

Inner core

• Solid

Inner Core

Outer Core

Mesosphere

Asthenosphere

Lithosphere

From Hamblin & Christiansen

Page 8: Natural Disasters Topic 2 (Geological Systems)

Convection

As a material is heated, it expands lowering its density. This causes the material to rise, where it cools, contracts, increases in density, and subsequently sinks.

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Photo by W. W. Little

Page 10: Natural Disasters Topic 2 (Geological Systems)

Mantle Convection

Rocks are heated in the mantle, causing them to expand. Expansion lowers density and weakens the rock’s structure. This allows the rock to flow and rise toward the earth’s surface.

convectioncell

convectioncell

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There are two major crustal components, continental masses and ocean basins. They differ in elevation, rock type, density, chemical composition, age, and history.

Crustal Features

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• Relatively low density rocks derived from granitic sources.

• Include oldest rocks on earth (up to 4.0 Ga).

• Divided into 4 major components; shields, stable platforms, mountain belts, and continental shelves.

Continents

Mountain Belt

Shield

StablePlatform

ContinentalShelf

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From Tarbuck and Lutgens

• Typically found along continental margins.

• Linear belts of folded sedimentary layers overlying intrusive igneous and metamorphic rocks.

• Highly deformed by horizontal and vertical forces.

• Characterized by internal zig-zag pattern

• E.g. Rockies and Appalachians

Folded Mountain Belts

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From Hamblin & Christiansen (2001)

• Regional surface of low relief with gentle warping (resembles the shape of a Greco-Roman shield).

• Metamorphic and intrusive igneous rocks formed deep within the earth and later elevated to the surface through isostacy as overlying material was eroded.

• E.g. Canadian shield

Shields

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From Hamblin & Christiansen (2001)

• Horizontal and slightly deformed sedimentary layers overlying the shields.

• Little deformation or vertical movement over hundreds of millions of years.

Can have broad domes and basins

• E.g. U.S. Midwest between the Rockies and the Appalachians.

Stable Platforms

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Continental shelf• Submerged, flat continental area• Very low seaward gradient• Thick sediment cover obscures complex underlying fault structures.

Continental slope• From edge of continental shelf to sea floor.• Steeper than shelf, but still relatively flat.• The base of the slope is known as the continental rise.• Include thick, fan-shaped deposits formed by submarine "avalanches."

Continental Shelves

From Tarbuck and Lutgens

Continental Shelves and slopes are considered to be parts of continents but form transition zones with ocean basins.

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From D. T. Sandwell & W. H. F. Smith, Scripps Institute of Oceanography

• Cover 2/3 of Earth’s surface.

• Little was known before the 1960’s.

• Relatively high density rocks derived from basaltic sources.

• Most rocks less than 150 Ma.

• Major features include oceanic ridges, abyssal floors, deep-sea trenches, and sea mounts.

Ocean Basins

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• Extend almost continuously from the Arctic basin through the Atlantic and Indian Oceans and across the Pacific Ocean.

• Broad, fractured swells.• Over 1400 km wide.• Peaks as high as 3000 m.• Central rift valleys.• Cut by fracture systems up to 4000 km long.

Oceanic Ridges

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From D. T. Sandwell & W. H. F. Smith, Scripps Institute of Oceanography

• Broad, relatively smooth areas flanking the ridges.

• Ave. depth of 3000 m.

• Hills up to 900 m high cover 80 - 85% of the floor and are the most widespread landform on Earth.

• Margins of abyssal floor covered with sediment to form the plains.

Abyssal Floors

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From Lamont-Doherty Earth Observatory

• Isolated volcanic peaks

• Often found in chains

• Can form islands

Seamounts

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• Lowest areas on Earth.

• Mariana Trench is 11,000 m below sea level.

• Located adjacent to continental folded mountain belts.

Trenches

From D. T. Sandwell & W. H. F. Smith, Scripps Institute of Oceanography

Page 23: Natural Disasters Topic 2 (Geological Systems)

• Combination of atmosphere and hydrosphere.• Driven by the Sun and modified by the earth’s rotation.• Responsible for storms, rivers, lakes, groundwater, sand dunes, glaciers, beaches, soil formation,

oceanic currents, global circulation patterns, climatic belts, and anything thing else involving water or air.

• Whereas, the tectonic system builds things up, the hydrologic system wears them down.• Primary processes include weathering, erosion, sediment transport, deposition, and lithification

Hydrologic System

From Okanagan University College, Department of Geography website

Page 24: Natural Disasters Topic 2 (Geological Systems)

Photo by NASA

The Water Planet

Over 2/3 of earth’s surface is covered by water.

The hydrologic system involves both surface and atmospheric processes.

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Surface Runoff

Much of the precipitation that falls to the earth returns to the sea as surface runoff associated with river systems.

Photo by W. W. Little

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In mountainous regions and in high latitude areas, surface runoff can be tied up as ice, which still flows seaward, but much more slowly.

Glaciers

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Temporary Impoundment

Water can be temporarily impounded in lakes and reservoirs as it makes its way back to the sea.

Photo by W. W. Little

Page 28: Natural Disasters Topic 2 (Geological Systems)

Photo by W. W. Little

Subsurface Flow

A significant amount of precipitation seeps into the ground and flows back to the sea below the earth’s surface as groundwater.

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Interaction Between Systems

Earth’s tectonic and hydrologic systems form a complex set of interactions. The local balance between these interactions is responsible for everything we see at the earth’s surface.

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Photo by NASA

Uniqueness of the Earth

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All bodies in the solar system are believed to have formed at the same time (4.6 Ga) from the same materials; however, their surface features and internal structures are significantly different from one another. These differences appear to be controlled by two factors, the body’s diameter and its distance from the Sun. Diameter affects the extent to which the tectonic system develops and how long it subsequently endures and whether or not there is sufficient gravity to maintain an atmosphere. Distance from the Sun determines whether or not water can exist in a liquid state.

Uniqueness of the Earth

Photos by NASA

Page 32: Natural Disasters Topic 2 (Geological Systems)

Terrestrial (Inner) Planets

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MercuryMercury has no atmosphere and, due to its small size, its tectonic system ceased to function long ago. As a result, craters, some of which formed billions of years ago, dominate the planets surface. There is no integrated drainage system and large mountain ranges are lacking.

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Due to its proximity to the Sun, water does not exist on Venus in the liquid or solid states, and the planet, therefore, lacks a hydrologic system. However, Venus is much larger than Mercury, though still smaller than the earth, and radar mapping of its surface reveals large-scale tectonic features, including mountain ranges. Impact craters are also scattered across the planets surface.

Venus

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Because of active tectonic and hydrologic systems, the earth's surface is in a constant state of change. Though the earth has undoubtebly been bombarded in its past by meteorites and asteroids, evidence of extraterrestrial impact is relatively sparse, having been subsequently erased. The earth's dominant features are continental land masses and ocean basins. Continental surfaces include well-developed mountain ranges as well as extensive plains. The most ubiquitous feature of land masses is the stream channel, which is present at all scales from small gullies to trunk rivers and shows a highly developed drainage pattern.

Earth

Photo by NASA

Page 36: Natural Disasters Topic 2 (Geological Systems)

The earth's moon is a similar size and character to the planet Mercury.

Earth’s MoonPhoto by NASA

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Mars is smaller than either the earth or Venus but much larger than Mercury. Features, such as volcanoes and rift valleys, indicate that Mars once had an active tectonic system, though this appears to no longer be the case as evidenced by common well-preserved ancient impact craters. Mars does have a thin atmosphere that generates huge dust storms. Large-scale channel networks are evidence that water once existed in a liquid state.

Mars

Page 38: Natural Disasters Topic 2 (Geological Systems)

Gaseous Giant (Outer) Planets

With the exception of Pluto (which is no longer considered to be a planet), the outer planets (Jupiter, Saturn, Uranus, and Neptune) consist mostly of thick atmospheres and relatively small rocky cores of which we know little.

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Moons of the Outer Planets

The moons of these planets have very interesting features that are quite different from those of the inner planets due to significantly different compositions. Some show evidence of "ice" tectonics, and impact craters are abundant.

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Comets/Pluto/Kuiper Belt

Comets are blocks of ice that form an elliptical orbit, mostly in the Kuiper Belt, around the Sun. The tail is formed by solar radiation that passes the comet and always faces away from the Sun. Pluto is thought by many to be a dead comet.

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The Scientific Method

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According to Webster's New World Dictionary (1984), science is “systematized knowledge derived from observation, study, etc."

From this, science can be considered as knowledge that is obtained through a deliberate method and then organized into an understandable system.

What is Science?

The Periodic Table and the Scientific Method

In this early depiction of the developing periodic table, there are spaces for elements that had been predicted to exist but had not yet been discovered.

Photographer Unknown

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“All science is based on the assumption that the

natural world behaves in a constant and

predictable manner. The overall goal of science

is to discover the underlying patterns in the

natural world and then to use this knowledge to

predict what will or will not happen, given

certain facts or circumstances (Tarbuck and

Lutgens, 2000).”

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Scientists make observations of an event or an object and then try to explain those observations by organizing them into a logical system. As rock bodies can cover extensive areas and represent vast periods of time, much of geological research cannot be done as controlled experiments in a formal laboratory setting. In these situations, we rely on the assumption that chemical and physical laws are constant. That is, the processes operating today are the same as those that operated in the past. For instance, since water flows down hill today, it must have done so in the past. Therefore, if we identify a body of rocks that exhibit characteristics similar to those found in modern geological environments, we assume that they must have formed in a similar manner.

Uniformitarianism

Ancient river channelsModern river channels

Photos by W. W. LittlePhotographer unknown

Page 45: Natural Disasters Topic 2 (Geological Systems)

1) Gather data (record observations).

2) Propose a hypothesis (a preliminary, untested explanation or prediction).

Often there will be more than one (multiple-working hypotheses).

3) Test the hypothesis (try to prove or disprove).

4) Interpret the test results.5) Accept the hypothesis as is (works repeatedly), discard the hypothesis (doesn't work at all), or, most likely, revise the hypothesis (some parts work well, others don't).

6) Test the revised hypothesis.

7) Make additional modifications.

8) More tests

9) More modification

10) More tests

11) Etc., etc., etc.

The Method

At some point, if the test results demonstrate to be consistently repeatable, the hypothesis can be elevated to the level of a theory, representing a high level of predictability. For instance, geologists consider the earth to have formed around 4.5 G years ago based on high consistency from numerous tests of radioactive material obtained from the earth, from the Moon, and from meteorites.

From the Central Arizona Regional Science and Engineering Fair

Page 46: Natural Disasters Topic 2 (Geological Systems)

Can a theory or law change over time?

Absolutely: the origin of the universe as an example.

Old law: The Sun, Moon, and stars revolve around the Earth.

New law: The Moon revolves around the Earth, the Earth revolves around the Sun, the Earth neither revolves around the stars nor the stars around the Earth.

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When dealing with science, it is important to distinguish between data and interpretations.

• Data are the facts and are recorded as observations.

Answer the question "what’s there?"• Interpretations are explanations for those observations.

Answer the question "how did it get there?"

Data vs. InterpretationThis illustration contains data showing fluctuations in temperature and CO2 levels over the past 400+ ka. Based on the similarity in pattern, it has been interpreted by most that there is a relationship between the two factors.

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Observation (data/fact): The earth’s surface is warmer during the day than it is during the night.

Explanation (interpretation): The Earth is warmed during the day by the Sun and cools down at night because the Sun is absent.

Example

Page 49: Natural Disasters Topic 2 (Geological Systems)

• In controlled experiments, usually only one factor will be tested at a time. This is called the variable.

• All other factors remain the same from experiment to experiment and are called constants.

• A "standard" has already been tested and has a known result and is, therefore, used as a comparison in each experiment.

Controlled Experiments

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An open system is one in which both energy and matter cross its boundaries. A closed system allows energy only to enter or exit.

Open vs. Closed Systems

Open System Closed (nearly) System

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Negative Feedback Systems

Negative feedback systems adjust internally to external forces.

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Positive Feedback SystemsPositive feedback systems feed upon, and ultimately eliminate themselves. External forces will enhance (accelerate) internal processes.

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System Complexity

Geological systems can be highly complex with numerous interdependent variables, making modeling extremely difficult.