• Internal Structure of the Earth
• Plate Tectonics
• Summary of the Three kinds of Plate Divisions
• Rock Types
• Folds and Faults
• Earthquakes
• Earthquakes: Further Information
• Volcanoes
• Canadian Volcanoes
• Tectonic Processes: Further Information and Discover Our Earth
• Geologic Survey of Canada

Plate Tectonics

"The earth's surface is broken into seven large and many small moving plates. These plates, each about 80 km thick, move relative to one another an average of a few centimetres a year. Three types of movement are recognized at the boundaries between plates: convergent, divergent and transform-fault [i.e. slip and slide]. At convergent boundaries, plates move toward each other and collide. Where an oceanic plate collides with a continental plate, the oceanic plate tips down and slides beneath the continental plate forming a deep ocean trench (long, narrow, deep basin.) An example of this type of movement, called subduction, occurs at the boundary between the oceanic Nazca Plate and the continental South American Plate. Where continental plates collide, they form major mountain systems such as the Himalayas. At divergent boundaries, plates move away from each other such as at the Mid-Atlantic Ridge. Where plates diverge, hot, molten rock rises and cools adding new material to the edges of the oceanic plates. This process is known as sea-floor spreading. At transform-fault boundaries, plates move horizontally past each other. The San Andreas Fault zone is an example of this type of boundary where the Pacific Plate on which Los Angeles sits is moving slowly northwestward relative to the North American Plate on which San Francisco sits. Plate tectonics, the branch of science that deals with the process by which rigid plates are moved across hot molten material, has helped to explain much in global-scale geology including the formation of mountains, and the distribution of earthquakes and volcanoes" (http://geology.er.usgs.gov/eastern/plates.html).

Courtesy: University of Wisconsin, Board of Regents.

A precursor to the theory of plate tectonics was the theory of continental drift put forward in 1912 by Alfred Wegener. The theory states that the continents were once joined together into a supercontinent called Pangaea which began to split up about 200 to 300 million years ago. Wegener presented six key proofs that the continents were once joined together.

• coastline fit
• geologic fit
• paleoclimatology
• paleoglaciation
• fossil correlation
• paleomagnetism
  
• For more information on these proofs, see the following websites:

USGS

National Geographic

Animation of continental drift over the last 220 million years.

Further Proof that the continents are moving discovered in the 1960s and 1970s:
SeaFloor Spreading and Magnetic Reversals (YouTube)

"Periodically, the Earth's magnetic field reverses. New rock formed from magma records the orientation of Earth's magnetic field at the time the magma cools. Study of the sea floor with magnometers revealed "stripes" of alternating magnetization parallel to the mid-oceanic ridges. This is evidence for continuous formation of new rock at the ridges. As more rock forms, older rock is pushed farther away from the ridge, producing symmetrical stripes to either side of the ridge. In the diagram below, the dark stripes represent ocean floor generated during 'reversed' polar orientation and the lighter stripes represent the polar orientation we have today. Notice that the patterns on either side of the line representing the mid-oceanic ridge are mirror images of one another. The shaded stripes also represent older and older rock as they move away from the mid-oceanic ridge. Geologists have determined that rocks found in different parts of the planet with similar ages have the same magnetic characteristics" (http://www.ucmp.berkeley.edu/geology/tecmech.html).

Courtesy: University of California, Berkeley

Rock Types

A rock is a collection of one or more minerals. Rocks are divided into three major groups.

1) IGNEOUS ROCKS - rocks of molten origin

a) Intrusive : magma cooled at depth: often forced into cracks or cavities in a molten condition.
Generally coarse-grained.
eg granite, quartz, feldspar, mica

b) Extrusive : lava cooled on the surface after flowing out of cracks or volcanoes as lava. Generally fine-grained.
eg pummice, obsidian

2) SEDIMENTARY ROCKS - are composed of the products of mechanical and chemical weathering. Sedimentary rocks are deposited usually in near-horizontal layers by the agents of transportation. eg wind, water, and ice. Burial produces lithification or hardening.

Sedimentary sources:

• deserts - aeolian deposits
• glacial deposits
• rivers - deltas
• shallow marine areas (fossils)
• deep marine areas
• reefs

Note: deposits from shallow and deep marine areas (i.e. organic deposits) eventually form coal, oil, and natural gas (i.e. fossil fuels)

a) Conglomerate: composed of large rounded fragments embedded in a fine-grained material
(easily eroded because made up of tiny particles and layers).

b) Sandstone: composed mainly of sand-sized particles, and these are mainly of quartz.

c) Arkose: composed mainly of sand-sized particles of quartz and feldspar, the latter making up more than 25% of the total.

d) Shale: composed of fine particles predominantly of clay size.

e) Limestone: composed of calcite either (1) chemically precipitated in water, or
(2) accumulated through the collection of organic remains such as
shells, or (3) the result of building up of organic rock through the action of organisms like corals.

3) METAMORPHIC ROCKS - formed by the influence of heat and pressure on other rocks resulting in changes in mineralogy and texture.

a) Gneiss: has a banded appearance with alternating composition; may be formed by
metamorphism of an igneous or sedimentary rock

b) Slate: fine-grained and platy; fissile; formed by the metamorphism of shale.

c) Schist: rock flaky minerals (eg mica) aligned in a parallel fashion; formed by the metamorphism of shale and slate.

d) Quartzite: hard, mainly composed of quartz; formed by the metamorphism of
sandstone.

e) Marble: a coarse, but even-grained rock formed by the metamorphism of limestone.

Courtesy: Geological Survey of Canada

The Rock Cycle

Courtesy: Department of Geology, College of William and Mary

Earthquakes

Within the earth, about a million tremors are produced each year. Of these, about 1000 are major earthquakes. Most of these do not make headlines because they occur in unpopulated areas or under oceans.

- Charles F. Richter devised a scale in 1935 that measures the amount of energy released by a shock or earth movement. The shock is graded on a scale of 1 to 10.

- Earthquakes are the result of a sudden release of stress across a fault zone . The stress takes many years to build up and then is suddenly released. Strain builds up until the blocks along the fault are moved a few centimetres or many meters. A slip at one weak point may trigger slips all along the fault. The sudden movement causes seismic waves that travel out from the centre of the slip. Example: 1906 San Francisco earthquake caused by a build up of pressure along the San Andreas Fault. The pressure had been building up on the Pacific side for 100 years.

- Shallow Earthquakes : cause the greatest destruction since their source of energy is very close to the earth's surface. Most quakes are of this variety. Shock lasts a fraction of a second to 10 seconds. Depth of shallow quakes varies from about 1.6 to 96 km.
- Deep Focus Earthquakes : Most of these occur near the deep oceanic trenches and are usually 6 to 7 in magnitude. Depth between 96 km and 960 km.

Note : Focus: where the earthquake occurs.
Epicentre: spot on earth's surface directly above the focus.

Major Effects
- They can cause vertical and lateral displacement of parts of the crust.

• eg in Sagami Bay (Japan) in 1923, parts of the bay were uplifted by 215 metres. This can then trigger a tsunami.
• eg in Alaska in 1899 some coastal rocks were uplifted by 16 metres.

- Near coastlines, harbors and rivers, they can make the wet, sandy soil jiggle, turning it temporarily from a solid to a liquid state. Heavy sand and rock sinks, while water and lighter sand bubble to the surface. The slurry spreads, often toward the water, and the surface shifts. This is known as liquefaction.
- They can cause landslides.
- They can cause the devastation of cities . (i.e. collapse of buildings, fires, death, disruption of utilities, spread of disease).
(eg Mexico City earthquake in 1985)

Causes of Earthquakes
- Two main belts: Pacific Rim and Portugal to S. Asia
1) In the Pacific, earthquakes are related to the giant convection currents moving the ocean floor.
2) The belt of earthquakes passing through the continental mountain ranges of the Alps, Andes, Himalayas could be the result of mountains growing and decaying - this is called orogenesis . Erosion wears down the mountains making them lighter. To reach a state of equilibrium (theory of isostasy ) other areas rise causing earthquakes.
Theory of Isostasy
- from Greek words iso (equal) and stasis (standing)
- can be used to explain the constant attempt of the continental blocks to achieve a state of equilibrium. This is like a block of ice floating in water - 11% of its mass will remain above water and 89% below. If part of that block of ice above water is chipped away, the block will readjust itself so that 11% of its mass continues to remain above water.
3) The movement of continents and ocean floors exerts pressure at the trenches, ridges, and throughout the faults in the earth's crust.
4) The earth wobbling on its axis may cause earthquakes. The earth wobbles a few centimetres each day totaling approximately 20 to 23 metres each year. These wobbles tend to maximize about every seven years.
5) Man's interference in the balance of nature.
eg water injection: pumping fluids at high pressures into oil-bearing rock formations underground - forcing more oil into production wells. Quakes as large as 4.3 on the Richter scale have been produced at the Eagle oilfield north of Fort St. John, BC.
eg underground nuclear bomb testing

Magnitude

Magnitude is the total amount of energy released by an earthquake, and consequently, the amount of ground motion it causes. Magnitude is measured using the Richter Scale. The scale grows by a power of 10 for each 1.0 increase in magnitude. Hence, a 5.0 earthquake is 1000 times (10 x 10 x 10) greater than a 2.0 earthquake.

Intensity

The intensity of an earthquake is a value that reflects the effect produced. Intensity is measured using the Modified Mercalli scale.

Volcanoes

Causes
- Almost all volcanoes happen near plate divisions. (Recall: Convective currents in the asthenosphere are the driving force for plate tectonics)
1) Destructive (violent) volcanoes usually happen near subducting plates. i.e. The Pacific Ring of Fire
2) Less violent volcanoes usually happen near diverging plates.

Types of Lava / Magma

Andesite: high viscosity, thick and slow moving, silica rich, greater risk of a violent eruption as gases are trapped by its thickness so pressure builds.

Basalt: low viscosity, more fluid and fast flowing, maganesium-rich, less risk of a violent eruption as gases can more easily escape.

Types of Volcanoes

1) Cinder Cone
- basaltic magma ejected under high pressure from a narrow neck cools to
form cinder and ash which falls back down to build the sides of the cone
- cone can build rapidly with steep sides i.e. 30 - 40 degree angle
eg the volcano Paricutin in Mexico in 1943 grew 500 m in 8 months
- if the narrow neck of the cone becomes plugged the volcano can explode violently

Photograph by J.P. Lockwood on December 1, 1975 (Courtesy: USGS)

2) Shield Cone
eg Kilauea in Hawaii
- often formed from basalt lava which has a low viscosity (i.e. basaltic) allowing the lava to move for some distance forming a broad gently sloped volcano i.e. 10 -20 degree angle
- less explosive tendency because the magma flows freely allowing the gases to escape

Photograph by M. Mangan on April 16, 1992 (Courtesy: USGS)

3) Composite Cone (or sometimes called Stratovolcanoes)
eg Mt. St. Helens, Mt. Baker, Mt. Rainier, Mt. Etna in Sicily
- consists of alternating layers of lava and ash
- lava layers permit the cone to attain considerable width while the cinder/ash layers encourage vertical growth
- as a result, some of the world's highest volcanic mountains are in this category
- lava is a mixture of the andesite type (i.e. silica-rich) and the basalt type
(i.e. magnesium-rich)
- can be very explosive spewing much pyroclastic material (i.e. cinders, ash, and rock fragments) high into the atmosphere and / or rapidly down the side of the volcano as a pyroclastic flow. Such a flow sears everything in its path and may also contain a
nuee ardente - a very hot cloud of gases (i.e. hydrogen, carbon monoxide, sulphur, and chlorine).
- mud flows, known as lahars, are frequently triggered as heat from the volcano melts snow and ice

Mt. St. Helens (Courtesy: Geological Survey of Canada)

The potential path of ash from a Mt. Baker eruption
(Courtesy: Geological Survey of Canada)

This exposed profile along the side of Highway 97 north of Vernon, B.C. near Monte Lake reveals ash
(also known as tephra) deposits from two separate eruptions. (Courtesy: P. Mleziva)

A closer view of the tephra from the Washington State volcano. (Courtesy: P. Mleziva)

A closer view of the Mt. Mazama tephra. This volcano is located in Oregon and has the famous caldera, Crater Lake, on top of it. (Courtesy: P. Mleziva)

STAGES

1) active: includes the cones that are erupting or show some signs of internal unrest.
2) dormant: currently inactive volcanoes that have erupted in historic times and could possibly erupt in the future.
3) extinct: no future activity appears possible.

VOLCANIC EXPLOSIVITY INDEX

STRUCTURES DUE TO VOLCANISM

1) volcanism: the process involving the transfer of magma from one area to another.

Extrusive Forms
- if large volumes of lava cover existing landforms, a plateau may result.
eg Columbia Lava Plateau in nothwestern USA covers over 500 000 sq. km
with lava up to 0.8 km in depth
- when a small opening is made in the crust and lava and other material gradually build up a cone-shaped structure surrounding the opening, this formation is known as a volcano
- the funnel-shaped opening at the top of the cone is known as the crater
- if a composite cone volcano erupts so violently that much of the magma from the cone is ejected, it is left with little support, resulting in an inward collapse of the cone leaving behind a depression known as a caldera .

eg Crater Lake in Oregon

Courtesy: USGS

Intrusive Forms
Due to magma that solidifies in cracks and cavities that it has made as it forces the surrounding rock apart.

wall or dike: magma that cools in a vertical crack
sill: magma that flows between strata of sedimentary rock forming a thin but extensive sheet
laccolith: dome of igneous rock formed from a large enough volume of magma to raise up the overlying strata
batholith: a huge storage basin of magma several kilometres thick.

Courtesy: University of Wisconsin - La Crosse

volcanic neck or plug: volcanoes, of course, are extrusive igneous features -- but part of a volcano cools underground and is considered an intrusive igneous feature. This feature is known as the volcanic neck or plug.

Courtesy: Texas A & M University

Note: It is possible for these features to become exposed at the surface over time due to weathering and erosion of the surrounding rock.

eg old volcanic neck

Courtesy: Texas A & M University

eg a dike uncovered

Courtesy: Texas A & M University


DESTRUCTIVE INFLUENCES OF VOLCANOES

- some eruptions can cause great loss of life eg Krakatoa in 1883 (caused great sea waves which drowned 40 000 people in neighbouring islands; Mt. Pelee in 1902 (outpourings of gases killed
30 000 people).

- some eruptions can cause great damage to property eg Vesuvius buried the city of Pompeii with ash

CONSTRUCTIVE INFLUENCES OF VOLCANOES

- some lava outpourings have weathered to give fertile soils eg in Java, the northwestern-part of the Deccan Plateau, and the plains around Etna. These regions are of important agricultural value.

- volcanic activity sometimes results in the formation of precious stones and minerals. These occur in some igneous and metamorphic rocks. eg copper deposits of Butte, USA, nickel deposits of Sudbury, Canada.

- some hot springs are utilized for heating and supplying hot water to buildings in New Zealand and Iceland.


HOT SPRINGS

- superheated water which flows quietly to the surface after being heated by volcanic rocks forms
hot springs


GEYSERS
- superheated water which is thrown out with great force and accompanied by steam is a geyser

Courtesy: National Parks Conservation Association

Volcanoes of the World Database