Lecture02-EngineeringGeology

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Lecture #02

A brief view

Engineering Geology

© Geotechnical Engineering, by L. Prieto -Portar, August 2004

View of the Earth and the Moon from Apollo 8, December 1968.

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Our planet is over 5,000 million years old, formed from gaseous origins that slowly cooled to the partly solid Earth of today. Originally 99% H and He and 1% all other elements, today it is the reverse. Aggregation started electrosta-tically and then gravity took over with heavier elements forming an early core.

About 4,000 million years ago a space object about the size of Mars impacted the young Earth, and tore out a terrestrial chunk of mantle into an Earth orbit, to become our Moon. The Moon’s low density is similar to our mantle. The Moon has been confirmed to be similar geologically to the Earth.

Only 65 million years ago, a smaller meteor, 16 km in diameter, struck our Earth again. The ensuing clouds of dust killed most of the vegetation, and lead to the extinction of dinosaurs.

An imaginary view of a meteorite hurling towards the Earth.

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Dimensions (in kilometers) of the three main components of the planet Earth.

The crust has a thickness of about 10 to 40 km. It is mostly made up of light silicates with an average ? = 3 g/cm 3.

The upper and lower mantle is about 3,000 km thick, and is made up of metallic silicates and sulfides with ? = 3.5 to 6 g/cm 3.

The liquid core is about 2,100 km thick, and is made up primarily of liquid Fe and Ni, with a ? = 10 g/cm 3. Finally, the solid inner corehas a diameter of 1,300 km and a ?= 13 g/cm 3.

Richard Oldham of Great Britain was the first physicist to study seis-mology. A few years later, the Cro-atian physicist Andrija Mohorovicicuse it to map the boundary between the mantle and the crust (now known as the “Moho” discontinuity.

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About 4,600 million years ago, a solid crust formed with temperatures in excess of 100ºC. There was no rain, but the Earth was shrouded in heavy clouds which was our proto-atmosphere.

Cooling below 100ºC brought rain and initiated weathering. But this occurred only 250 million years ago! There was little oxygen, since today’s atmosphere is largely a product of photosynthesis.

These early soils are called protosoils. Some are still around in Africa and northern Europe as granites, 3,500 million years old.

Sun’s core as a comparison to the Earth’s.

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Sir Edward Bullard of Cambridge University prepared this 1965 computer fit of the single Pangeacontinent.

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India first made contact with the southern coast of Asia about 55 million years ago. Since then it has advanced a further 2000 kilometers.

In doing so, it has squeezed and thickened the continental crust in a region which is now severalthousand kilometers wide, pushing up the mountains.


Geology is the study of rocks, which are divided according to their origin into three types:igneous, sedimentary and metamorphic.

The ancestor rock is igneous (from Latin ignis = “fire ”). They form on the Earth surface when the hot molten magma from the mantle reaches close to the surface, or actually breaks out and cools.

Depending where the magma cools, it forms a batholith , a laccolith dome, a sill, dikes, lopolith , a volcano, lava flows or ash deposits.

The crystals in a batholith are usually very large, because cooling has taken place slowly during several million years. In contrast, lava cools in a matter of days, resulting in tiny crystals, so small that some rocks resemble glass (for example, obsidian).

Quarks + gluons

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A geologist’s view of early Earth, 3.5 billion years ago, showing intense volcanic activity, and primitive life-forms that built mound-like structures called stromatolites in shallow water or near bubbling hot springs.

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The large yellow crystals are the uranium ore called autunite.

Uranium is found in minute quantities in many rocks. Geologists can use its predictable rate of decay as a clock to date the age of the rock.

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Volcanic cones create gap in Djibouti.

This satellite photo shows the location of Mount Etna, Europe’s highest active volcano, at elevation 11,000 feet MSL. It is located at the NE corner of the island of Sicily.

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A closer view from a Landsat satellite of Mount Etna when it started to erupt in July, 2000. The plume of ash seen in this photo reached across the Mediterranean Sea into Libya (National Geographic).

These shots were taken on 17 July, 2000 showing the ash plume and jets of lava emitted by Mount Etna. Borings taken at the flanks of the volcano show eruption deposits as old as 15,000. Plato sailed from Greece in 387 B.C. to see its eruptions, and Odysseus claimed to have been attacked by a Cyclops with lava boulders.

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Eruption of the Kilauea volcano in Hawaii (1984).


Devil’s Tower in Wyoming shows the columnar jointing of basalt in a vo lcanic neck.

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A piece of obsidian about 10 cm across. Although it has the same mineralogy as a gabbro or a granite, its rapid cooling in a volcanic environment has turned it into a glass.

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Pyrite

Magnetite

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Calcite

Orthoclase feldspar

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Igneous rocks at the Earth ’s surface are subjected to weathering. This process is complex, and may consist of one or more of the following forces acting simultaneously:

1) Gravity,2) Ice and water (freeze-thaw cycle),3) Wind,4) Glaciation,5) Tectonic action,6) Organic action,7) Chemical (hydration, hydrolisis, disolution and oxidation).

Weathering breaks down the igneous rocks into soils. Depending on the particle size and other properties, soils are classified as gravels (G), sands (S), silts (M) or clays (C).

Soils may either (1) remain in place to form a residual soil, or (2) be transported away and deposited in lakes, oceans or flood plains.

These deposits harden into a new rock, called sedimentary (from Latin sedimentum = “settling”). These rocks display the characteristics of its parent soil and the weathering actions.

Finally, sedimentary rock may be subjected to heat and/or pressures from Earth forces, and reform into newer rocks, called metamorphic (from Greek, meta = “change ” and morphe = “form”). The remelting of metamorphic rocks at great depths reforms the material back to an igneous rock, and the cycle continues forever (“All is flux”, Heraclitus, circa 505 BC).


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Conglomerates at Montserrat, Catalunya, Spain.

Layered limestone at Trenton Falls, New York.

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Dolomite, a sedimentary rock, is the main formation of the Alps between northeastern Italy and Austria.

This formation was formed from 60 million years ago when the European and African continental plates collided.

Himalayas, at Zanskar, rise over 8,000 meters above sea level.

These mountains have been pushed up as the Indian island ploughed northward into Asia.

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Cross bedding of the Navajo sandstones, Utah.

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Precambrian conglomerate from Great Slave district, Canada.

A Jurassic era sedimentary rock formed 150 million years ago from the fossilized remains of ammonite shells.

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Gneiss of South Island, New Zealand.

Banded gneiss.

The original homogeneous granite has been banded by metamorphism. The light colored bands are quartz and potassium feldspars. The darker bands are Biotite and other ferromagnesian minerals.

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A thin section of a biotite gneiss under a microscope.

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Lecture02-EngineeringGeology