Topic B - Geologic Processes on Earth

Chapter 6 - ELEMENTS OF GEOLOGY

6-1 The Original Planet Earth

Planet Earth formed out of the original gas and dust that prevailed at the origin of the solar system some 4.6 billion years ago. It is the only known habitable planet so far. This is due to the concurrence of special conditions such as its position with respect to the Sun giving it the right temperature range, the preponderance of necessary gases and a shielding atmosphere that protects it from lethal solar radiation. Early Earth has however not always been so welcoming to life. Initially Earth was rich in silicon, iron and magnesium oxide. Heat trapped inside Earth along with radioactive decay which tends to produce more heat helped heavier elements to sink to the depths leaving lighter elements closer to the surface. Within the first 500 million years, an inner core formed of mostly solid iron surrounded by a molten iron outer core. The mantle formed of rocks that can deform. The thin outer crust that sustains life is composed mostly of silicate rocks. The various natural processes inside and on the surface of Earth make it a dynamic system which has evolved into what we know now. These include the oceans and the continents, the volcanoes that form the mountains and erosion that erodes the landscape, earthquakes that shape the topography and the movement of earth’s crust through the plate tectonics process.

mantle

outer core

crust inner core

35 700 2885 5155 6371 Depth in km

Figure 6-1: Schematics showing the Earth’s solid inner core, liquid outer core, mantle and curst. The crust consists of continental and oceanic crusts. 2

Early Earth was full of toxic gases. Volcanoes released sulfur along with water vapor trapped in the magma buried deep inside. Water condensation formed oceans and the original atmosphere saturated with carbon dioxide. This created the right conditions for basic life to start in the oceans. Primitive algae (called Cyanobacteria) that formed in the oceans absorbed carbon dioxide and water, which along with sunlight produced oxygen and carbohydrates (essential for life) a couple of billion years ago. Earth hosted some life forms for that long and animal life for the last 600 million years. Humans have been around for around one million years. Homosapians have been around for some 200,000 years.

Earth is a “live” planet because of its internal heat engine that drives most natural processes. Planets like Mars and Mercury are dead because they lack such dynamic internal heat mechanism. Heat convection inside Earth drives volcanoes and earthquakes that formed the oceans and the continents. Igneous rocks form when lava cools down. Plate tectonics and volcanoes form mountains. Weathering breaks down rocks into smaller components like pebbles and dirt. Glaciers and rivers transport sediments from mountains to lakes and oceans where sedimentary rocks form. The driving process for earthquakes and the formation of mountains is the movement of tectonic plates on the ocean floor. These are rigid plates that spread apart, collide into each other and slide one under the other at a subduction process.

The continental crust is composed of granitic rocks while the oceanic crust is composed of basaltic rocks. The mantle is composed of peridotite rock. These are solid rocks that can deform and sustain convective flow under extreme heat. The core is composed mostly of iron and nickel. The outer core is liquid whereas the inner core is solid; silicon and oxygen makeup most of the elements in the crust and the other elements make up the rest. Silicon, oxygen and magnesium makeup most of the mantle composition. The only liquid part of the Earth interior is the outer core.

6-2 Plate Tectonics

Outline of the coast on the eastern part of South America is similar to that of West Africa. Identical rock fossils on both sides pointed to the drift of the two continents over time. Actually, the continents of South Africa, South America, Oceania and Antartica formed a huge continent called Gondwana. Two supercontinents, Gondwana in the south and Laurasia in the north formed the original landmass called Pangea that existed from 510 to 180 million years ago. The breakup of Pangea some 180 million years ago gave the five continents that we know now. Continents have drifted apart over time.

Laurasia

equator

Gondwana

Figure 6-2: Representation of the two original supercontinents (Gondwana and Laurasia) that were part of Pangea some 200 million years ago (during the Triassic period).

Figure 6-3: Some 130 million years ago, the west coast of the US was located in Idaho

Changes in the shape of continents over time includes the shrinking of the Pacific Ocean and the widening of the Atlantic Ocean, the continent of Africa running into Europe, Australia crashing into Asia, the widening of the Red Sea and Mount Everest getting taller.

World War II (1939-1945) brought about great deal of technological advances. Sonar radars were developed to detect submerged submarines. Sonar radars were used in the 1950s to map out the ocean floor. A mid-ocean ridge was discovered in the Atlantic Ocean floor formed by the flow of lava. Moreover, it was discovered in the 1960s that the sea floor was spreading at this ridge. Rocks become magnetized when they form from cooled lava at the mid-ocean ridge. Rocks close to the mid-ocean ridge contain stripes of weak and strong magnetic fields as well as the evidence of magnetic field reversals. The fact that the magnetic field pattern was the mirror image on both sides of the ridge center along with the other fact that drilled cores grew older as one moved away from the ridge is evidence of sea floor spreading. The reversal of Earth’s magnetic field occurred multitude of times, the last one going back to some 700,000 years ago. The ocean crust is not older than 200 million years while fossils on continents can be older than 700 million years. Old ocean crust gets covered over through the sea floor spreading process which acts as a conveyor belt laying new geologic material covering old one. Spreading is currently at a rate of a couple of centimeters per year.

The occurrence of earthquakes, volcano eruptions, the formation of mountain ranges and ocean basins are all a consequence of plate tectonics, which became a unifying theme in geology.

6-3 Eras in Geologic History

The geologic time line is divided into four main eons. (1) the Haddean from 4.6 to 3.8 billion years ago, (2) the Archean from 3.8 to 2.5 billion years ago, (3) the Proterozoic from 2.5 billion years ago to 542 million years ago, and (4) the Phanerozoic from 542 million years ago to now.

This last eon, the Phanerozoic, is itself divided into three eras. (1) the Paleozoic from 542 and 251 million years ago, (2) the Mesozoic between 251 and 65 million years ago and (3) the Cenozoic from 65 million years ago to the present. Each era is itself is divided into many periods and each period is divided into epochs. Since these names are used in the literature, it’s nice to get exposed to them at least this once.

Figure 6-4: The geologic time scale is divided into eons, eras, periods and epochs.

The Hadean eon started with the formation of Earth some 4.6 billion years ago. This date is based on dating meteorites and moon rocks. It took some 700 million years for heavy bombardment to stop and for Earth to cool down enough. It took another 375 million years for liquid water to form oceans. During the Archean eon, the atmosphere was rich in carbon dioxide. The oldest known rocks date back to 3.8 billion years ago and is the boundary between the Hadean and Archean eons.

The Proterozoic eon saw an early form of life, which dates back to 2.5 billion years ago and is the boundary between the Archean and Proterozoic eons. An early life form (cyanobacteria) absorbed carbon dioxide and produced oxygen. Stromatolites are sheet-like sedimentary rocks formed from layers of cyanobacteria over past history; these are found in shallow water environments. During the Archean eon, photosynthesis started some 3.5 billion years ago. Then, it took some 1.5 billion years for basic life form to take hold in oceans at first. Multicellular organisms appeared some 1.25 billion years later. Plants and animals became abundant some 375 million years later. The first snowball Earth took place some 2.3 billion years ago during the 6

Proterozoic eon. Another major glaciation period took place between 800 and 635 million years ago. The next (Phanerozoic) eon saw the development of rich eras and periods due to the development of bony and hard shell creatures that left rich fossil evidence.

Life expanded tremendously during the Paleozoic era, which is divided into the Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian and Permian periods. There was an outburst of complex life during the Cambrian period including the appearance of the first vertebrates. This is the referred to as the Cambrian explosion, which happened 530 million years ago. Continents were drifting apart and the climate was getting warmer. The world was full of shallow seas where life thrived at first. Over time, plants and animals made it out of the ocean and started to populate the land. The first vertebrate land animals appeared 380 million years ago. Oxygen levels increased due to photosynthesis. Collision between landmasses created mountains, which diversified the ecosystem.

The Mesozoic Era corresponds to the age of the reptiles including the dinosaurs and is divided into the Triassic, the Jurassic and the Cretaceous periods. Multitudes of life forms dominated the Earth, each better suited to their environment. Corals filled continental margins. Reptiles joined the fish first in the oceans then spread on land. Flowering plants, fruit trees, birds, and mammals enhanced diversity. Dinosaurs dominated the Earth during the Jurassic and Cretaceous periods and went extinct some 65 million years ago in another mass extinction. Mass extinctions are due to natural cataclysms such as supervolcanoes or impact of huge meteorites. A dust and ash cloud may have cooled the atmosphere enough to change drastically the climate. Fossil records document this era reasonably well.

The Cenozoic Era corresponds to the age of the mammals and is divided into the Tertiary and Quaternary periods. Animal populations increased and spread throughout Earth. Intense tectonic activity affected evolutionary trends. For example, Australia drifted alone; became disconnected and trapped some species that evolved in isolation. Humans appeared during the last million years.

The geologic time line is determined using radiodating. Some elements are radioactive and decay over time with a characteristic half-life (time it takes for half the atoms to decay). For example, uranium-235 experiences radioactive decay with a half-life of 700 million years. Note that this is not the fission nuclear reaction of uranium, but its radioactive decay nuclear reaction. Moreover, uranium-234 has a half-life of 233,000 years. Measuring the ratio of the parent to the daughter element content determines the age of the rock formation. This radiodating method is also used to date organic objects using carbon-14, which is also a radioactive isotope with a half-life of 5730 years.

Chapter 7 – PROCESSES ON EARTH

7-1 Glaciation Periods

Five documented glaciation periods (ice ages) have occurred over Earth’s history. These have respectively taken place (1) between 2.4 and 2.1 billion years ago, (2) between 800 and 635 million years ago, (3) between 450 and 420 million years ago, (4) between 360 and 260 million years ago and (5) finally over the past 2.58 million years. Earth iced over completely during the second glacial period giving it the name Snowball Earth.

Possible causes of glaciation are related to astronomical cycles, geologic activity (plate tectonics and volcanoes), as well as changes in atmospheric composition and in ocean currents. For example, an increase in overall carbon dioxide (greenhouse gas) in the atmosphere is associated with a decrease in oxygen leading to global warming which ends glaciation periods. The last ice age ended approximately 10,000 years ago. During that ice age, North America became connected to North Asia through the Behring Sea, which was frozen over. That was when American Indians crossed from Siberia to Alaska and moved down the west coast of the Americas. Ocean levels dropped during glacial periods and rose during warming periods. Ending of the last ice age created the Great Lakes in North America. An increase of ocean levels by as much as 12 cm has taken place over the past 100 years during the current global warming period. Recent melting of arctic ice is amplifying this phenomenon.

Studies of ice cores composition helped map out atmospheric changes over time and better understand past climates. Such studies have helped document pollen count and volcanic eruptions over hundreds of thousands of years. The recent increase of greenhouse gases (carbon dioxide, nitrogen, methane) has contributions from manmade pollution that has been increasing due to the burning of coal and petroleum fuels that feed our energy intensive needs.

ice sheet

North America

Figure 7-1: Ice sheet covering part of North America during a glaciation period.

7-2 Earth’s Internal Heat Engine

Temperature increases with depth inside Earth.

Parts Temperatures

crust Ambient to 600 oC upper mantle 600 oC to 1600 oC

lower mantle 1600 oC to 4000 oC

outer core 4000 oC to 6000 oC

inner core 6000 oC to 6650 oC

Figure 7-2: Temperature increases with depth inside Earth.

If the Earth were of basketball size, the top crust would be as thin as a sheet of paper wrapping it. The continental crust forms the top 10 to 70 km and is granitic in composition while the oceanic crust forms the top 2 to 10 km and is of basaltic composition. The lithosphere is 100 km thick and the asthenosphere is 250 km thick underneath it. These two layers are composed of rocks but rocks in the asthenosphere are softer and can flow over time. The boundary between the lithosphere and the asthenosphere is a layer at 1300 oC temperature. The solid lithosphere “floats” over the asthenosphere. The crust is the top part of the lithosphere. Although the mantle temperature exceeds the melting temperature of the rocks, these remain mostly solid due to the tremendous pressures exerted by the overwhelming lithosphere.

When oceanic lithosphere subducts underneath the continental lithosphere, it heats up when it dives deep into the asthenosphere and causes water to get released into the overlying asthenosphere. Water reduces the melting temperature of the rocks thereby producing magma. The magma is less dense than the surrounding rocks and rises to the surface through convection. Magma becomes lava that flows out of a volcano to form a mountain top.

seafloor spreading trench subduction zone oceanic crust continental crust volcano

lithosphere magma chamber asthenosphere

Figure 7-3: Schematics of the seafloor spreading, subduction and the formation of magma.

Rocks originate as lava in the lithosphere, which is the 100 km thick upper portion of the mantle. The mantle is very hot due to ongoing radioactive decay of elements like uranium, thorium and potassium as well as the original heat left over from the formation of the Earth. Melted rocks rise up inside the mantle, cool down by convection close to the surface then dive back down again in

subduction in a cycle driven by Earth’s internal heat engine. Molten lava escapes at mi-ocean ridges and at hot spots in volcanoes. This geothermal convection plays an essential role in the motion of tectonic plates.

continental crust seafloor spreading volcan oceanic crust subduction zone o

crust lithosphere

magma convection currents asthenosphere

mantle magma

Figure 7-4: Tectonically active mantle and crust, showing two plates moving apart away from the seafloor spreading ridge. The lithosphere consists of the crust and the top part of the upper mantle. Convection heat currents keep rocks in the upper mantle moving. When the lithosphere dives deeper into the mantle below a subduction zone, rocks melt and change into magma.

7-3 Tectonic Plates

The slow movement of molten mantle rocks at the mid-ocean ridges keeps plates moving. There are oceanic plates (formed of basalt) and continental plates (formed of granite). Since basalt is heavier than granite, the oceanic crust dives under the continental crust. Two plates pushing against each other form mountains. A plate diving under another while pushing it creates uplift. The Rocky Mountains formed when the Pacific and the North American plates collided. The collision zone where two plates meet is geologically active with earthquakes such as the San Andreas Fault in California. Oceanic crust sinks underneath the continental crust thereby renewing oceanic crust and making continental crust older. Most mountain ranges are close to continental margins where an oceanic plate meets a continental plate. Mountains form in oceanic crust as well, but the top of such mountains erodes over geologic time due to wave erosion.

Asia North America Europe Asia Atlantic Ocean Pacific Ocean Africa

South America Oceania

Indian Ocean

Figure 7-5: Boundaries of tectonic plates are shown as dashed lines

Figure 7-6: Overhead photo of the San Andreas Fault.

Interaction between plates occurs at the boundaries. The mid-ocean ridge is the center from which plates spread apart by a couple of centimeters every year. Spreading forms new oceanic

crust. Collision between plates leads to thickening of the continental crust and buildup of mountain ranges.

Pacific Ocean Atlantic Ocean Africa South America

moving plate moving plate moving plate magma mantle mid-ocean ridge subduction zone

Figure 7-7: Schematics of the tectonic plates process

Iceland is the only site where plate tectonics collide on land. This is why it’s geologically so active with its geysers and volcanoes. Iceland gets a great deal of its energy needs from geothermal.

Figure 7-8: Geysers in Iceland

7-4 Continental and Oceanic Crusts

The distribution of crust is very different above and below sea level. The part of the continental crust that is above sea level is as high as around 4 km. A small fraction (like mountains) are even higher like Mount Everest that goes up to 8.8 km height; these, however constitute a small fraction of the landmass. Part of the continental crust is under sea level. The continental shelf drops slowly at first then drops fast to a couple of kilometers depth below sea level. The abyssal plains are flat parts of the ocean and delimit the boundary between the continental crust and the oceanic crust. The oceanic crust bottoms at around 5 km depth but can go down to 10 km depth at places. The lowest zone in the ocean is the mid-ocean ridge that goes down to 8 km depth. The deepest part of the ocean is called the Mariana Trench at 10 km depth. Ocean depths are determined using sonars whereby sound waves are emitted from a ship and reflected from the ocean floor. The ship receives the reflected signals and interprets the depth profile. Sound waves travel at 300 m/s in air and travel 3 to 4 times faster in water. Satellites in orbit also gather information about ocean currents and sea floor topography.

30 below sea level

oceanic 25 crust continental crust 20

10 Relative Fraction (%) Relative Fraction

over land 5

0 -10 -8 -6 -4 -2 0 +2 +4

Vertical Dimension (km)

Figure 7-9: Distribution of the continental and oceanic crusts with respect to the sea level

Water fills the lowest part of Earth’s crust. Because of the abundance of water in the oceans, water submerges not only the oceanic crust but also part of the continental crust. Water level has fluctuated over geologic time. It has gone down during glaciations periods and gone up during warm periods.

mid-oceanic ridge continental shelf seamount mountains mountains continental slope trench abyssal plain sea level

Figure 7-10: Schematics of the continental and oceanic profiles

Figure 7-11: Aerial survey of the magnetic field taken over the west coast produces a field orientation map.

7-5 Volcanoes and Earthquakes

Magma accumulates in the upper mantle due to the high temperature gradients and to the convection currents of soft rocks in the asthenosphere. It should be emphasized that magma does not originate in the outer core (which is the only liquid part of Earth’s interior); it comes from the partial melting of rocks in the upper mantle instead. Since magma is of lower density than the surrounding rocks, it rises up toward the surface. Pressure builds up inside a magma chamber until the volcano erupts thereby releasing lava flow and spewing ash that can travel long distances. There are some 500 active volcanoes on continents and thousands in oceans. Volcanoes tend to be located at plate boundaries. Those at convergent plate boundaries tend to erupt more explosively than the ones at divergent plate boundaries due to higher pressure buildup. Volcano ash forms fertile soil. Volcano eruptions are predicted through the monitoring of sudden changes in ground activity.

lava flow

crust magma chamber

upper mantle

lower mantle

Figure 7-12: Pressure builds up in the magma chamber leads to volcanic eruption.

Figure 7-13: Volcano spewing lava that flows down the flanks

Earthquakes happen when pressure builds up at plate boundaries as well. In order to relieve the pressure, a plate slides past another erratically and suddenly. Earth shakes violently for tens of miles around the earthquake’s epicenter. Primary waves (p-waves) travel fast (7 km/sec) and are followed by secondary waves (s-waves) which are slower (3 km/sec); they all originate at the quake epicenter. The time difference between these two types of waves recorded at different spots on the surface of Earth points to the location of the epicenter deep down in the mantle. S- waves are very destructive because they involve oscillating shear stresses. Epicenters can be kilometers to tens of kilometers deep.

A B

epicenter C

Figure 7-14: Recording from various seismographs point to the epicenter location by triangulation

Earthquakes are classified following the Richter magnitude scale, which varies logarithmically. The US Geological Survey (USGS) operates a network of seismometers spread out in order to detect even minor tremors. Despite a great deal of progress, earthquakes cannot be forecast precisely. The best predictive ability is achieved at the San Andreas Fault in California where a warning to the public is sent out when there is at least 30 % chance that a magnitude 6 earthquake may happen over the next 3 days. A short term warning is, however, not possible.

Figure 7-15: Seismometer

Figure 7-16: Seismograph showing earth shaking waves

earthquake

s-waves

p-waves core

mantle

Figure 7-17: Earthquake produces seismic waves that propagate through Earth. P-waves can cross the core while s-waves are absorbed and reflected from the core.

Earthquakes are highly destructive especially in areas where earthquake-proof building codes have not been followed. In 1976 an earthquake in China caused some 240,000 deaths, another one in Sumatra in 2004 caused 228,000 deaths while more recently (2010) an earthquake in Haiti caused some 316,000 deaths. Strong earthquakes can engender tsunamis when their epicenter is at sea. A plate sliding abruptly vertically at a fault line to release stress displaces great deal of water that forms a tsunami wave that causes devastation when it reaches coastal regions. A tsunami in the Indian Ocean caused the death of some 300,000 people in 2004.

Earthquake waves are used as diagnostic probes to investigate the composition of Earth’s core and mantle. Earth-shaking devices are used to investigate rock structures and formations deep underneath the ground. This is performed routinely to prospect for oil deposits.

Figure 7-18: Earth-shaking device

Chapter 8 – THE ROCK CYCLE

8-1 Types of Rocks

Silicon is the second most abundant element in Earth’s crust after oxygen. Sand consists mostly of silica (silicon dioxide) in the form of quartz. Over 90 % of the crust is composed of silicates. Rock types are classified according to a set of criteria. These include texture, color, hardness, mineral composition, density and crystalline structure. There are three types of rocks, igneous, sedimentary and metamorphic depending on the way they form.

Igneous rocks form from the cooling of lava. This takes place when magma from Earth’s mantle reaches the crust. These form either below the ground surface (called plutonic) or on the surface (called volcanic). Basalt and granite are examples of plutonic and volcanic rocks respectively. Basalt has dark color and contains low silicon content while granite has light color and contains high silicon content. Basalt forms most of the oceanic crust while granite forms most of the continental crust. Granitic rocks on continents are of lower density than basaltic ones on oceanic floors. Igneous rocks are harder than sedimentary rocks.

Figure 8-1: Pictures of basalt (left) and granite (right) igneous rocks.

Sedimentary rocks form by the sedimentation of minerals and organisms in oceans (mostly) but also in riverbeds, lakes and other bodies of water. Minerals dissolve in water through weathering and erosion before settling down. Mechanical and chemical weathering break rocks into smaller components while erosion carries minerals to lower grounds. Erosion takes many forms including by water, by wind and through the movement of glaciers. Sedimentary rocks formed through the deposition of horizontal layers under the influence of gravity. Examples of sedimentary rocks are sandstone and limestone. Sandstone has coarse grain and contains mainly quartz while limestone has fine grain and fizzes (releases carbon dioxide) when exposed to acidic environments (like rain). Oil shale is also a sedimentary rock that contains a large fraction of organic material in the form of kerogen. Sandstones are porous while shale stones are not.

Figure 8-2: Pictures of sandstone (left), limestone (middle) and oil shale (right) rocks which are sedimentary rocks. Sandstone and limestone are porous rocks while shale rocks are non-porous.

Metamorphic rocks form by subjecting igneous and sedimentary rocks to geologic pressure and temperature treatments. This results in changes in the physical properties and chemical composition of the rocks. This process requires pressures found underground such as when mountains form through the plate tectonics process. Slate and marble are examples of metamorphic rocks. Slate has dark color and flaky texture while marble has light color and hard texture.

Figure 8-3: Pictures of slate (left) and marble (right) rocks which are metamorphic rocks.

8-2 Mountains

Tectonic motion and volcanoes create mountains and erosion grinds them down in a perpetual cycle on a geological timescale. Erosion by ocean waves pushes the continental coast back and truncates the top of small islands. Mountains have finite lifespan. The buildup and erosion of mountains is a cycle that could take rock material formed on the ocean floor to the top of mountains on continents and back. For example, marine limestone can be found on Mount Everest. These sedimentary rocks formed on ocean floors some 450 million years ago. Plate movement can cause considerable deformation of rock layers and contribute tremendous uplift of mountains.

Mountains start young and are high at the time they form. They become deeply eroded with age and end up being buried. Young mountains are found close to the edge of continents where tectonic plates come together while old eroded mountains are found in the middle of continents far away from plate boundaries. Plate boundaries involve subduction whereby a plate dives underneath another. This forms mountains that rise up as well as forms deep roots under them. The crust under mountains can be as much as 70 km thick. Higher mountains have thicker crust. Continental crust undergoes constant deformation.

mountain buildup

rock folding rock faulting continent ocean magma sliding plate

lithosphere Converging plate boundary asthenosphere

Figure 8-4: The process of mountain formation

Plate tectonics generate lava at mid-ocean ridges. Ocean floor spreading pushes the newly formed rocks to form the oceanic crust which becomes continental crust when uplifted. Older mountains are found at the middle of tectonic plates (middle of continents) and younger ones are found at plate boundaries (close to coasts). Erosion and weathering tend to grind down mountains and level the landscape. Rivers move ruble down to oceans where sedimentation takes place. Rocks have a cycle from their origin in oceans to their formation of mountains down to their endpoint in ocean of order of tens to hundreds of millions of years. This is the timescale of formation and erosion of mountains.

8-3 Layering Features

Sedimentary rocks could take tens of millions of years to form. Sedimentation layers contain clues about the sequence of deposition and of time history of the site. Sedimentary layers are horizontal at the time of their formation since gravity is the driving mechanism. Older layers are deeper than younger ones. Lava buildup and internal pressure can push rock formations upward. A horizontal layer is younger than any vertical one that it truncates. Vertical layers are obtained when rocks fold and meander.

Recent sedimentary rocks 30 million years Older sedimentary rocks between 40 and 50 million years

Older than 50 million years Older rocks 40 million years

50 million years Oldest rocks Older than 50 million years

Figure 8-5: Buildup of rock layers over tens of millions of years. The top horizontal layer is younger than the vertical ones.

Continuous movement of tectonic plates deforms the horizontal sedimentation layering into folded layers of arbitrary orientation. A fault is observed when two layers of different orientations meet. When younger sedimentary rocks cover older igneous ones, the folding pattern is broken. A break in the stratified record is referred to as a disconformity. The Earth’s crust is constantly changing due to uplift, deformation and erosion. For example, the presence of glassy rocks is evidence of the rapid cooling of lava. These are clues to infer past geologic history. Based on such observations, geologists can read a terrain or landscape.

The Colorado River has carved the Grand Canyon one mile deep. The canyon cliffs are full of landscape features such as folds and faults uncovering deep layers. The dating of old rocks is performed through the analysis of fossil remains and through radiometric dating. The age of fossils present must be consistent with that of the surrounding rocks. The Grand Canyon provides a record of geologic history over billions of years. The Grand Canyon itself was, however carved out by the Colorado River over the past 6 million years.

Figure: 8-6 Photo of the Grand Canyon showing revealed rock layers

limestone Permian (about 270 million years ago) sandstone shale

Pennsylvanian (about 310 million years ago)

Mississippian (about 340 million years ago) limestone Devonian (about 380 million years ago)

shale Cambrian (about 500 million years ago) sandstone schist Pre-Cambrian (older than 2.5 billion years) granite

Figure 8-7: Rock formation layers in the Grand Canyon and the time of their formation

8-4 Weathering and Erosion

Water streams carry rocks and pebbles downstream; this causes mechanical weathering. They also dissolve minerals that become sediments. Dissolved salts are carried to the sea making it salty. Rivers carve meandering channels, canyons and valleys continuously changing the landscape. Rainfall feeds water streams and contributes to water erosion.

Figure 8-8: Water erosion

Rainwater gets absorbed in soil and seeps through porous rocks to form aquifers and channels that feed the groundwater table. The saturated part of the soil underground can run as deep as hundreds of meters. It communicates with surface streams and water wells.

unsaturated water zone surface stream well

water table

saturated water zone

Figure 8-9: Groundwater reservoirs.

Caves are testament to the persistent erosive action of water on limestone over long periods. The Luray Caverns in Virginia is a vast network of caves open to the public with their stalactite and stalagmite formations. Rainwater becomes acidic when it reacts with carbon dioxide. This acidic rainwater running underground dissolves limestone to form the caverns.

Figure 9-10: The Luray Caverns in Virginia

Erosion takes place in oceans and along the coast as well. The rotation of Earth creates circular ocean currents. The Gulf Stream is a current in the Atlantic Ocean that brings equatorial water to warm the eastern part of the US coast. Attraction by the Moon creates regular tides, which cause wave erosion.

Downslope mass movement of rocks and rubble occurs when the downward pressure buildup and underground water saturation level become overwhelming. This triggers landslides that level the terrain. Over time, erosion tends to flatten landscape features.

Wind causes erosion on land and in deserts. Local wind patterns carry dirt and sand particles and transports soil and sediments. Desertification is caused by climate change. Ancient rock paintings in the Sahara Desert attest that this region used to be lush with vegetation some 10,000 years ago. Global wind patterns have been documented around the globe.

Figure 8-11: Sandstorms reduce visibility and can last for days

Warm air rises close to the equator and cool dry air descends around the tropics. This along with the Coriolis effect of Earth’s rotation create regular wind patterns that carry clouds and affect global weather. Tremendous levels of wind are involved in tropical storms such as hurricanes and typhoons that start in oceans and become highly destructive when they move to land. Hurricanes engender flooding as well.

The downward movement of glaciers from mountaintops to the coastline is another form of terrain erosion. Glaciers cover as much as 10 % of Earth’s surface and dominate the Arctic and Antarctic circles. They carry as much as three fourth of the fresh water found on Earth. Solid ice as well as rocks that get carried down scrape the bottom of glaciers thereby carving deep channel beds.

Figure 8-12: Advancing glaciers erode the landscape

The Greenland ice sheet is as much as 3 km thick and never melts. It constitutes an undisturbed record of the climate on Earth for millions of years. Ice core samples helped in the understanding of past climates including glaciation periods and volcanic eruptions history. Many glaciations have occurred during the Pleistocene epoch (over the past 2 million years). Glaciations are due to long-term climate variation caused by continental drifts as well as changes in Earth’s astronomical factors such as slight variations in Earth’s eccentricity, tilt axis and wobble on its axis. Atmospheric factors include a noted decrease in the level of greenhouse gases (such as carbon dioxide and methane) during glaciation periods and an overall increase of these gases during global warming periods.

8-5 The Geology of Fossil Fuels

Fossil fuels like petroleum, gas and coal form deep underground. Geologic aspects are covered here.

The formation of petroleum involves the presence of buried organic matter from dead organisms. These organisms included plankton and bacteria originally deposited (along with clays) in oceans. These organisms were rapidly buried before oxidation took place. Slow chemical reactions transformed the deeply buried organic material into hydrocarbons that form petroleum.

This formed oil shale rocks buried at depths of 2 to 4 km. Heating at these depths broke the organic matter into kerogen. Formation temperatures between 90 and 150 oC formed oil and gas while temperatures higher than 150 oC formed gas only. Compaction and geologic activity forced oil and natural gas to migrate into reservoir rocks where they were trapped. It is more difficult to extract oil and gas from shale rocks than from oil reservoirs. These reservoirs are found in permeable (i.e., porous) rocks like sandstone or limestone surrounded by impermeable (non- porous) rocks like shale or mudstone.

well platform

gas

oil

water

Figure 8-13: Fossil fuels migrate to pockets trapped between impermeable (non-porous) rocks like shale or mudstone and permeable (porous) rocks like sandstone or limestone.

Most petroleum was formed in rocks from the Cenozoic era (58 %) with some from the Mesozoic era (27 %) and the Paleozoic era (15 %). Recall that the Cenozoic is the most recent era of the geologic past (last 65 million years) while Mesozoic goes back to earlier times (250 to 65 million years) and the Paleozoic is the oldest era (from 550 million to 250 million years ago).

Exploring for oil and natural gas involves generating seismic waves using earth-shaking devices or explosives and recording the seismic waves reflected from different rock formation interfaces back to the ground surface. Deep underground strata can be reconstructed and analyzed for potential oil reservoirs. Once such potential reservoirs are located, drilling is performed to access these reservoirs. When an oil reservoir is found, drilling stops and steel casing is used to line the well and prevent its collapse. The natural pressure inside an oil reservoir is usually sufficient to pump out oil without added pressurization. This permits the extraction of some 30 % of the well reserve. When the bottled pressure is too weak, artificial pressurization methods are used. These consist in pumping-in fluids like steam or carbon dioxide to increase well pressure. Fracking consists in pumping other chemicals like small amounts of surfactants.

Coal is a sedimentary/metamorphic rock produced in swamps where there was huge accumulation of plant matter. This was from the Devonian to the Permian periods of geologic history between 400 and 250 million years ago. Most continents were close to the equator and covered with shallow seas, which favored vegetation growth and rapid burial. As plants died, they formed peat that became covered and compacted to yield lignite. Further heating and compaction produced bituminous coal and high-grade coal called anthracite.

8-6 Summary of the Main Points

Planet Earth is geologically active. Its internal heat engine drives many processes that affect the geography, the topography of the continents and the local landscape. A flat landscape would not be rich in life diversity and renewal. Internal heat from the mantle escapes at mid-ocean ridges where lava cools down to form oceanic crust. Mid-ocean floor spreading (about a couple of centimeters per year) rolls out the new crust like a conveyor belt in both directions. Adjacent tectonic plates interact strongly. They push each other creating uplift that forms mountains. The lithosphere (top part of the mantle) from one plate dives underneath another plate (subducts) at plate boundaries. When the diving part reaches deep into the hot mantle, rocks soften then melt forming magma. Magma has lower density then rocks in the crust and therefore rises up. When the rise of magma is blocked, it creates bottled pressure that gets released either as earthquakes at fault lines or as lava in volcano eruptions.

The tectonic plates process builds up mountains. Oceanic crust is constantly getting renewed and is therefore younger (less than 200 million years old). The continental crust is formed through the rolling of oceanic crust and its uplift. The continental crust is therefore much older (700 million years). Older mountains are found at the middle of continents while younger ones are found at plate boundaries at the edge of continents. Geologic processes build mountains up and erosion and weathering grind them down in a long term (millions of years) cycle. Lava at a mid- ocean ridge forms the oceanic crust in the middle of the ocean where the rock cycle starts. Weathering breaks down rocks on mountains. Erosion carries rubble down to the ocean and sedimentation finishes this cycle.

Natural disasters include earthquakes, volcanoes and tsunamis. These reap havoc on human civilization. Tropical storms like hurricanes and typhoons affect many regions of the world. Flooding and landslides add to the regular challenging threats. Mitigation to these challenges include enforcement of earthquake-proof building codes to avoid catastrophic collapse of buildings, the development of evacuation plans in case of emergency, the construction of dykes to avoid major flooding, active involvement of regulating agencies to discourage construction too close to unstable coastal areas, downstream from dams, on the slopes of volcanoes, etc.

Online Material

Search Wikipedia (http://www.wikipedia.org) using keywords like “Plate Tectonics”, “Volcanoes”, “Earthquakes”, “Rocks”, “Geologic Time”, etc.

Watch the videos at (http://www.learner.org), then Science, College/Adult, then Earth Revealed.

Check the website at (http://www.pbs.org/nova/). Watch the videos about Planet Earth

Lecture notes for course found online at http://www.tulane.edu/~sanelson/eens1110/

Check the website (http://education.usgs.gov/videos.html). Watch the video titled “The Living Rock: Earth’s Continental Crust”

Questions

1.Is Earth’s mantle liquid?

2. What is the temperature at the boundary between the lithosphere and the asthenosphere?.

3. What is the circumference of Earth in km?

4. Does lava come from Earth’s liquid core?

5. How do we know the composition of Earth’s core?

6. What are tectonic plates?

7. What is a subduction zone?

8. What is an evidence of continental drift?

9. What are two mechanisms that play in mountain formation?

10. When did the Cambrian explosion happen?

Answers

1. No. It’s solid rock.

2. The temperature at the boundary between the lithosphere and the asthenosphere is around 1300 oC.

3. The circumference of Earth is 40,000 km.

4. No! Earth’s lava does not come from Earth’s liquid core. It comes from the mantle.

5. Information is known about Earth’s core through the reading of seismographs around the world during earthquakes.

6. Tectonic plates are moving slabs on Earth’s crust.

7. A subduction zone is the diving of one tectonic plate underneath another.

8. An evidence of continental drift is the spreading at mid-ocean ridges.

9. Two mechanisms that play in mountain formation are tectonic plate collisions and volcano eruptions.

10. The Cambrian explosion took place some 530 million years ago.

Additional Questions

11. How could one tell that continents drifted over time? By how much per year.

12. What causes tectonic plates to move?

13. How old is the bottom layer on the Grand Canyon cliff? How deep is it?

14. What are the three types of rocks? What is the difference?

15. Name the common type of volcanic rock on the oceanic crust? How about on the continental crust? Where do both come from?

16. Is the oceanic crust older than the continental one? Why?

17. What is the main effect of erosion on the landscape? Could erosion make mountains taller?

18. Name a well-known ocean current that warms up the east coast of the US.

19. Could earthquakes be predicted reliably?

20. Could volcanic eruptions be predicted? How far in advance?

Watch Online Videos Questions

Q1. Google USGS and “The Living Rock”. Watch this video. Answer the following questions: How far back did rocks form on Earth (08:32)? Tufa towers are made of what mineral on the shore of in Mono Lake in California (15:05)?

Q2. Go to http://www.learner.org. Select Teacher Resources, then Science, then College/Adult, then Earth Revealed. Watch the video “3. Earth’s Interior”. Answer the following questions: How many earthquakes are recorded per year? How many seismic stations monitor seismic activity?

Q3. Go to http://www.learner.org. Select Teacher Resources, then Science, then College/Adult, then Earth Revealed. Watch the video “5. The Birth of a Theory”. Answer the following questions: What is the name of the scientist that put the theory of plate tectonics forward in 1912? What causes reversal of the magnetization of rocks on the sea floor?

Q4. Go to http://www.learner.org. Select Teacher Resources, then Science, then College/Adult, then Earth Revealed. Watch the video “7. Mountain Building and the Growth of Continents”. Answer the following questions: What is a craton? What is the Greenstone Belt?

Q5. Go to http://www.learner.org. Select Teacher Resources, then Science, then College/Adult, then Earth Revealed. Watch the video “8. Earth’s Structures”. Answer the following questions: How are the two angles named that determine the orientation of geologic layers? What is an unconformity?

Q6. Go to http://www.learner.org. Select Teacher Resources, then Science, then College/Adult, then Earth Revealed. Watch the video “11. Evolution Through Time”. Answer the following questions: How does the jellyfish absorb oxygen (09:48)? When did animals develop a hard shell (10:07)?

Q7. Go to http://www.learner.org. Select Teacher Resources, then Science, then College/Adult, then Earth Revealed. Watch the video “12. Minerals”. Answer the following questions: How many minerals have been identified? Does quartz dissolve in acids?

Q8. Go to http://www.learner.org. Select Teacher Resources, then Science, then College/Adult, then Earth Revealed. Watch the video “13. Volcanism”. Answer the following questions: Volcanic ash could remain suspended in the atmosphere for how long? The Capital of Iceland is heated using what form of energy?

Q9. Go to http://www.learner.org. Select Teacher Resources, then Science, then College/Adult, then Earth Revealed. Watch the video “15. Weathering and Soils”. Answer the following questions: Is weathering faster in cold or warm climate? Weathering produces what?

Q10. Go to http://www.learner.org. Select Teacher Resources, then Science, then College/Adult, then Earth Revealed. Watch the video “19. Running Water 1: Rivers, Erosion and Deposition”. Egypt is referred as the (what?) of the Nile? Answer the following questions: Dykes are constructed for what purpose?

Q11. Go to http://www.learner.org. Select Teacher Resources, then Science, then College/Adult, then Earth Revealed. Watch the video “25. Living with Earth – Part 1”. Answer the following questions: During the 1989 San Francisco earthquake, the ground was shaking for how long? What was the dollar amount of the damage during that earthquake?