geologic processes on...

GEOLOGIC PROCESSES ON EARTH

OUTLINE1. Elements of Geology

The Original Planet Earth

Plate Tectonics

Eras in Geologic History

2. Processes on Earth

Glaciation Periods

Earth’s Internal Heat Engine

Tectonic Plates

Continental and Oceanic Crusts

Volcanoes and Earthquakes

3. The Rock Cycle

Types of Rocks

Mountains

Layering Features

Weathering and Erosion

The Geology of Fossil Fuels

1 – Elements of Geology

The Original Planet Earth

- Earth is 4.6 billion years old- Only known habitable planet

- Right temperature, atmosphere,necessary gases for life, andmagnetic shield

- Heat trapped inside Earth - Heavier elements (iron) in core,

lighter elements (rocks) closer to the surface

- Thin outer crust (silicates)35 700 2885 5155 6371 Depth in km

inner core

mantle

crust

outer core

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.

Early Earth

- Early Earth was full of toxic gases. Volcanoes released sulfur.

- Water vapor trapped in magma got released to form oceans.

- Original atmosphere was saturated with carbon dioxide.

- Primitive algae absorbed carbon dioxide and water and produced oxygen and carbohydrates.

- Favorable conditions for basic life to start in oceans 2 to 3 billion years ago.

- Earth hosted animal life form for 600 million years.

- Humans have been around for less than 1 million years.

Earth’s Heat Engine

- Early Earth has an internal heat engine (original heat plus radioactive decay)- Planets Mars and Mercury lack such heat engine.

- Heat convection inside Earth drives volcanoes, earthquakes and plate movement that formed the oceans and continents.- Plate tectonics and volcanoes form mountains.

- Igneous rocks form when lava cools down.- Weathering and erosion grind mountains down and transport sediments to the ocean forming sedimentary rocks.

- Tectonic plates move (spread apart, collide and slide under each other).- The continental crust is composed of granitic rocks. The oceanic crust is composed of basaltic rocks.

Plate Tectonics

- Outline of the coasts of south America and West Africa are similar.

- Continents drift over time.

- Pangea supercontinent before 180 million years ago. Broke up 180 million years ago giving 5 continents.

Laurasia

Gondwana

equator

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

Plate Tectonics

- Continents drifted apart as well as into each other over time.- World War II brought sonar technology.- In the 1950s, ocean floor got mapped out.- Mid-ocean ridge was discovered. - Sea floor spreading at this ridge- Evidence of magnetic field reversal.- Oceanic crust is less than 200 million years old.- Continental crust can be older than 700 million years.

Figure 6-3: Some 100 million years ago, the western part of the US was very different.

Eras in Geologic History

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

Era of the vertebrates

Era of the reptiles and birds

Era of the mammals

Early Earth History

- Radiodating over geologic time. Uranium-235 decays with half-life of 700 million years. Uranium-234 has half-life of 233,000 years. Carbon-14 has half life of 5730 years.

- It took 700 million years for heavy bombardment to stop and for Earth to cool down enough.

- It took 375 million years for liquid water to form oceans.

- During the Archean eon, the atmosphere was rich in carbon dioxide. The Proterozoic eon saw an early form of life (cyanobacteria) which absorbed carbon dioxide and produced oxygen.

- Photosynthesis started some 3.5 billion years ago.

- Basic life form started in the ocean. Multicellular organisms appeared. Then plants and animals.

Early Earth History

- Life expanded tremendously during the Paleozoic era. Outburst of life during the Cambrian period. Age of the vertebrates land animals.

- Oxygen levels increased due to photosynthesis. Ecosystem started diversifying. Appearance of flowering plants, fruit trees, birds and mammals.

- Mesosoic era corresponds to the age of the reptiles including the dinosaurs. Jurassic and Cretaceous periods. Dinosaurs disappeared at the end of the Cretaceous period (some 65 million years ago).- Dinosaurs got extinct due to a supervolcano eruption or due to the impact of a huge meteorite.

- The Cenozoic era corresponds to the age of the mammals. Humans appeared during the last million years.

- Two major extinctions at the end of the Paleozoic and Mesosoic eras.

2 – Processes on Earth

Glaciation Periods

- 5 glaciation periods are known.

- 700 million years ago. Snowball Earth.

- Last glaciation period over past 2.6 million years.

- Glaciations are caused by astronomical cycles, geologic activity, change in the levels of green house gases.

- The last ice age ended some 10,000 year ago.

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

ice sheet

North America


- If Earth were of the size of a basketball,Its crust would be a thin sheet of paper.

- Continental crust is 10 – 70 km thick. Granitic in composition.

- Oceanic crust is 2 – 10 km thick. Basaltic in composition.

crustupper mantle

lower mantle

outer core

inner core

Ambient to 600 oC

4000 oC to 6000 oC

6000 oC to 6650 oC

Temperatures

600 oC to 1600 oC

1600 oC to 4000 oC

Parts

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


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

- Lithosphere is 100 km thick. Asthenosphere is 250 km thick.

- Oceanic crust subducts under the continental crust. Heats up when it dives into the asthenosphere.

- Magma is produced.

oceanic crust

seafloor spreading trench

volcanosubduction zone

lithosphere

asthenosphere

magma chamber

continental crust


- Mantle is very hot due to radioactive decay of uranium, thorium and potassium and due to original heat.- Rocks melt into lava.- Molten lava escapes at mid-ocean ridges and from volcanoes.- Geothermal convection drives the tectonic plates motion.

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.

oceanic crust

continental crust

crust

volcano

subduction zone

seafloor spreading

magma

asthenosphere

mantle

lithosphere

magmaconvection currents

Tectonic Plates

- Plates pushing into each otherform mountains.

- Plate diving under another creates uplift.

- Plate collision zone is prone toearthquakes. Indian Ocean

Africa

AsiaEurope

Atlantic

South AmericaOceania

Pacific Ocean

North AmericaAsia

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

Tectonic Plates

- Pacific plate collides against North American plate.

- San Andreas fault in California.

- Seafloor spreading forms new oceanic crust.

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

Tectonic Plates

Figure 7-7: Schematics of the tectonic plates process

South America

Africa

Convergent Boundary

Divergent Boundary

Asthenosphere

Pacific Ocean

Atlantic Ocean

Lithosphere

UpwellingSinkinglithosphere

Tectonic Plates

- Iceland is the site of a “mid-ocean ridge” on land.

- Iceland is famous for its geysers.

- Lots of geothermal energy.

Figure 7-8: Geysers in Iceland


- Continental crust is 4 km high.

- Mountains are higher. MountEverest is 8.8 km high.

- Part of the continental crust is under sea level.

0

5

10

15

20

25

30

Re

lati

ve

Fra

cti

on

(%

)

Vertical Dimension (km)

-10 +40-4-6-8 -2 +2

over land

below sea level

oceanic

crust

continental

crust

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


- The continental crust dropsslowly at first. Then drops fast to 2-3 km at the continental slope.

- The abyssal plains are flat and 5 km deep.

- The mid-ocean trench goes down to 8 km depth.

- Deepest part of the ocean is the Mariana trench at 10 km depth.

continental shelf

abyssal plain

continental slope

mid-oceanic ridge

trench

sea levelmountains

seamount

mountains

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


- Magnetic field lines fromrocks changes at the plate boundary.

- Conveyor belt from mid-ocean ridge.

- Continental crust is much older than oceanic crust.

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

Volcanoes and Earthquakes

- Magma accumulates in the upper mantledue to high temperatures and to convectioncurrents in the asthenosphere.

- Magma rises up since it has lower density.

- Pressure builds up in the magma chambertill the volcano erupts.

upper mantle

crust magma chamber

lava flow

lower mantle

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

Volcanoes

- There are 500 active volcanoes on continents. Thousands in oceans.

- Volcanoes tend to be locatedat plate boundaries.

- Volcano eruptions can bepredicted.

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

Earthquakes

- Earthquakes happen at plate boundaries.

- Earth shakes for thousands of kilometers.

- Epicenters can be km to tens of km deep.

A

B

Cepicenter

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

Earthquakes

- The USGS operates a network of seismometers.

- Richter scale to classify earthquakes.

- Earthquakes cannot be forecast.

Figure 7-15: Seismometer

Earthquakes

- At the San Andreas fault, a warning issent out when there is at least 30 % chance that a magnitude 6 earthquakemay happen over the next 3 days.

Figure 7-16: Seismograph showing earth shaking waves

Earthquakes

- 1976 earthquake in China caused 240,000 deaths- 2004 earthquake in Indonesia caused 228,000 deaths.- 2010 earthquake in Haiti caused 316,000 deaths.

- 2010 Earthquake in Indian Ocean caused Tsunamiand resulted in 300,000 deaths.

p-waves

mantle

core

earthquake

s-waves

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. This helps probe the composition of the core and mantle.

Earthquakes

- The Earth-shaking device helpsinvestigate rock structures and formationsdeep underneath the ground.

- This is performed routinely to prospect for oil deposits for instance.

Figure 7-18: Earth-shaking device

3 – The Rock Cycle

Types of Rocks - Igneous

- Silicon is the most abundant element in Earth’s crust after oxygen.

- There are 3 types of rocks:igneous, sedimentary andmetamorphic.

- Igneous rocks form when lavacools down.

- Basalt forms most of oceanic crust while granite formsmost of continental crust.

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

Types of Rocks -Sedimentary

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.

- Sedimentary rocks form fromthe sedimentation of minerals.

- Sandstone contains mainly quartz.

- Limestone releases carbondioxide when exposed to rain.

- Oil shale contains kerogen

Types of Rocks -Metamorphic

- Metamorphic rocks form by subjecting igneous and sedimentaryrocks to geologic temperature and pressure treatments.

- Slate is flaky while marble is hard.

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

Gems and Metals

- Gems form deep underground in magma at high temperatures and pressures.

- Rubies are red, emeralds are green and sapphires are blue.

- Inclusions (impurities) in diamond helped date the origin of the tectonic plates process to some 3.2 billion years ago.

- Metals heavier than iron formed in supernovae explosions (not in the Sun).

- Gold is an inert metal. It is soft.

- Copper is another soft metal.

- Bronze is an alloy of copper (90 %) and tin (10 %). Smelting (mixing) at high temperatures.

- Steel is an alloy of iron and carbon.

Mountains

- Tectonic plates and volcanoes create mountains and erosiongrinds them down.

- Rock material starts at mid-ocean ridge, forms continental crust,is uplifted to form mountains, then eroded back down to the ocean. Cycle takes 100s millions of years.

- Marine limestone can be found onMount Everest. Young mountains are found close to the edge of continents.

rock folding

rock faulting

oceancontinent

mountain buildup

sliding plate

asthenosphere

lithosphere

Converging plate boundary

magma

Figure 8-4: The process of mountain formation

Layering Features

Recent sedimentary rocks

Older rocks

30 million years

50 million years

40 million years

Older than 50 million years

between 40 and 50 million years

Oldest rocks

Older than 50 million years

Older sedimentary rocks

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

- Sedimentary rocks take 10s millions of years to form.

- Sedimentary layers are horizontal.

- Vertical layers help date layers.

- Geologist can “read” a terrain.

Layering Features

- The Grand Canyon on the Colorado River.

- Rock and fossil dating through radiodating.

- Provides rock history over billionsof years.

- It took 5 million years to carve it.

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

Layering Features

Devonian (about 380 million years ago)

Permian (about 270 million years ago)

Pennsylvanian (about 310 million years ago)

Mississippian (about 340 million years ago)

Cambrian (about 500 million years ago)

Pre-Cambrian (older than 2.5 billion years)

limestone

sandstoneshale

limestone

shale

schistsandstone

granite

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

Weathering and Erosion

- Water streams carry rubble downstream.

- Rivers carve the landscape.

- Dissolved minerals become sediments.

- Dissolved salts are carried to the seamaking it salty.

Figure 8-8: Water erosion

Erosion

- Rainfall contributes to water erosion.

- Rainwater seeps through porous rocks.

- Feeds the water table.saturated water zone

water table

surface stream wellunsaturated water zone

Figure 8-9: Groundwater reservoirs.

Caverns

Figure 9-10: The Luray Caverns in Virginia

- Persistent water erosion produces caves.

- Slightly acidic water dissolveslimestone to form caverns.

- Feeds the water table.

Sand Erosion

- Wind causes erosion.

- Desertification is caused byclimate change.

- The Sahara desert used to belush 10,000 years ago.

- Wind patterns carry clouds andaffect global weather.

- Tropical storms (hurricanes andtyphoons) create havoc.

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

Glacier Erosion

- Glaciers cover 10 % of Earth’s surface.

- They dominate the Arctic and Antarctic circles.

- They carry ¾ of the fresh water.

- The Greenland ice sheet is 3 km thick.It constitute a record of the climate formillions of years.

- Glaciations are due to climate change.

Figure 8-12: Advancing glaciers erode the landscape


- Buried organic matter formed petroleum.

- Slow chemical reactions deep underground transformed hydrocarbons into petroleum.

- Oil shale rocks buried to 2-4 km depth.

oil

gas

water

well platform

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.


- Temperatures between 90 oC and 150 oC formed oil and gas.

- Compaction and geologic activity forced oil and natural gas to migrate to reservoir rocks where they got trapped.

-Reservoirs are found in porous rocks like sandstone and limestone which are surrounded by non-porous shale rocks or mudstone.

- 58 % of petroleum was formed in the Cenozoic era (past 65 million years), 27 % In the Mesozoic era (250 to 65 million years ago) and 15 % in the Paleozoic era (550 to 250 million years ago).

- Exploring for oil involves generating seismic waves and recording the reflected waves to map out deep strata.

- Coal was produced in swamps. They formed during the Devonian and Permian periods (400 to 250 million years ago). Continents were close to the equator and full of shallow seas.

Summary

GEOLOGY AT WORK

Earth’s internal heat engine drives geology.

Lava at mid-ocean ridges leads to mid-ocean floor spreading.

Oceanic crust is constantly forming and is younger (200 million years).

Continental crust is older (700 million years).

Earthquakes occur at plate boundaries (edge of continents).

Geology builds up mountains from crust that starts in the ocean.

Weathering and erosion grind mountains down back to the ocean.

NATURAL DISASTERS AND MITIGATION

Earthquakes, volcanoes, tsunamis.

Flooding and land slides.

Building codes, evacuation, dikes construction, etc.

geologic processes on...

Documents