PURDUE UNIVERSITY

GEOS 105-THE PLANETS

Prof. Robert L. Nowack

 

Lecture 10

 

 

Earth - The Home Planet

 

View from Space

 

 

 

 

From space, the Earth would have a partially blue surface from reflectance of water.  It would have white ice caps, as well as brilliant white clouds reflecting sunlight.  Also, it has a few brown land masses.  More remarkable is the Earth's atmospheric chemistry.  Nitrogen (N2) gas makes up 78%, but the highly reactive oxygen (O2) gas is second at 21%.  Without some active processes going on, oxygen would get trapped in surface rocks.   Equally surprising is the lack of carbon dioxide (CO2) gas other than just a trace.  CO2 is the primary constituent of the atmospheres on the planets Venus and Mars.  Although not obvious from space, one of the most distinctive aspects of Earth is the abundant and diverse forms of life.

 

 

 

Surface Features on Earth

 

3/5 of the Earth's surface is underwater.  In fact, there’s enough water to cover the entire globe to a depth of 3 kilometers.  Most of the land area is concentrated in 6 large continents.  Near the edges of some continents, mountains rise as high as 9 kilometers above sea level.  While in the ocean basins, there are trenches as deep as 11 kilometers below sea level.

 

Earth is a very geologically active planet with volcanoes, frequent earthquakes, geologically "young" mountain ranges with sharp ridges and peaks.  In addition, erosion by water, wind, and ice is active in tearing down mountains, weathering surface rocks, and layering down vast beds of sediment.

 

From space, one could also measure that the Earth has a significant magnetic field generated deep in the Earth's interior resulting from an active metal core.  Hence the Earth is a differentiated planet with a rocky exterior and an iron core.

 

 

 

Probing of the Interior

 

Earth has an average radius of 6371 kilometers (slightly bulged at the equator to 6378 kilometers).  The deepest well ever drilled is to a depth of only 13 kilometers (by the Russians) from the surface – not even through the skin of the Apple!).

 

How do we investigate the Earth's interior structure?  First, we can appeal to bulk density, about 5500 kg/m3 (or 4500 uncompressed).  Most surface rocks have densities from 2600-3000 kg /m3.  (Note:  Water has a density of 1000 kg/m3).  Heavier material settled or differentiated from Earth's iron core.  Both temperature and pressure increase with depth in the Earth.

 

 

 

Seismology

 

A powerful technique for investigating the Earth's interior is to use vibrations caused by earthquakes.  Catastrophic slipping or breaking of rock deep inside the Earth causes earthquakes.  This causes vibrations or "waves" to propagate through the Earth.  The largest of earthquakes can cause the whole Earth to ring like a bell.

 

 

"Seismograph"

 

 

 

 

"Seismogram"

 

 

 

 

There are two types of seismic vibrations:  Compressional or "P wave" vibrations and Shear or "S wave" vibrations.  “S wave” vibrations cannot travel through a liquid.

 

 

 

Seismic Waves propagating through the Earth from a large earthquake.

 

 

 

 

An important early observation was that S waves cannot vibrate through the central core of the Earth.  At least the outer part of the Earth's core must be a liquid (iron), but not like any liquid on the surface!!  This is consistent with the Earth having a strong (and slowly varying) magnetic field – this requires a liquid iron core.

 

 

 

 

There are six major divisions in depth in the Earth

 

(1)     The Magnetosphere - 200 kilometers up from Earth's surface

-     The Earth's magnetic field traps charged particles in space.

 

(2)     The Atmosphere

-     A layer of gas composed mainly of oxygen and nitrogen beginning at the surface up to about 200 or more kilometers.

 

(3)     The Oceans

-     A layer of liquid and frozen water covering 3/5 of the Earth's surface

 

(4)     The Crust – The solid surface of the Earth

-     Continental crust - 50 kilometers thick

-     Oceanic crust -  6 to 7 kilometers thick

 

(5)     The Mantle

-     the major rocky layer of the planet under the crust and down to 2900 kilometers in depth.

 

(6)     The Core

-     dense metal outer liquid core/inner solid core

 

 

A More Detailed View of the Earth's Interior

 

 

 

 

The above diagram of the interior of the Earth represents current understanding.  A liquid layer surrounds a small solid core, which in turn gives way to the mantle.  Rapid convection in the liquid layer of the core generates the Earth’s magnetic field.  Convection at geologic time scale in the mantle causes the slow motions of crustal plates moving the continents, forming mountains, and subducting eroded material.

 

The crust is the Earth's uppermost solid layer and has 2 distinct types:

 

(1)  Ocean basin crust is thin, only about 6 kilometers thick, composed primarily of young basaltic rock less than 200 million years old.  (Recall that Moon basalts are all greater than 3,000 million years old or 3 billion years old).  Earth basalts are formed continuously along long sub-ocean ridges.  They are ultimately recycled by sinking back into the mantle along subduction zones.

 

(2)  Continental crust is thicker, older and less dense than oceanic crust.  A primary constituent is an igneous crystalline rock called granites.  Thickness ranges from 20 to 70 kilometers.  In addition, the continental crust contains a great deal of sedimentary rocks as well as deformed metamorphic rocks.  The oldest continental rocks form the centers of continents and can have ages greater than 3 billion years.

 

 

 

The Mantle

 

The mantle is largely made up of iron rich silicate rocks in 3 distinct layers:

 

(1)     The lithosphere - upper 100 km of rigid mantle mechanically attached to the crust.

 

(2)     The upper mantle – From the crust down to 700 kilometers.  A more ductile layer which mechanically acts in a plastic fashion over geologic time.

 

(3)     The lower mantle - 700 to 2900 kilometers.  Because of pressure, these silicate rocks are in a denser phase.

 

 

 

The Core – 2900 km to a depth of 6371 kilometers (the Earth's center)

 

The Earth's core has a diameter of 6942 kilometers (larger than Mercury).  The outer core is liquid.  Still deeper is a solid inner core from 5200 to 6371 kilometers.  Pressures in the core range from 1.3 to 4.0 million bars (surface pressure = 1 bar) and core temperatures ranging from 4500 to 5000 K.

 

 

The Earth's Geomagnetic Field

 

 

 

 

The Earth’s magnetic field is primarly a dipole field with one geomagnetic pole in Antarctica (78°30’ South latitude; 110°10’ east longitude).  The other geomagnetic pole is on the northwest coast of Greenland (78°30’ North latitude; 68°50’ west longitude).  The geomagnetic axis forms an angle 11°30’ with the rotational axis.

 

Heat drives turbulent convection cells in the outer liquid core which sets up the Earth's magnetic field.  In fact, the Earth's magnetic field slowly moves (over hundreds of years) and even regularly completely reverses (over 10's of thousands of years!!)

 

The Earth's magnetic field reversed 980,000 years ago to what it is today.

 

 

 

 

 

980,000 years ago                                   Today

 

A “Stone Age” Boy Scout would have his compass point in the opposite direction!

 

 

 

The Geologic Time Scale

 

How old is the Earth?  Early Christian theologians used the Story of Genesis.  This culminated in an estimate in 1642 by John Lightfoot of the time of creation at exactly: 9:00 AM, October. 23, 4004 B. C.  Unfortunately, it became increasingly apparent that fossils in rocks were the remains of once living plants and animals and this would require much more than just a

few thousand years.  In addition, sedimentary rocks must have taken very long periods to deposit.

 

Early Ordovician Trilobite Fossil 500 million years old

 

 

 

 

Middle Devonian Fern Fossil 385 million years old

 

 

 

 

In 1830-1833, Charles Lyell wrote his "Principles of Geology".  He extended the ideas James Hutton had on uniformitarianism.  Lyell believed in the uniformity of natural geologic forces (rain, river, wind, etc.) over geologic time  Geologic features today resulted from forces in the past similar to those working today.

 

Strict uniformitarianism implied never-ending cycles of mountain building and erosion.  (or the Earth as a "clock" or a solar system machine).  Applied to geological structures, this principal required long periods of time for the raising and tilting of once horizontal rocks to form mountains and then eroding them away.  Geologic structures can be age dated in a relative sense by determining which rock units are older.

 

 

 

 

Darwin used the long time spans uniformitarianism implied to develop his theory of evolution of animal species in 1859; even though the theory of uniformitarianism is quite different from evolution.  In 1862, the famous physicist, Lord Kelvin, attempted to compute the age of Earth based on cooling of an initially molten uniform Earth to its present temperature.  His estimate was about 100 million years.

 

100 m. y. is a long time but not nearly enough to be consistent with what the geologic record required!  Thus, a 50-year controversy occurred between the physical models of Lord Kelvin and geologists’ observations!  This controversy was resolved when natural radioactivity was discovered (this allowed for an additional source of heat in the Earth which could extend the age estimates of Kelvin) and was put forward by Lord Rutherford in 1904 and by Lord Boltwood in 1906.

 

In 1911, Arthur Holmes developed a dating method based on radioactive decay resulting in radioactive age dating of rocks based on relative proportions of "parent" and "daughter" products.

 

The oldest known rocks on Earth are about 3.8 billion years (although small inclusions in other rocks have been dated older).

 

 

 

 

Meteorites have ages as old as 4.6 billion years.

 

The age of the Earth is now thought to be similar in age at about 4.6 billion years.  Even before absolute age dating, geologists devised a relative Geologic Time Scale based on fossils and the geologic record.  Later these divisions were correlated with the absolute dates.

 

General Divisions:

 

I Precambrian:

 

 

 

(i) Archean and Hadean

» 2,500 m. y.

 

 

 

 

(ii) Proterozoic

2,500 to 570 million years ago
oxygen rich atmosphere.

 

 

 

II Phanerozoic: 

 

 

 

(i)Paleozoic 

Fish, land plants, early reptiles

570 to 248 million years ago.

 

 

 

 

(ii) Mesozoic 

Dinosaurs, early mammals, flowering plants

248 to 65 million years ago.

 

 

 

 

(iii) Cenozoic 

Mammals - to present

 

Even the finer distinctions of the Geologic Time Scale are commonly used.  For example, the time for extinctions of the dinosaurs is at the Cretaceous/Tertiary boundary (which is also the boundary between the Mesozoic and Cenozoic). 

 

 

 

 

There appear to be times in Earth's history where large numbers of species have ceased to exist - Mass Extinctions.

 

Major and minor mass extinction events marked by arrows during the past 500 million years.

 

 

 

 

There are many theories for these extinctions - large impacts, etc.  Still, whatever the causes, the Earth has had a number of “catastrophic" episodes.

 

 

 

General Summary of Ways of Looking at Geologic Time (All Viewpoints Necessary)

 

(1)     Uniformitarianism

Geologic processes happening in cycles (also forces acting in the past same as they do today.

 

Example: the cycles of mountain building and erosion.

 

(2)     Geology as a slow "evolving" process:

 

Example: Increase of oxygen in atmosphere

Example: Progression from simple single celled to multi-celled organisms.

 

 (3)    Catastrophism

Short-term geologic catastrophic events and processes.

 

Example:  Impacts on Earth

Example:  The extinction of dinosaurs

 

 

 

20th Century Revolution in Geology

 

Plate Tectonics

 

(1)     The crust beneath oceans all appears to be "young", less than 200 million years (continents can be much older).

 

(2)     Sources of earthquakes occur along fractures in the Earth. These sources of earthquakes map out belts which separate large "plates" of the Earth's surface.

 

 

 

 

(3)     In 1915, the German meteorologist, Alfred Wegener, put forth the theory that the Americas, Europe and Africa were once adjacent and then moved apart -  Continental Drift.

 

However, a problem with Wegener's theory was, How do you get solid rock to move in the underlying mantle?  A debate lasting 30 years went on over this theory.

 

Solution:  Over long geologic times, at mantle pressures and temperatures, solid rock in the mantle can slowly deform.

 

 

Supercontinent of Pangaea 200 Million Years Ago

 

 

 

 

Continents and Oceans Today

 

 

 

 

 

In the meantime, geologists were mapping the ocean floor and found linear belts of ocean bottom mountains called Ocean Ridges.  These correlated well with sub-ocean seismicity.  This mapping of the ocean floor took place during and after WWII (primarily to look for submarines!). 

 

Mid-Oceanic Ridge Mountain System

 

 

 

 

Another piece of evidence that the Earth is constantly recycling its outer surface is that the number of craters on Earth is very low in comparison to the Moon.  Thus, the Earth’s surface is constantly eroding and changing.

 

 

Meteor Crater in Arizona

 

 

 

 

The diameter of Meteor Crater is about ¾ mile and is about 50,000 years old.  Slow erosion in arid Arizona maintains the crater.

 

With regard to ocean ridges, the Mid-Atlantic Ridge bisects the oceanic crust beneath the Atlantic Ocean. 

 

Tectonic Plates of the World

 

 

 

 

In the 1950's, magnetic field orientations frozen in the rocks supported the hypothesis that continents had moved apart.  But the question remained - how can continents move on a fixed sized Earth?

 

 

 

Diverging Plates Boundary

 

New ocean crust develops and forms along mid-ocean ridges.  These are divergent zones, pulling older ocean crust apart. 

 

 

 

 

Converging Plate Boundary

 

The ocean crust thickens and cools and finally sinks under its own weight back into the Mantle along zones called subduction zones. 

 

 

 

 

Behind subduction zones, a line of Andesite volcanoes form such as the Cascades (i.e., Mount Saint Helens) and the Andes in South America.

 

 

(Note:  Massive shield volcanoes such as Hawaii, also form which result from direct penetration of material from the mantle.)

 

 

Transform Plate Boundary

 

There is one more type of plate boundary called transform faults in which one plate moves laterally past another. 

 

 

 

 

The San Andreas Fault in California is a well-known example of a transform fault.  The North American Plate moves laterally past the Pacific Plate by about 6 cm/yr.  Some areas along the fault are "stuck" and then slip catastrophically resulting in an earthquake.

 

 

 

 

The San Andreas Fault suddenly slipped six meters in the famous 1906 San Francisco Earthquake.

 

The final plate tectonics model is that the outer crust and lithosphere of the Earth is more rigid and moves laterally as plates.  Oceanic crust will ultimately sink back into the Mantle along subduction zones.  Granitic continental crust is not dense enough to sink and remains on the surface - hence its greater geologic age.

 

 

 

 

But what causes the outer crust and rigid lithosphere to move?  The theory is that heat causes the mid-Mantle to slowly deform and "convect" having cooler material sink and hotter material rise.  Convection cells in the Mantle over geologic time allow heat to escape the Lower Mantle and Core below.