Chapter 11:  The Dynamic Planet

Geologic Time Scale – The history of the Earth showing the evolution of all plants and animals through time.

 

            Uniformitarianism – The present is the key to the past.  Proposed by James Hutton in 1795 and amplified by Charles Lyell in 1830.

 

Earth’s Structure and Internal Energy

We are able to map the internal structure of the Earth using Seismic waves from earthquakes and underground nuclear tests.  The shock waves transmit through solid objects, can be deflected at boundaries, and generally do not pass through liquid layers.

 

Earth’s Core – Made up of a solid inner core and a liquid outer core.  The core is mainly composed of iron.  Fluid flow in the outer core generates 90% of the Earth’s magnetic field.  This field experiences magnetic reversals when the north and south polarity switch.  This transition takes from 4000 to 8000 years thousand years to occur.

 

While we are in a transition phase, the surface of the Earth is not protected from the solar wind and the cosmic radiation associated with it.  Current rates of the magnetic field decay suggest that we are 2000 years from the next reversal. 

 

Each magnetic phase can last from several thousand to tens of millions of years.  We can use the magnetic field that is preserved in rocks to reassemble past continental arrangements.

 

Earth’s Mantle – The mantle makes up about 80% of the Earth’s total volume.  The mantle is composed of oxides of iron, magnesium, and silicates.  The mantle is divided into the upper and lower mantle.  The upper mantle has three divisions, the upper mantle, the asthenosphere, and the uppermost mantle.  The uppermost mantle is rigid and seismic waves pass quickly through this zone.  Next is the asthenosphere which is a plastic layer.  About 10% of the asthenosphere is molten which results in an area of slow convection.  This also gives rise to hot spots.

 

Earth’s Lithosphere – The lithosphere includes the crust and the uppermost mantle to a depth of about 70km.  The crust is has two very different compositional types, the continental crust and the oceanic crust.

            Continental Crust – Mostly granite composed of silica, aluminum, potassium, calcium, and sodium crystals.  It is of low density.

            Oceanic Crust – Mostly basalt composed of silica, magnesium, and iron.  It is more dense than continental crust.

 

Buoyancy is the principle that a less dense object, such as wood, floats in something denser, such as water.  A high mountain range, like the Himalayas, is less buoyant than a thin layer of continental crust, like the Canadian Shield.  Therefore, mountain masses can have 50-60km of crust below them whereas the continental interiors have about 30km of crust below them.

 

Isostasy – Combines the idea of buoyancy and balance.  Just as a raft floating in a lake with a person sitting on it will sit lower in the water than after the person jumps off of the raft, the same is true with the continental land masses.

As glaciers melt, removing their mass from the land, the continental masses can rise up.  This process contributes to mountain building and vertical uplift of the Earth’s crust.  It is called Isostatic Rebound.

 

Geologic Cycle – The process of building and eroding landforms.  It is powered by the Earth’s internal heat and solar energy.  Gravity continually works to level the result structures.

            The Geologic cycle is composed of:

                        The Hydologic Cycle which rearranges the Earth’s materials through erosion, transportation, and deposition.

                        The Rock Cycle, through processes in the atmosphere, crust, and mantle it produces three types of rocks, igneous, sedimentary, and metamorphic.

                        The Tectonic Cycle beings heat energy and new material to the surface and recycles old materials to the mantle, creating movement and deformation of the crust.

 

Composition of the Earth’s crust.

Ø      99% of the Earth’s crust is composed of only 8 natural elements. 

Ø      Oxygen and Silicon account for 74.3% of the crust. 

Ø      Oxygen is the most reactive gas in the lower atmosphere and combines readily with other elements. 

Ø      The percentage of oxygen in the crust (47%) is greater than that in the atmosphere (21%).

 

 

Element

Percentage of the Earth’s Crust by Weight

Oxygen (O)

46.6%

Silicon (Si)

27.7

Aluminum (Al)

8.1

Iron (Fe)

5.0

Calcium (Ca)

3.6

Sodium (Na)

2.8

Potassium (K)

2.6

Magnesium (Mg)

2.1

All others

1.5

 

Minerals and Rocks

            Mineral – An inorganic natural compound having a specific chemical formula and possessing a crystalline structure.  The combination of the elements and the crystal structure give each mineral its characteristic hardness, color, and density.

            There are approximately 3000 minerals but only 20 of them are common rock-forming minerals.

            Mineralogy is the study of the composition, properties, and classification of minerals.

 

            Rock – An assemblage of minerals bound together (such as granite), or as a mass of a single mineral (such as rock salt).

            Thousands of different rocks have been identified but all of then can be classified into three difference kinds; igneous (melted), sedimentary (from settling out), and metamorphic (altered).

 

Igneous Processes – Igneous means fire-formed in Latin.

            Igneous Rock – A rock that solidifies and crystallizes from a molten state (i.e. granite, basalt, and lava).

            Magma – A molten rock beneath the surface which can intrude and harden into crustal rocks or extrude onto the surface and form lava.

            The cooling history of an igneous rock – how fast it cooled and how steadily its temperature decreased – determines its crystalline properties.  It will be coarse-grained if it cooled slowly and fine-grained and glassy if it cooled quickly.

 

Intrusive and Extrusive Igneous Rocks

            Pluton – Intrusive rocks that cool slowly in the Earth’s crust.  This is a general term fro any intrusive igneous rock body regardless of size.

 

            Batholith – The largest type of pluton.  It is an irregular-shaped mass with a surface greater than 100km2.  Batholiths form the mass of many large mountain ranges such as the Sierra Nevada mountains in California or the Coast Range mountains in Washington state and British Columbia.

 

            Other pluton formations are:

Sills – Intrusive rock that forms parallel to sedimentary rock layers.

Dikes – Intrusive rock that crosses sedimentary layers.

Laccolith – A lens-shaped mass of intrusive rock.

Volcanic neck – A vertical column of solidified magma.

 

Classifying Igneous Rocks – Igneous rocks are usually categorized by their mineral composition and texture.

Felsic – Rocks made up primarily of feldspar and silica.  Felsic minerals include silica, aluminum, potassium, and sodium.  Felsic minerals are generally light in color and are less dense than Mafic mineral rocks (i.e. granite or rhyolite)

Mafic – Rocks made up primarily of magnesium and ferric minerals.  Mafic minerals are low in silica and high in magnesium and iron.  These rocks are generally darker than Felsic mineral rocks (i.e. basalt)

 

Sedimentary Processes

Sedimentation is driven by solar energy and gravity, with water as the principal medium.

            Sedimentary rock – A that forms from existing rocks that have been weathered, transported, and deposited. Examples are sandstone, limestone, and coal.

            Lithification – The process of cementation, compaction, and hardening of sediments.

            Stratigraphy – The study of a sequence, thickness, and spatial distribution of layers of rock.

 

Clastic Sedimentary Rocks – Rocks that form from other rocks that have been weathered, transported, and redeposited.  Particle sizes may vary from clay to boulders.

 

Unconsolidated Sediment

Grain Size

Rock Form

Boulders and cobbles

> 80mm

Conglomerate (breccia if pieces are angular)

Pebbles and gravel

2 to 80mm

Conglomerate

Coarse Sand

0.5 to 2.0 mm

Coarse Sandstone

Medium-to-fine sand

0.062 to 0.5mm

Medium Sandstone

Silt

0.002 to 0.062mm

Siltstone

Clay

<0.002mm

Shale

 

Chemical Sedimentary Rocks – Rocks formed from dissolved minerals, transported in solution, and chemically precipitated.

            Limestone – The most common chemical sedimentary rock formed from calcium carbonate CaCO3.

            Dolomite – Lithified calcium-magnesium carbonate (CaMg(C03)2).

 

Metamorphic Processes

            Metamorphic Rock – Any rock, either igneous or sedimentary, that is transformed by temperature and pressure.  Metamorphic rocks can be changed physically and chemically from the parent rock type.

 

Parent Rock

Metamorphic Rock

Shale (Clay minerals)

Slate

Granite, Slate, Shale

Gneiss

Basalt, Shale, Peridotite

Schist

Limestone, Dolomite

Marble

Sandstone

Quartzite

 

Plate Tectonics

A Brief History

            As early as 1596, scientists have noticed that the continents seem to fit against each other like pieces in a puzzle.  In 1912, Alfred Wegener first presented the theory of continental drift, the idea that, over vast amounts of time, the continents move around the globe.

            Wegener also postulated that all of the landmasses used to be united in one supercontinent called Pangaea.  His ideas were based on the geologic record around the world.  At the time his ideas were so counter to the current thought that they were often ignored.  As modern research advanced, we found more evidence in support of continental drift, so that Wegener’s theory was revived in the 1950s and 1960s.

 

Plate Tectonics – Refers to changes in configuration of Earth’s crust as a result of internal forces.  It include processes such as upwelling magma, crustal plate movement, earthquakes, volcanic activity, Subduction, warping, folding, and faulting of the crust.

 

Sea-Floor Spreading and Production of New Crust

            In the 1960s, Harry Hess and Robert Dietz proposed that sea-floor spreading was the mechanism that builds the great undersea mountains chains (called mid-ocean ridges) and drives continental movement.  They proposed that these ridges were due to upwelling magma from the upper mantle and the asthenosphere. 

 

            Magnetic Reversals recorded in the sea floor – As the hot iron-rich magma flows out of the mid-ocean ridges and cools, the iron molecules align themselves with the current magnetic field. 

 

Subduction of the Crust

            As the dense ocean crust (3.0 g/cm3) slowly collides with the relatively lighter continental crust (2.7 g/cm3), the ocean floor will move underneath the continental crust forming a Subduction zone.  The Subduction zones form deep trenches that are the deepest features on the surface of the earth.  The Mariana Trench near Guam is 36,198 ft below sea level

            The subducted crust is dragged down into the mantle where it remelts and will eventually rise again to form new ocean crust.

 

The Formation and Breakup of Pangaea

            The supercontinent of Pangaea existed 225 million years ago.  Most of what we know about plate tectonics starts at that time and progresses through today.  That means that we only understand the plate tectonics for the 1/23 of the Earth’s existence.

 

Plate Boundaries

            Divergent Boundary – Regions of sea-floor spreading.  This is where new crust is formed and creates a region of tensional forces, pulling the crust apart.

            Convergent Boundary – Collision zones, where oceanic crust is subducted under continental crust.  These are zones of compression and often times mountains are formed.

            Transform Boundary – A region where plates slide laterally past one another at right angles to a sea-floor spreading center.  Usually not volcanic activity is associated with these boundaries.

 

Earthquakes and Volcanic Activity

            Plate boundaries are the primary location of earthquakes and volcanic activity.  The Pacific Basin is surrounded by the “ring of fire”, which is named from it’s frequent volcanic events.

 

Hot Spots – Individual sites of upwelling magma arriving at the surface from tall plumes from the mantle.  The Earth has 50-100 hot spots, which generally remained fixed relative to the center of the Earth.  This means that the plates move over the top of the hot spots, making lines of islands like Hawaii.