Bio 120: Lectures 3 and 4 – Geology; Finish Ecology
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Our Planet Earth
•
Earth’s layers
1. Inner Core
• Hottest
• Densest
• Solid iron and nickel
• 1,228 km radius
2. Outer Core
• Very hot
• Liquid iron and nickel
• 2,260 km
3. Mantle
• Getting cooler
• Molten
• 2,390 km
4. Crust
8-32 km
The earth’s outer layer, the crust, is where we live. It is a solid layer, made of rock, that
“floats” on top of the molten (liquid) mantle. The earth’s crust varies in density,
thickness, and composition. The crust underlying oceans, called oceanic crust, is
generally denser and thinner than the crust of the continents, continental crust. Oceanic
crust is often called basalt, whereas continental crust is often called granitic.
• Oceanic crust
– Main crust under oceans
– ~ 8 km thick
• Continental crust
– Forms continents
– Rests on oceanic crust
– Up to 32 km thick
The Lithosphere is the rigid material that floats on the earth’s surface.
The lithosphere has two components: 1. the outer layer, which is made of crust
(“lithos” means rock), and 2. the transition layer, which lies just beneath the
crust and is considered to be the outermost layer of the mantle.
The Asthenosphere (root meaning a = without, sthenos = strength) is the molten
magma under the lithosphere. It is also part of the mantle.
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Convection currents in the molten asthenosphere move the lithosphere. As magma
deeper in the mantle heats up, it becomes less dense and rises. After it has risen to
nearer to the Earth’s crust, it cools and again sinks deeper within the mantle. This rising
of hot magma and falling of cooler magma creates currents in the asthenosphere.
These currents do not run in the same direction in all areas of Earth. Direction of flow of
convection currents in a region can be very complex (http://www.tectonicforces.org/pt04.htm). These convection currents are very influential, as they determine
the direction of plate movement.
Note: Images of the earth’s layers, as well as information about each layer, can be
found at:
http://volcano.und.nodak.edu/vwdocs/vwlessons/lessons/Earths_layers/Earths_layers7.
html. Images and some information used in lecture were taken from this site.
Plate Tectonics
The concept that the Earth’s lithosphere is made up of a number of rigid mobile pieces
called plates, that move on top of the liquid asthenosphere, is called Plate Tectonics.
There are various types of movement of one plate against another. Three main types of
movement are represented by transform boundaries, divergent boundaries, and
convergent boundaries:
1. Transform boundaries: At transform boundaries, plates slide against each other.
An example of a transform boundary is the San Andreas Fault, which forms the
boundary between the Pacific and North American Plates. Transform boundaries are
often the sites of earthquakes, as two plates move against each other. It is estimated
that there are around 10,000 earthquakes a year along the San Andreas Fault.
2. Divergent boundaries: At divergent boundaries, plates are moving apart. When
plates diverge, the movement of plates away from each other creates a spreading
center. At the spreading center, new crust is created when magma from the mantle
flows into the space between diverging plates and cools to become rock. Spreading
centers are the source of Oceanic Ridges or Rises.
As plates move apart at oceanic ridges, new crust fills in and there is often a lower
lying area between divergent plates called a Rift Valley. Rift valleys are often
sites of high volcanic activity, as very hot magma close to the crust breaks through
areas of weakness in the very thin crust layer.
An aside: Rift valleys on continents, such as Africa’s Great Rift Valley, can be formed
as plate divergence (in this case, the divergence of the African and Asian plates)
creates forces that cause areas of land between fault lines to sink down. In the case
of the African Rift Valley, this lowered land area extends for 6,000 miles between
Lebanon and Mozambique. As the crust in these rift valleys sinks, molten magma is
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often forced up in volcanic eruptions. Mount Kilaminjaro and Mount Kenya are
volcanoes resulting from plate divergence.
The creation of new crust at spreading centers further pushes the diverging plates
apart, and this movement causes collisions with other crustal plates.
Plates collide at convergent boundaries:
3. Convergent boundaries: At convergent boundaries, plates are moving together.
Convergence of plates can result in either buckling of one or both plates, or
subduction of one plate beneath the other.
- Buckling can result from the pressure produced when two plates collide. Buckling
refers to the bending, or folding of crust that can occur when an area is compressed
due to squeezing.
- Subduction occurs at convergent boundaries when one plate moves beneath
another and eventually is melted to form part of the mantle. Often denser ocean
plates subduct beneath lighter continental plates. Subduction of one plate beneath
another forms a trench. Deep sea trenches are sites of plate convergence.
Note: The coast ranges of California were formed at the site of subduction of the Farallon
Plate beneath the Pacific Plate. As the oceanic crust was moving beneath the continental
crust, sediments and matter on top of the oceanic crust were scraped off onto the continental
crust. These scraped off materials accumulated and formed the Coast Ranges.
See this website for a good simulation of subduction:
California subduction: http://emvc.geol.ucsb.edu/forteachers/flashmovies/CAsubduction.swf
Basic Geology:
Types of Rock:
1. Igneous
– Volcanic rock
– Not easily eroded
– Ex: granite, basalt
2. Metamorphic
– Produced by heat or pressure changing other rock
– Resistance to erosion varies
– Ex: marble, gneiss
3. Sedimentary
– Produced by deposition of sediments, followed by compaction (consolidation)
– Usually easily eroded
–
Ex: shale, sandstone
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The Rock Cycle:
Igneous rock and Sedimentary rock can be changed through heat and pressure into
Metamorphic rock. All rocks can erode to produce sediments. And so on… The rock
cycle summarizes the processes that lead to the creation and destruction of the various
rock types.
(There are several images of the rock cycle online. A good one can be found at:
http://rst.gsfc.nasa.gov/Sect2/Sect2_1a.html. I encourage you to refer to the drawing we made in
Wednesday’s class for a simple rock cycle diagram.)
Formation of sedimentary rocks:
There are two main processes involved in the formation of sedimentary rocks: 1.
deposition of sediments, and 2. consolidation of layers.
Sediments settle in layers:
• In ocean basins
• Or in land basins
The Principle of Superposition states that as sedimentary rock layers accumulate,
older layers are buried beneath younger layers.
The Principle of Original Horizontality states that sedimentary rock layers are
originally horizontal. This is because sediments settle out at the lowest point, due to
gravity.
The Principles of Superposition and Horizontality make statements about how
sedimentary layers should be, given that they have not been altered. However,
movement of the Earth’s crust (Plate Tectonics) and other environmental forces can
significantly alter the form of sedimentary layers:
Forces that upset horizontal layering:
1. Folding
2. Faults
3. Erosion
1. Folding (or buckling)
- results from plate movement
- due to compression
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Types of folding:
1. Anticline
- an upward-buckling fold (like an arch)
2. Syncline
- a down-buckling fold (like a U)
2. Faults
-
Fractures in earth’s crust, resulting from plate movement
Here is the url for great USGS webpage that illustrates and further explains types of faults:
http://www2.nature.nps.gov/geology/usgsnps/deform/gnormal.html. Go to this site to see
pictures of normal and reverse vertical faults.
Types of faults:
a. Horizontal (strike-slip) faults
- movement is lateral
b. Vertical faults
- movement is up and down
Types of Vertical Faults:
1. Normal Faults
- Normal faults result from expansive forces.
2. Reverse Faults
- Reverse faults are the result of compressional forces.
3. Erosion
- Erosion is the breaking down of the earth’s crust into sediments. Erosive forces
often carry these sediments, along with ions, away from their original source.
Sources of Erosion:
 Wind
 Rain
 Rivers
 Glaciers (ice)
 Waves
Sources used for these notes:
1. http://volcano.und.nodak.edu/vwdocs/vwlessons/lessons/Earths_layers/Earths_layers7.html
2. Anderson, Genny and Friesen, LJ 2006. Natural History online lessons.
3. Alden, Peter and Heath, Fred 1998. National Audubon Society Field Guide to California.
4. Harden, Deborah R. 2004. Californian Geology.
5. Campbell et al 2005. Biology: Concepts and Connections.
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Finishing up Ecology:
Community trophic structure
Terrestrial and aquatic food chain (also on handout)
Simple food chains are rare
Most trophic interaction more closely resemble a food web (see handout)
Ecosystems
• Include biotic and abiotic factors
• Ecosystem dynamics
– Energy Flow – energy flows through ecosystems
– Chemical Cycling – chemicals cycle within ecosystems
Energy transfer through ecosystems is inefficient
• Roughly 10% is transferred up to a higher trophic level
(recall example from lecture regarding corn, cows, and humans)
Chemicals (gases and nutrients) cycle within ecosystems.
The Carbon cycle
Carbon atoms from carbon dioxide are used by plants to make food
(glucose) during photosynthesis.
Photosynthesis –
Carbon atoms are released back into the atmosphere as food (glucose) is used
up during the process of Cellular Respiration.
Cellular respiration –