Structure and Composition of the Earth

• The solid earth can be divided into:

_______________

• (water, organic substances and skeletal matter) - solid and liquid - and includes all forms of life (e.g. plants and animals) and their products (e.g. skeletons) both on land and in the sea

_______________

• (fresh and salt water, snow and ice) - mainly liquid, some solid - includes all forms of water

Internal structure of the Earth

• Which includes: – _______________ (normal silicate rocks

such as granite and basalt) - solid – _______________ (ferromagnesium-rich

silicate rocks) - solid – _______________ (iron-nickel alloy) - liquid

upper part and solid lower part

Crust

• The crust makes up only 0.5 % of the Earth's total mass and can be subdivided into two main parts:– _______________– _______________

• Both differ in thickness, density and composition.

• Although oceanic crust covers approximately 61 % of the Earth's surface, it only comprises some 30 % of the crustal mass, as the continental crust is much thicker.

Continental Crust:

• From the surface of the Earth down to 30 km - 50 km.

• _______________appears to be stratified (layered) and becomes denser with depth.

Oceanic Crust:

• From the surface of the Earth down to 10 km - 12 km

The Moho

• The _______________ between the crust and mantle is known as the Mohorovicic Discontinuity (Moho).

Mantle

• The _______________ is thought to be primarily composed of rocks rich in magnesium and iron.

• We can subdivide it into an – upper mantle– transition zone– lower mantle

Upper Mantle

• Measured from the base of the crust down to 400 km

• 10 % of the Earth's total mass

Transition Zone

• 400 km - 1000 km below the Earth's surface

• 17 % of the Earth's total mass

Lower Mantle

• 1000 km - 2900 km below the Earth's surface

• 41 % of the Earth's total mass

The Core • The _______________ is marked as that

point within the Earth where S-waves cannot penetrate.

• It is believed to be composed primarily of a nickel-iron alloy, consisting of a _______________ outer zone, and a _______________inner zone.

• It is also marked by an abrupt increase in _______________.

• Outer core – 2900 km - 5000 km below the Earth's

surface– 30 % of the Earth's total mass

• Inner core– 5000 km - 6370 km below the Earth's

surface– 2 % of the Earth's total mass

Cross Section of the Earth

• Not to scale

History of Plate Tectonics

• Plate tectonic theory had its beginnings in 1915 when Alfred Wegener proposed his theory of "_______________."

• He was one of the first to realize that the Earth's surface has changed through time, and that continents that are separated now may have been joined together at one point in the past.

• Paleontologists had also found that there were fossils of similar species found on continents that are now separated by great geographic distance.

• Wegener's ideas were very controversial because he didn't have an explanation for why the continents moved, just that there was observational evidence that they had.

• At the time, many geologists believed that the features of the Earth were the result of the Earth going through cycles of heating and cooling, which causes expansion and contraction of the land masses.

• Although Wegener's "continental drift" theory was later disproved, it was one of the first times that the idea of crustal movement had been introduced to the scientific community; and it laid the groundwork for the development of modern plate tectonics.

Plates

• _______________deals with the study of the motion and deformation of the Earth's crust.

• Plate tectonic theory tries to account for the movement of the crust throughout geologic time.

• _______________ are rigid bodies of rock that essentially float atop a region of partial melt called the _______________.

• The plates comprise the _______________, composed of the crust (which is the rigid, outermost layer of the Earth) and the solid portion of the upper mantle.

Plate Boundaries

• _______________are found at the edge of the _______________ plates and are of three types:– Convergent– Divergent– Conservative

• The three boundaries are characterized by their distinct _______________.

Divergent Boundary• The first sort of plate

boundary is called a divergent boundary, or spreading center.

• At these boundaries, two plates move away from one another.

• As the two move apart, mid-ocean ridges are created as magma from the mantle upwells through a crack in the oceanic crust and cools.

• This, in turn, causes the growth of oceanic crust on either side of the vents.

• As the plates continue to move, and more crust is formed, the ocean basin expands and a ridge system is created.

• Divergent boundaries are responsible in part for driving the motion of the plates.

• As you can imagine, the formation of the new crust on either side of the vents would act to push plates apart, as we see at the Mid-Atlantic Ridge, which helps to move North America and Europe further and further apart.

Convergent Boundaries

• These are plate margins where one plate is overriding another, thereby forcing the other into the mantle beneath it.

• These boundaries are in the form of trench and island arc systems (volcano).

• Convergent boundaries also explain why crust older than the Cretaceous cannot be found in any ocean basin-- it has already been destroyed by the process of subduction.

• Subduction zones are the location of very strong earthquakes, which occur because the action of the down going slab interacts with the overriding slab.

• The "Ring of Fire" around the margins of the Pacific Ocean is due precisely to the subduction zones found around the edges of the Pacific plate.

• Sometimes, when there is a convergent boundary between two continental plates, subduction cannot occur.

• Since continental crust is more buoyant, or less dense, than oceanic crust, one plate does not easily override the other.

• Instead, the plates crumple as they plow into one another, and a very high mountain range is created.

• The Himalayas in India are the result of two continental plates (the Indo-Australian and Eurasian plates) colliding head on.

Conservative Boundary

• It is called conservative because plate material is neither created nor destroyed at these boundaries, but rather plates slide past each other.

• The classic example of a transform plate boundary is the San Andreas fault in California.

• The North American and Pacific Plates are moving past each other at this boundary, which is the location of many earthquakes.

Faults

• _______________ are surfaces along which rocks have fractured and been displaced.

• The tectonic stresses caused by plate motions build up over time and eventually cause breaks in the crust of the Earth along which the rocks sporadically grind past one another.

• When this happens, earthquakes occur.

Earthquakes

• Earthquakes are the Earth's natural means of releasing _______________.

• When the Earth's plates move against each other, stress is put on the _______________.

• When this stress is great enough, the lithosphere breaks or shifts.

• There are many different types of earthquakes: – Tectonic– Volcanic– Explosion– Collapse

• The type of earthquake depends on the region where it occurs and the geological make-up of that region.

• _______________ - occur when rocks in the earth's crust break due to geological forces created by movement of tectonic plates

• _______________ - occur in conjunction with volcanic activity

• _______________ - small earthquakes in underground caverns and mines

• _______________ - result from the explosion of nuclear and chemical devices

Seismology

• _______________ is the study of the origin and propagation of elastic waves through planetary bodies.

• It was originally regarded as the study of earthquakes.

• There are three types of waves that are created when stress is released as energy in earthquakes: – P– S– Surface waves

P Waves• P-waves: Primary or Compressional waves are the _______________ type of wave and have velocities of 5 km/s at the top of the crust, 8 km/s at the top of the mantle, and 14 km/s at the bottom of the mantle.

S Waves• S-waves: Shear or Transverse waves that travel at slower speeds than P-waves.

Surface Waves

• Slowest• These waves move

close to or on the outside surface of the ground.

Richter Scale• Using a

seismogram, the time difference between the recording of the P wave and the S wave is determined and matched to a corresponding distance value.

• Close-up of Government Hill Elementary School Anchorage, Alaska. The school was destroyed by the 1964 Prince William Sound earthquake, magnitude 9.2. Note the person in the upper right portion of the image(in the red box) for scale.

• Fence offset 8 ½ feet by the 1906 San Francisco, California earthquake. Magnitude 7.3