1_Earth Materials Flashcards
What are the five properties that define minerals?
- Solid
- Naturally occurring
- Inorganic
- Specific chemical composition
- Possesses a long-range, geometric arrangement of constituent atoms or ions.
What is the radius of the Earth?
6380 km
What are the layers within Earth defined largely on the basis of chemical composition?
- Crust (subdivided into continental and oceanic crust)
- Mantle
- Core
What is the thickness range of the thin crust and what percentage of Earth’s volume does it occupy?
5 to 80 km
Occupies < 1% of Earth’s volume.
What is the average radius of the mantle and what percentage of Earth’s volume does it occupy?
∼ 2885 km
Occupies ∼ 83% of Earth’s volume.
What is the radius of the core and what percentage of Earth’s volume does it comprise?
∼ 3480 km
Comprises ∼ 16% of Earth’s volume.
What are the layers within Earth defined principally on the basis of mechanical properties?
- Lithosphere
- Asthenosphere
- Mesosphere
- Core: liquid outer core and solid inner core
What is the average depth of a strong lithosphere and what does it include?
∼ 100 km
Includes all of the crust and the upper part of the mantle.
What is the depth range of the weaker asthenosphere?
100 to 660 km
Includes a transition zone from ∼ 400 to 660 km.
What is another term for the lower mantle?
Mesosphere
What is the depth range of the mesosphere or lower mantle?
∼ 660 to 2900 km
How is the core divided?
- Liquid outer core: ∼ 2900 – 5150 km
- Solid inner core: ∼ 5150 km to the center of Earth (6380 km)
What is the outermost layer of the geosphere?
Earth’s crust (extremely thin)
What separates the crust from the underlying mantle?
The Mohorovicˇic´ (Moho) discontinuity
What is oceanic crust primarily composed of?
Dark-colored, mafic rocks enriched in oxides of magnesium, iron, and calcium (MgO, FeO, and CaO).
What is responsible for the dark color and elevated density of oceanic crust?
Iron content
What is the average depth to the Moho in oceanic crust?
5 – 7 km
Under some oceanic islands, its thickness reaches 18 km.
What rocks principally compose oceanic crust?
Basalt and gabbro, composed largely of the minerals pyroxene and calcic plagioclase.
What layers of oceanic crust comprise mafic rocks?
Layers 2 and 3 of oceanic crust topped with sediments that comprise layer 1.
What is the composition of oceanic crust?
- Dark-colored, mafic rocks enriched in MgO, FeO, and CaO
- Averages ∼ 50% SiO2
What is the density of oceanic crust?
- Higher; less buoyant
- Average 2.9 – 3.1 g/cm³
What is the thickness of oceanic crust?
- Thinner; average 5 – 7 km thickness
- Up to 15 km under islands
What is the age of oceanic crust?
Up to 180 Ma for in-place crust
∼ 3.5% of Earth history.
What is the composition of continental crust?
- Complex; many lighter colored felsic rocks
- Enriched in K2O, Na2O, and SiO2
What is the density of continental crust?
- Lower; more buoyant
- Average 2.6 – 2.9 g/cm³
What is the thickness of continental crust?
- Thicker; average 30 km thickness
- Up to 80 km under mountains
What is the age of continental crust?
- Up to 4000 Ma
- 85 – 90% of Earth history
Where does the oldest ocean crust occur?
Along the western and eastern borders of the north Atlantic Ocean and in the western Pacific Ocean.
What fragments of oceanic crust may be preserved on land?
Fragments of oceanic crust, perhaps as old as 2.5 Ga, may be preserved in the form of ophiolites.
What is the general composition of continental crust?
Granitic in composition, enriched in K2O, Na2O, and SiO2 relative to average crust.
What is the average thickness of continental crust?
30 km
Under areas of very high elevation, such as the Himalayas, its thickness approaches 80 km.
Where are the thinnest portions of continental crust found?
Below sea level, most frequently along thinned continental margins and rifts.
What is the age of the oldest well-documented continental crust?
4.03 Ga rocks from the Northwest Territories of Canada
Approximately 4 Ga rocks also occur in Greenland and Australia.
How far back may greenstone belts date?
4.28 Ga
Suggesting that crust began forming 300 million years after Earth’s birth.
What is the age of individual zircon grains from metamorphosed sedimentary rocks in Australia?
4.4 Ga
What is the depth of the Earth’s mantle?
2900 km
What is the mantle rich in?
MgO (30 – 40%) and, to a lesser extent, in FeO.
What is the average percentage of SiO2 in the mantle?
40 – 45%
Some basic rocks such as eclogite occur in smaller proportions.
What silicate minerals are dominant in the upper mantle?
Olivine and pyroxene are dominant; spinel, plagioclase, and garnet are locally common.
What is the dominant group in the upper mantle?
Peridotite
What occurs under higher pressure conditions deeper in the mantle?
Similar chemical components combine to produce dense minerals with tightly packed structures.
What does the uppermost part of the mantle and the crust together constitute?
The relatively rigid lithosphere
Strong enough to rupture in response to stress.
At what depths does a discrete low velocity zone (LVZ) occur within the upper mantle?
∼ 100–250 km below the surface
What does the top of the LVZ mark?
The contact between the strong lithosphere and the weak asthenosphere.
What is the behavior of the asthenosphere when subjected to stress?
It is more plastic and flows slowly, rather than rupturing.
What explains the anomalously low P-wave velocity of the LVZ?
Small amounts of partial melting.
This is supported by laboratory studies suggesting that peridotite should be very near its melting temperature at these depths due to high temperature and small amounts of water or water-bearing minerals.
What happens to seismic wave velocities below the base of the LVZ?
Seismic wave velocities increase
Indicating that the materials are more rigid solids.
At what depths do seismic discontinuities marked by increases in seismic velocity occur within the upper mantle?
∼ 410 and ∼ 660 km
What is the interval between the depths of 410 and 660 km called?
The transition zone between the upper and lower mantle.
What do sudden jumps in seismic velocity record?
Sudden increases in rigidity and incompressibility.
What transformation occurs at approximately 410 km depth?
Olivine is transformed to more rigid, incompressible spinel (β-spinel), also known as wadleysite (Mg2SiO4).
What is wadleysite transformed into within the transition zone?
Ringwoodite (Mg2SiO4)
What occurs at ∼660 km depth?
Ringwoodite and garnet are converted to very rigid, incompressible perovskite [(Mg,Fe,Al)SiO3] and oxide phases such as periclase (MgO).
What is the chemical formula for ringwoodite?
Mg₂SiO₂
At what depth does ringwoodite convert to perovskite?
At ∼660 km depth (∼24 GPa)
What are the major minerals in the lower mantle?
- Perovskite, 2. Periclase [(Mg,Fe)O], 3. Magnesiowustite [(Mg,Fe)O], 4. Stishovite (SiO₂), 5. Ilmenite [(Fe,Mg)TiO₂], 6. Ferrite [(Ca,Na,Al)Fe₂O₄]
What is the depth range of the mesosphere?
From 660 km to the mantle-core boundary at 2900 km depth.
What is the depth range of the D″ discontinuity?
∼ 130 to 340 km above the core-mantle boundary.
What did Williams and Garnero propose in the lowermost mantle?
An ultra low velocity zone (ULVZ) based on seismic evidence.
What is the composition of Earth’s core?
Iron: ∼ 85%, Nickel: ∼ 5%, Lighter elements: ∼ 8 – 10% (such as oxygen, sulfur, and/or hydrogen).
What occurs at the Gutenberg discontinuity?
A dramatic decrease in P-wave velocity and the termination of S-wave propagation.
What is the density of Earth’s outer core?
∼ 10 – 12 g/cm³
What produces Earth’s magnetic field?
Circulating molten iron in Earth’s outer core.
What is the outer/inner core boundary called?
Lehman discontinuity at 5150 km.
What is the density of the solid inner core?
∼ 13 g/cm³
What is the seismic nature of the inner core?
Anisotropic; seismic velocity is faster in one direction than in others.
What are triple junctions?
Locations where three tectonic plates meet.
What are the three major types of plate boundary segments?
- Divergent plate boundaries, 2. Convergent plate boundaries, 3. Transform plate boundaries.
What marks horizontal extension in continental lithosphere?
Continental rift systems.
What is an example of a continental rift system?
The East African Rift.
What type of faulting occurs in continental rift systems?
Normal faulting.
What is the process called when a continental lithosphere is rifted into two separate continents?
Sea floor spreading.
What is the most recent example of complete continental rifting?
The separation of the Arabian Peninsula from Africa to produce the Red Sea basin.
What is the oceanic ridge system?
Earth’s largest mountain range covering roughly 20% of Earth’s surface.
What is the average elevation of the oceanic ridge system above the surrounding sea floor?
∼ 3 km
What is the average slope of the oceanic ridge system?
0.4 degrees.
What is the spreading rate of the East Pacific Rise?
∼ 6 – 18 cm/yr.
What is the spreading rate of the Mid-Atlantic Ridge?
∼ 2 – 4 cm/yr.
What type of slopes do warmer, faster spreading portions of the ridge have?
Gentler slopes.
What marks the central portion of the ridge system?
A rift valley.
What is the process called that occurs as the solid, unmelted portion of the asthenosphere spreads laterally?
Underplating.
The process by which the leading edge of a denser lithospheric plate is forced downward into the underlying asthenosphere.
Subduction
What are the surface expressions of subduction zones?
Trench-arc systems.
Deep, elongate troughs in ocean floors marked by water depths that can approach 11 km.
Trenches
What generates earthquakes along the inclined seismic zone?
Stresses in the cool interior of the subducted lithosphere.
What are the three largest magnitude earthquakes in the past century?
- Chile (1909), 2. Alaska (1964), 3. Sumatra (2004).
What earthquake produced the devastating Banda Aceh tsunami?
Sumatra Earthquake in 2004.
To what depth do earthquakes occur in subducted slabs?
660 km.
True or False: Subducted slabs may sink to the core-mantle boundary.
True.
What are examples of composite volcanoes in Pacific Ocean subduction zones?
- Mt St. Helens, 2. Mt Pinatubo, 3. Mt Fuji, 4. Krakatau.
What are long mountain belts formed along convergent plate boundaries called?
Orogenic belts.
What does the increasing weight of a thickening orogenic belt produce?
Foreland basin.
What is the most striking example of a modern orogenic belt?
The Himalayan Mountain range.
What has resulted from the convergence of the Indian microcontinent with Asia?
The Tibetan Plateau.
What type of rock near the summit of Mt Everest was formed on the floor of the Tethys Ocean?
Limestone.
What does the Ganges River represent?
A modern foreland basin.
What type of boundaries are characterized by horizontal motion?
Transform plate boundaries.
What type of fault system do transform fault systems represent?
Strike-slip fault system.
Who first envisioned transform faults?
J. T. Wilson (1965).
What type of fault system do transform faults belong to?
Transform fault systems are a type of strike-slip fault system.
Who first envisioned transform faults to explain seismic activity along ocean floor fracture zones?
J. T. Wilson (1965).
What are curvilinear zones of intensely faulted, fractured oceanic crust called?
Fracture zones.
What are the portions of fracture zones called that are no longer plate boundaries?
Healed transforms or transform scars.
Where do transform plate boundaries also occur?
In continental lithosphere.
What are some modern examples of continental transforms?
- San Andreas Fault system in California
- Alpine Fault system in New Zealand
- Anatolian Fault systems in Turkey and Iran.
What may develop in places where strike-slip faults bend or overlap?
Deep pull-apart basins.
What are long-lived areas in the mantle where large volumes of magma are generated called?
Hotspots.
What direction of plate motion is suggested for the Hawaiian chain over the last 45 million years?
West – northwest.
What do hotspots represent the surface expression of?
Fixed, long-lived mantle plumes.
What are mantle plumes hypothesized to be?
Columns of warm material that rose from near the core-mantle boundary.
What are superplumes hypothesized to influence?
Extinction events, the initiation of continental rifting, and the supercontinent cycle.
