ESCI Test 1
What outlines the lithospheric plates?
distribution of earthquakes epicenters; deep earthquakes restricted to convergent subduction zones
Continental Drift
The concept that the continents were located in other positions in the geologic past and have moved (or "drifted") into their present positions, a concept presented by Alfred Wegener in 1915
Additional evidence that the continents have moved comes from Paleomagnetism:
The magnetic signature in a Fe-bearing rock has three components: 1. Its intensity 2. Its azimuth (compass direction or declination) 3. Its inclination in a vertical plane is a function of the latitude at which the rock formed
Continental Drift controversy
a mechanism(s) to drive the motion of continents was missing
Lithosphere
crust and upper mantle; the solid, rocky outermost layer of the Earth that is divided into plates
Plant-tectonic theory
emerged 1968-1970; it satisfied the combined lines of evidence for continental drift, sea-floor spreading, locations of earthquakes and the internal layering of the Earth
Sea Floor Spreading
emerged in the 1960's, the concept that ocean floor is created at midocean ridges and that the oceans grow wider with time
Mantle transition zone
extends 400 to 660 km depth
Lithospheric mantle
from base of crust to variable depth of ~100-200 km (62-217 miles)
Evidence for lithospheric plates of 100-200-km thickness
is based on seismic data suggesting the presence of the low-velocity zone
Magnetic inclination
used to determine the ancient latitude at which a rock formed -- Is also used as an additional post-World War II line of evidence for movement of the continents
Magnetic sea-floor anomalies
-- How does a rock acquire a magnetic signature? -- Evidence that the Earth's magnetic field reverses -- Sea-floor magnetic anomalies occur throughout the ocean floors but lie parallel to the crests of the midocean ridges, which is where the youngest rocks are located
Coring of the Sea Floor
-- recognition of a pervasive sequence consisting of a sediment veneer overlying the rock of volcanic lava flows, called basalt -- recognition that the sea floor is not the same age everywhere based on fossils in the sediment as well as dating of the underlying basalt -- recognition that the youngest ocean floor corresponds to the crests of the mid-ocean ridges
Key elements of plate-tectonic theory
-the Earth's outer layer is divided into rigid lithospheric plates, ~100-200 km thick, whose interiors are relatively inactive
P-Waves
...
S-Waves
...
oceanic crust
0-10 km depth; average 4 km thick [2.5 miles]
continental crust
variable thickness of 20-70 km thick [12.5-45 miles]
4 Lines of evidence for Continental Drift known before World War II (proposed 1912):
1) Fit of continental shorelines 2) Geology matches among the continents 3) Fossils of the same age match among the continents 4) Deposits generated under specific ancient climates
Core
1) Liquid outer core = 2900 km to 5155 km depth [3270 miles] 2) Solid inner core = 5155 to 6371 km depth [3959 miles]
Sea-Floor Spreading views
1) One view is that if the oceans are growing through time, then the Earth must be expanding 2) Alternative view is that oceans are ephemeral -- Concept suggests oceans are generated at mid-ocean ridges and are consumed/destroyed by subduction at the deep oceanic trenches (1962) -- Thus the ocean floor is recycled -- Oldest proper oceanic crust (oceanic floor) is about 164 million years old, whereas the continents exceed 3.9 billion years in age and contain mineral grains as old as 4.1 billion years old
Lower mantle
660 km [or 440 miles] to 2900 km depth [1800 miles]
Low-velocity zone
Asthenosphere = extends 100-400 km [62-217 miles] ("aestheno" = "weak")
Post-World War II brought:
Coring of the Sea Floor; continued bathymetry studies; magnetic sea-floor anomalies
Evidence from Earthquakes:
Energy released from earthquakes (seismic energy) is used to locate each earthquake, but also to interpret the structure of the Earth's interior
Crust-mantle boundary
Mohorovicic discontinuity or Moho for short
World War II brought:
Sea-floor water-depth data (bathymetry of the oceans) -- mountains exist under the sea; Geomagnetics - first extensive use of the magnetometer
Continued bathymetry studies
recognition of Mid-Ocean Ridges
Also known before World War II -Earthquakes
tend to occur in specific areas
data from post WWII suggested
the crust beneath the oceans is created at the mid-ocean ridges