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