Download Intro to Earth

The Structure of the Earth
Composed of
Core
Mantle
Crust
The Earth’s Interior
The Core
The center of the earth
3500 km in radius
Divided into
• inner core – solid
• outer core – liquid
Composed of iron with some nickel
3000 to 5000°C
The Earth’s Interior
The Mantle
2900 km thick
Composed of mafic minerals similar
to olivine and perhaps ultramafic
minerals such as peridotite
2800 to 1800°C
The Earth’s Interior
The Crust
8 to 40 km thick
Mostly igneous rock, with significant
proportions of metamorphic rock and a
thin layer of sedimentary rock.
The boundary between the mantle and the
crust is sharply defined and is the
Mohorovicic discontinuity or Moho
The Crust
Continental crust is composed of two zones
Lower, continuous, mafic layer (more dense)
Upper, continuous, felsic layer (less dense)
• Composed mainly of granitic rock
The ocean basins are composed of only one
layer
Almost entirely mafic rocks - gabbro and basalt
The crust is much thicker beneath the
continents (35km) than the ocean basins (7km)
Lithosphere & Asthenosphere
Lithosphere
The outer shell of rigid, brittle rock which is broken
into lithospheric plates
Includes the earth’s crust and the upper part of the
mantle
60 to 150 km thick
Thickest under the continents and thinnest under
ocean basins
Asthenosphere
a weak plastic layer located beneath the lithosphere
The lithosphere moves on the asthenosphere
Temperature reaches 1400°C
Outer Layers
of the Earth
Crust
continental
ocean
Lithosphere
Asthenosphere
Continental
Drift
Continental Drift
Theory proposed by Alfred
Wegener, in the early 1900’s
Evidence included:
Distribution of fossils and plant
species
Glaciations
Geologic Structures and Deposits
Continental Drift
Fossil of a Glossopteris leaf
Fossils of this land plant have been found in South
America, Africa, India, Australia, and Antarctica and
provide evidence that all of these landmasses were
joined as one continent in the Permian
Continental Drift
Wegener’s 1915 map
of the fit of the
continents bordering
the Atlantic Ocean
His explanation of
the way the
continents moved
was weak and highly
criticized
Pangaea
Plate Tectonics
http://pubs.usgs.gov/publications/text/dynamic.html#anchor19309449
Major Concepts
Continental Drift (early 1900’s) supported by
a variety of geologic evidence; however, this
theory did not adequately explain the forces
that caused the plates to move
Major breakthrough in the development of a
complete theory of the earth’s dynamics
occurred in the early 1960’s, when the
topography of the ocean floors was mapped
and magnetic and seismic characteristics
were determined
Evidence
Symmetrical
magnetic banding
found on both
sides of the MidAtlantic ridge
General
paleomagnetic
stratigraphy for
the past 160
million years
Major Concepts
The lithosphere is divided into plates,
bounded by oceanic ridges, trenches,
mountain ranges, and transformed faults
Plates separate where convection currents in
the mantle rise and magma makes its way to
the surface where it spreads at oceanic ridges
Plates descend into the mantle beneath
trenches and are consumed
Plate Tectonics
Refers to all forms of breaking and
bending of the lithosphere (this
includes the crust)
Tectonic Processes include
Extension → pulling apart → faulting
Compression → crushing, squeezing
together → folding and faulting
Cross Sections
Plate Motions & Interactions
Two types of lithospheric plates
Ocean lithosphere and continental lithosphere
Lithosphere floats on the asthenosphere
Ocean and continental lithosphere ‘float’ at
different levels
Plates are simultaneously undergoing both
accretion and consumption
Rates of each determine the size of the plate
Plate Motions –
Spreading Boundaries
Plate spreading allows magma to rise to the surface to
form new crust
Basaltic magma congeals in the rift
At depth under the rift, magma solidifies into gabbro
Basalt and gabbro form new oceanic crust
Plate Motions - Subduction
Thin ocean plate is subducted under the thicker less
dense continental plate
Leading edge of the plate is cooler and denser and sinks
under its own weight once subduction is initiated
Eventually the plate melts, denser mantle rocks become
part of the mantle
Less dense oceanic and continental sediments become
magma and rise to the surface forming chains of
volcanoes
Plate Motions –
Transform Boundaries
Plates not converging or diverging but
sliding past each other along a
transform fault
Associated with mid-oceanic ridges
Earthquakes
Plate Boundaries
Transform
boundary
Spreading
boundary
Converging
boundary
Plate Boundaries Summarized
Spreading boundaries
New lithosphere is created
Converging boundaries
Sea-floor spreading along the axial rift
Lithosphere is being consumed – subduction
Along active continental margins
Transform boundaries
Plates are moving horizontally past each other along
a transform fault
Driving Force for Plate Tectonics
Heat produced by radioactive decay of
unstable isotopes in the crust and mantle
Continents of the Past
During the 1960s, seismologists showed
lithospheric plates are in motion
Evidence based on magnetic data from the
rocks on either side of plate boundaries
Continents are moving today and have for at
least the past 2 billion years
Tectonic Features of the World