Lecture 3: Earth's Structure, Plate Tectonics, and the Rock

Cycle

Our Hazardous Environment

GEOG 1110Dr. Thieme

Structure of Earth

Average density ofthe solid Earth = 5.5 g/cm3

Rocks of continental crust average 2.5 to 3.0 g/cm3

Dense (Fe, Ni) core required by planetary motions

What Is Inside What Is Inside Earth?Earth?

Thickest Thickest layer: layer:

mantlemantle

Upper layer is Upper layer is crust; crust; two types:two types: continental continental oceanicoceanic

Lowest layer: iron-nickel core Lowest layer: iron-nickel core (molten outer core; solid inner (molten outer core; solid inner

core)core)

Relief and Tectonics

• Ocean Basins are spreading away from mid-ocean ridges

• Continental collisions build mountains and increase land surface elevation:

• ocean-ocean

• ocean-continent

• continent-continent

Earth's crust is divided into 7 major and 20 smaller plates

Plate boundaries are defined by areas of seismic activity

Tectonic Cycle: Earth's crust is constantly being recycled as lithosphere is created at spreading ridges, rides on aesthenosphere, and is subducted into the mantle

Tectonic Cycle

• New Ocean Crust is produced at spreading ridges

• Different spreading rates along the ridge are accomodated by Transform Faults

• Compressional Stress at Convergent Boundaries results in Folding and Faulting of Crustal Rocks

• Earthquakes are responses to tectonic stresses

Divergence at Spreading Ridges

- most important area for creation of new crust

- sea floor spreading apart at gradual and constant rate

- ocean floor subducted at trenches, remains geologically young

Ocean-Continent Convergence

- deep sea trenches on the ocean floor

- denser plate of oceanic crust is "subducted" beneath continental crust

- ocean floor remains geologically young

Convergent Boundary - Oceanic-Oceanic

-both plates are the same density

- Aleutian, Mariana, and Tonga Islands in Pacific

- Antilles in Caribbean

Convergent Boundary - Continent-Continent

- both plates are the same density

- edges of colliding plates crumple into mountains

- Himalayan Mountains in Nepal and India

divergent plate boundary

convergent plate boundary

transform faults/triple junctions

Strike-slip (Transform)fault

Reverse (Thrust)fault

Normalfault

Transform Boundary

• most transforms offset spreading ridges of oceanic crust

• the San Andreas fault zone in California is a transform plate boundary

• the Pacific plate is sliding horizontally to the northwest past a segment of the North American plate

• transform plate motions result in the strongest shaking by earthquakes at the Earth's surface

Continental Transform Fault

San Andreas Transform Fault running across the Carrizo Plain in California

Figure 1.18a

San Andreas Fault impounds drainage used by palm trees

Figure 1.18b

Marsh in Pool impounded by San Andreas Fault

Rock Cycle

• all types of rocks can be changed into other types by:

• time

• heat

• pressure

• beginning the cycle with igneous rocks (high temperature) is arbitrary

Igneous Rocks - crystallized from magma which either intruded deep beneath the surface ("plutonic") or extruded to the surface ("volcanic")

Sediment - particles and dissolved substances which settle out of a liquid (oceans, river floodplains, swamps), also windblown (dunes)

Sedimentary Rocks - sediment compacted by weight of overlying layers, cemented by percolating water, or chemically precipitated.

Metamorphic Rocks - Igneous rocks, sediment, or sedimentary rocks altered by being subjected to temperature or pressure conditions above those at the Earth's surface.