Lecture 3: Earth's Structure, Plate Tectonics, and the Rock Cycle Our Hazardous Environment GEOG 1110 Dr. Thieme

Structure of Earth Average density of the solid Earth = 5.5 g/cm 3 Rocks of continental crust average 2.5 to 3.0 g/cm 3 Dense (Fe, Ni) core required by planetary motions

What Is Inside Earth? Thickest layer: mantle Upper layer is crust; two types: continental oceanic Lowest layer: iron-nickel core (molten outer core; solid inner 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 Normal fault

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.