Sept 9 Lecture (Cont Drift, Plate tectonics) Flashcards
magnetic north has changed throughout geological time via ___ ___
magnetic reversal
define normally magnetized rocks
rocks crystallizing at times when earth’s magnetic field was in the same orientation as today
define reversely magnetized rocks
rocks crystallizing when the field was oriented the opposite direction as it is today
___ ___ explains magnetic patterns on the seafloor
seafloor spreading
explain the magnetic patterns on the seafloor and how it happened
- newly formed basalts contain ferromagnesian minerals, so attain magnetism when cooled. the basalt is continuously extruded at mid-ocean ridges
- as the seafloor splits/ moves away from the ridge, half of the newly magnetized material moves to 1 side and half to another: 2 symmetrical magnetized bands formed
- new material fills in the crack, continuing the process
- so, the ocean floor has developed an alternating pattern of normal and reversely magnetized rocks
topographic maps of the seafloor have shown:
3
- there are ridges running N-S in both Atlantic and pacific oceans
- along the margins of some continents there are trenches several km deep
- the other continents are bounded by shallow water shelves
define lithosphere
- what do stresses cause?
earth’s crust and uppermost mantle (solid)
- stresses cause brittle and elastic deformation
asthenosphere
plastic layer beneath the lithosphere
- lithosphere plates can move over this plastic layer; plates carry the continents
some plates are moving towards each other = ___ ___ ___
- some plates are separating = ___ ___ ___
- to have this happen on a spherical planet, plates must be able to slide past one another (___ ____)
convergent plate boundary
divergent plate boundary
transform boundary
at transform- fault boundaries:
plates slide horizontally past each other
at divergent boundaries:
plates move apart and create new lithosphere
at convergent boundaries:
plates collide and one is pulled into the mantle and recycled
define black smokers
(occur at divergent boundaries- mid-ocean ridges)
- hot springs; sites of large biological communities that live off the volcanic heat/ gases (not sunlight)
how is a new lithosphere formed at divergent boundaries?
at mid-ocean ridges, the release of pressure as the plates move apart allows hot basalt magma from the asthenosphere to reach the ocean floor. It cools & crystallizes to form a new lithosphere
explain continental rifting at divergent boundaries
- plate separation can also occur on continents- characterized by long rift valleys w/ basalt eruptions
- sometimes, continental rifting allows a new ocean basin to open up
continental lithosphere is (more/ less) dense than the asthenosphere (mantle)
less
oceanic lithosphere is more similar in density to asthenosphere, so it’s easily forced under continental material when the plates converge. this is called ___
subduction
at convergent boundaries, the subducted plate is heated/ melted, and reincorporated into the ___
- this applies to ___/___ convergent boundaries
asthenosphere
ocean/ continent
how do andesite volcanoes form at ocean/ continent convergent boundaries?
they form where the melted material rises up through overlying plate to the surface
why do mountains form at ocean/ continent convergent boundaries?
the continental crust is compressed, folded, and faulted. This deformation= mountains form
in a continent/ continent collision, the 2 landmasses come together to form a ____
why?
mountain
- b/c both plates too buoyant to sink into mantle
explain transform boundaries
b/c of spherical shape of earth, the spreading ridges cannot be long/ continuous. They’re in numerous short segments (2 opposite plates scrape past each other).
- lots of stress here, leads to eq
explain hot spots
- unusual volcanic eruptions
- arise when isolated plumes of magma rise from deep within the asthenosphere to the surface
- plumes are fixed in the mantle, so as the plates move over it, there becomes a progressive trail of volcanoes at earth’s surface
