geol unit1 pt3

Crust

The outermost layer of Earth, primarily composed of igneous and metamorphic rocks, remaining 5% is sedimentary

Base of crust (crust-mantle boundary)

Moho (Mohorovicic seismic discontinuity

Oceanic Crust (dense)

Thin (7-10 km), young (less than 200 million years), and composed mainly of mafic igneous rock.Homogenous composition

Continental Crust (light)

Thick (30-70 km), old (up to 4.4 billion years), and heterogeneous in composition. Felsic, intermediate, and mafic igneous rocks, Sedimentary rocks, Metamorphic rocks

Mantle

The layer beneath the crust, constituting 82% of Earth's volume and 68% of its mass. more dense than crust.

mantle is ____ km thick

2900

crust

Mantle contains more iron, magnesium, and calcium than the ___

peridotite

The upper mantle is typically composed of ______, a rock dominated by the minerals olivine and pyroxene

peridotite is the most _____ rock in our planet!

abundant

Core

The innermost layer of Earth, composed of a dense metallic core primarily made of iron and nickel.

outer core (liquid)

Circulation of liquid iron alloy in the outer core generates Earth’s magnetic field

inner core (solid)

Has the properties we would expect of solid iron mixed with a small percentage of nickel. Solid due to Extremely high pressure and heat.

the core is __% of Earth’s volume, and __% of its mass

16, 32

Plate Tectonics

The theory explaining the movement of Earth's lithospheric plates and the interactions at their boundaries.

Continental Drift

The hypothesis proposed by Alfred Wegener that continents have moved over geological time.

What led to the development of the Theory of Plate Tectonics in the late 1960s?

Technological advancements

Are not

The positions of the continents and ocean basins ____ fixed

How many years did it take for the Theory of Plate Tectonics to be accepted?

50

Why did it take so long for the Theory of Plate Tectonics to be accepted?

1. True revolution in thinking about Earth- It was difficult for many geologists to accept

2. Political gulf between the main proponent of continental drift (German Alfred Wegener) and the geological establishment of the day, which was mostly in the UK and US

3. Key evidence and understanding of Earth that would've supported plate tectonics theory simply did not exist until the middle of the 20th century (we had to wait for technology to catch up)

What was Wegener's Idea?

*Continents wander about

*they bump into each other, collisions causing mountains

*Continents were once all joined together in one supercontinent (Pangaea), containing all of Earth's Landmasses, and slowly moved to their current positions

*During the Mesozoic Era (200 million years ago) Pangaea began drifting or splitting apart

Supporting Evidence of Continental Drift

1. Fit of the Continents

2. Distribution of Climate Belts

3. Past Glaciations

4. Fossil Evidence

5. Matching Rock Units

Pangaea

The supercontinent that existed around 200 million years ago, consisting of all Earth's landmasses.

 No oceanic crust older than ___million years old

180

what drives plate motion?

convection

convection

type of heat transfer that involves the actual movement of a substance

convergent boundaries

two plates come together, results in either oceanic lithosphere descending beneath an overriding plate, eventually to be reabsorbed into the mantle, collision of two continental blocks to create a mountain belt

Divergent and convergent plate boundaries each account for about __% of all plate boundaries. Transform faults account for the remaining __%

40,20

Oceanic-Oceanic Divergent Oceanic Ridge (mid-ocean ridge)

• broad, linear swell (mid ocean ridge) along a divergent plate boundary 

• Longest topographic feature on Earth 

• Occupy elevated positions 

• Segments offset by transform faults (strike-slip motion) 

• Extensive faulting and earthquakes (shallow focus) 

• Rift valley (deep, down-faulted structure) exists on axis of most ridges 

•  Volcanoes (new ocean crust created here)

Why Are Ocean Ridges Elevated?

Newly created lithosphere is hot and less dense than surrounding rocks. As the newly formed crust moves away from the spreading center, it cools and increases in density

When lithosphere is thick and cold, rifts are _____

narrow

very wide

When the lithosphere is thin and hot, rift can be…

Evolution of an Ocean Basin 

• Divergent plate margins first develop in continents 

• New ocean basin begins with formation of continental rift 

• elongated depression (rift valley) where lithosphere is stretched and thinned

East African Rift Valley 

• Continental rift extending through eastern Africa 

• Consists of several interconnected rift valleys 

• Normal faulting led to grabens (down-faulted blocks) 

• Area has expensive basaltic flows and volcanic cones (volcanoes present here!)

• lithosphere is thick and cold, rifts are narrow

Red Sea Rift 

• Formed when Arabian Peninsula rifted from Africa beginning ~30 million years ago 

• Fault scarps surrounding Red Sea similar to structures seen in East African Rift 

•  If spreading continues, Red Sea will grow wider and develop an elongated mid-ocean ridge

Atlantic Ocean 

• After tens of millions of years, the Red Sea may develop into a feature similar to the Atlantic Ocean

• As new oceanic crust was added to the diverging plates, rifted margins moved further from region of upwelling 

• These margins cooled and subsided below sea level

New Madrid Seismic Zone (NMSZ) 

• prolific source of intraplate earthquakes (earthquakes within a tectonic plate) in the southern and midwestern US 

• responsible for the 1811–12 New Madrid, MO earthquakes 

• M 7-8 (?) 

• has the potential to produce large (M 7-8?) earthquakes in the future!

• Reelfoot Rift formed during breakup of supercontinent Rodinia ~ 750 million years ago) 

• resulting rift system failed to split the continent, but has remained as zone of weakness deep underground

Oceanic-Oceanic

•  two oceanic plates move toward one another 

• one oceanic plate bends and sinks down into the asthenosphere beneath the other plate 

• Geologists refer to the sinking process as subduction 

• These types of convergent boundaries are also known as subduction zones 

•  EARTHQUAKES and VOLCANOES have the potential to cause TSUNAMIS at subduction zones

Major Features of Subduction Zones

Volcanic island arc, Deep-ocean trench, Forearc region, Back-arc region

Paleomagnetic records show…

Apparent polar wandering (evidence that continents have moved), Paleomagnetic Reversals 

Evidence from Paleomagnetism

• Basaltic rocks contain magnetite (Fe3O4), an iron-rich mineral influenced by Earth’s magnetic field 

• When the basalt cools below Curie point, iron-rich minerals become magnetized and align with existing magnetic field 

• magnetite is then “frozen” in position and, like a compass needle indicates the position of the magnetic north pole and magnetic inclination angle at the time the lava solidified

increase

sediments ______ in age with increasing distance from the ridge crest 

Thickest

sediments almost absent on ridge crest; sediments _____ furthest from the spreading center 

Harry Hess

Seafloor Spreading

Evidence from Seismic Surveys

Seismic surveys use acoustic waves to create images of the subsurface

mantle plume

a cylindrically shaped blob of molten rock

hot spot

surface expression of a mantle plume

Volcanic Island Arc (ex: Aleutian Islands in Alaska)

subducting slab partially melts the overlying mantle wedge. Melt migrates upward through the overlying oceanic lithosphere and forms a growth called a volcanic island arc or island arc

forearc region

area between the trench and the volcanic island arc

back-arc region

• located on the side of the volcanic island arc opposite the trench 

• both regions consist of pyroclastic material and eroded sediments 

• tensional forces prevalent in these regions, causing stretching

Trenches created when….

oceanic lithosphere bends as it descends into the mantle

subducting lithosphere

Trench depth is related to the age of the…

Old lithosphere is…..

cold and dense

Young lithosphere

warm and buoyant

Subduction Zones

Areas where one tectonic plate sinks beneath another, often associated with earthquakes and volcanic activity.

Divergent Boundaries

Plate boundaries where two plates move apart, leading to the formation of new oceanic crust.

Transform Boundaries

Plate boundaries where two plates slide past each other without creating or destroying lithosphere.

Hot Spots

Areas of volcanic activity that are not located at plate boundaries, often associated with mantle plumes.Waning volcanic activity produces individual volcanoes that are carried away by plate motion to form a chain of extinct volcanoes.

Geologic Time Scale

A system of chronological dating that relates geological strata to time.

Minerals

Naturally occurring, inorganic solids with a definite chemical composition and crystalline structure.