BHS Earth & Space Science Plate tectonics BHS Earth & Space Science

Evidence of crustal movement The rock layers at Earth’s surface are constantly in motion. Principle of original horizontality – all rocks form in horizontal layers called strata (singular: stratum). But we often see deformed strata. (Oh noes!)

Strata

Evidence of crustal movement There are three types of deformed rock. 1) Folded rock strata Lateral forces push together and deform folded rock either upwards or downwards. An anticline forms an arch. A syncline forms a bowl-like depression.

Syncline

Evidence of crustal movement 2) Tilted rock Strata tilt angularly upwards or downwards.

Tilted rock

Evidence of crustal movement 3) Faulted rock A fault is where a large crack occurs in the rock and the strata move up or down along the fault. Normal fault Hanging fault

Top: Normal fault Bottom: Hanging or reverse fault

Evidence of crustal movement Location of fossils in sedimentary rock reveals that the Earth’s crust is dynamic, e.g. marine fossils on Mt. Everest. Benchmark elevation markers are placed all over the world. Measurements taken over time reveal that positions of the markers are changing. Satellite data can show how fast tectonic plates are moving.

Evidence of crustal movement A geosyncline: a large shallow ocean basin where sediments accumulate near the edges of the continents. The weight of the sediments causes continent to rise upward.

Geosyncline

Evidence of crustal movement Isostasy: the Earth’s crust is in a state of equilibrium. The lighter continents float on the mantle, and the denser ocean basins sink into the mantle. If a plate sinks somewhere, a plate somewhere else must rise. It’s like squeezing a water balloon.

Isostasy

Continental drift Alfred Wegener, a meteorologist, in 1912 formulated a theory called continental drift that explained how continents were connected and drifted apart. He studied fossils from Africa and South America and found many of the same species (indicator species).

Distribution of the fern Glossopteris

Continental drift Geologists shrugged and hypothesized that ancient land bridges connected the continents but disappeared long ago. Rock types were also similar and scars were left on rocks by glaciers, with the direction of the scarring matching on both continents.

Wegener’s theory was rejected for two reasons: Continental drift Wegener’s theory was rejected for two reasons: He was a meteorologist, not a geologist. People can be protective of their niche. He failed to provide the mechanism for drift – what could move a continent? His theory was dismissed, and he died in 1930. In 1960, it turned out he was right. Oops.

Underwater mapping revealed mountain ranges called mid-ocean ridges. Seafloor spreading After WWII, the US military used sonar technology (developed to find enemy submarines) to take a look at the ocean floor. Underwater mapping revealed mountain ranges called mid-ocean ridges. New crust was being formed at the center and moving outward from the ridge.

Seafloor spreading Evidence included alternating bands of magnetic crystals – and magnetic north and south reversed periodically.

Seafloor spreading Scientists dated rock from mid-ocean ridges and the older rocks were farther from the ridge center and cooled as they moved outward.

Seafloor spreading This indicated large convection cells within the Earth’s mantle, and proved Wegener’s theory right.

The theory of plate tectonics Earth’s surface is composed of large, moving plates, floating on semi-liquid asthenosphere. The plates’ edges are called plate boundaries, and can be detected by presence of volcanoes and earthquakes. 15 major plates and many more minor ones. They move 1 to 10cm/yr.

Divergent plate boundaries Two plates moving away from each other that create new crust. Divergent boundaries form mid-ocean ridges underwater, and rift valleys on land.

Convergent plate boundaries When two boundaries collide: 3 types. Continental-Continental: forms mountains. Oceanic-Oceanic: Subduction forms trenches and volcanic arcs. Oceanic-Continental: Subduction forms trenches and volcanic mountain ranges In the battle of the plates, the bigger you are, the more you win! If you are more dense than your competitor, you lose!

Transform plate boundaries When two plates slide along one another (a lateral fault) Great tension is built up along the fault due to friction, then released as an earthquake! San Andreas fault, CA Golden fault, CO In the battle of the plates, the bigger you are, the more you win! If you are more dense than your competitor, you lose!

Heated material rises, spreads out, cools, then sinks. Mantle convection Heated material rises, spreads out, cools, then sinks. The Earth is like a giant lava lamp. With people and stuff. Hot spots: where magma rises and breaks through crust. Some are along plate boundaries (mid-ocean ridges), others are in the middle of plates. Yellowstone, Iceland & Hawaii In the battle of the plates, the bigger you are, the more you win! If you are more dense than your competitor, you lose!

Mantle convection In the battle of the plates, the bigger you are, the more you win! If you are more dense than your competitor, you lose!

Hot Spot A spot where magma reaches the surface in the central part of a plate instead of along the edge

Which State in the U.S.A is a result of a hot spot under a moving plate?

Which National park in the U. S Which National park in the U.S.A is a result of a hot spot under a moving plate?

several plate boundary features