Earth Science Test 3

What is the difference between magma and lava?

Magma is molten rock below Earth's surface. Lava is molten rock that reaches the surface.

What is magma viscosity?

Viscosity is magma's resistance to flow. High-viscosity magma flows slowly.

How does silica content affect magma viscosity?

Higher silica content generally makes magma more viscous.

Why can gas-rich, viscous magma erupt explosively?

Viscous magma slows gas escape. Expanding gas can build pressure and fragment the magma.

What is an effusive eruption?

An eruption dominated by lava flowing from a vent.

What is a shield volcano?

A broad, gently sloping volcano built mainly by fluid lava flows.

What is a stratovolcano (composite volcano)?

A steep-sided volcano built by repeated eruptions of lava and fragmented volcanic material.

What is a cinder cone?

A small, steep volcano built from loose fragments that fall around a vent.

How does a caldera form?

The ground collapses after enough magma leaves a reservoir that the roof above it can no longer stay supported.

What is volcanic ash?

Fragments of rock, minerals, and volcanic glass less than 2 millimeters across.

What is a pyroclastic flow?

A fast-moving, hot mixture of gas and volcanic fragments that travels along the ground.

What is a lahar?

A flowing mixture of water and volcanic debris that often follows river valleys.

Why do volcanoes form above subduction zones?

The sinking plate releases water into the mantle above it, lowering the mantle rock's melting temperature and helping produce magma.

A volcano is broad with gentle slopes and fluid basaltic lava. What type is it?

A shield volcano.

Why can a volcano form far from a plate boundary?

A hot spot can supply magma within a tectonic plate, as in Hawaii.

What is relative dating?

Putting rocks or geological events in order from older to younger.

What is numerical (absolute) dating?

Estimating an age in years, usually with an uncertainty.

What is the law of superposition?

In a sequence of sedimentary layers that has not been overturned, the oldest layer is at the bottom.

What is the principle of original horizontality?

Sedimentary layers usually form roughly horizontally. Strong tilting generally happens after deposition.

What is the principle of cross-cutting relationships?

A fault or intrusion is younger than the rocks it cuts.

What is the principle of inclusions?

A rock fragment enclosed in another rock is older than the rock that contains it.

What is an unconformity?

A surface that represents missing geological time due to erosion, a pause in deposition, or both.

What is an angular unconformity?

A surface separating older tilted or folded layers from younger layers deposited at a different angle.

What makes a fossil useful as an index fossil?

Its species lived for a relatively short time and was geographically widespread.

How does radiometric dating estimate age?

It uses the measured amounts of radioactive parent isotopes and their decay products, together with a known decay rate.

What is a half-life?

The time it takes for half of the radioactive parent atoms in a sample to decay.

If 25% of a radioactive parent isotope remains, how many half-lives have passed?

Two half-lives. The remaining amount went from 100% to 50% to 25%.

An isotope has a half-life of 10 million years. How old is a closed sample with 12.5% of its original parent isotope left?

30 million years. Three half-lives have passed.

Why is carbon-14 dating useful for recent wood but unsuitable for dinosaur fossils?

Carbon-14 dates once-living material from roughly the last 50,000 years. Nonavian dinosaur fossils are millions of years older.

How can volcanic ash layers help date a fossil-bearing sedimentary layer?

Radiometric ages from ash below and above the fossil layer can bracket the age of deposition.

What causes most tectonic earthquakes?

Stress builds until rocks suddenly slip along a fault, releasing energy as seismic waves.

What is elastic rebound?

Rocks deform as stress builds and spring toward a less strained shape when a fault slips.

What is an earthquake's focus (hypocenter)?

The point underground where the rupture begins.

What is an earthquake's epicenter?

The point on Earth's surface directly above the focus.

How do P waves move particles?

They compress and stretch material in the same direction the wave travels.

How do S waves move particles?

They move material side to side or up and down, perpendicular to the direction the wave travels.

Which seismic waves arrive first, and which can travel through liquid?

P waves arrive first and can travel through solids and liquids. S waves travel through solids.

Where do surface waves travel?

Along or near Earth's surface, where they can produce strong shaking.

How does the P-to-S arrival time gap change with distance from an earthquake?

It generally gets longer as distance increases because P waves travel faster than S waves.

How can three seismic stations locate an epicenter in the standard classroom method?

Use each station's P-to-S arrival gap to estimate a distance. Draw circles with those radii around the stations and look for their intersection.

How do earthquake magnitude and intensity differ?

Magnitude measures overall earthquake size. Intensity describes the shaking and effects at a specific location.

What does a one-unit increase in earthquake magnitude represent?

About 10 times the recorded wave amplitude at comparable distances and about 32 times the energy released.

What is a normal fault?

A fault where the hanging wall moves down relative to the footwall, commonly under tension.

What is a reverse fault?

A fault where the hanging wall moves up relative to the footwall, commonly under compression.

What is a strike-slip fault?

A fault where blocks slide mainly horizontally past each other.

How can an undersea earthquake trigger a tsunami?

Sudden vertical movement of the seafloor can displace the water above it and generate waves.

What is a tectonic plate?

A rigid piece of lithosphere that moves relative to neighboring plates.

What makes up the lithosphere?

Earth's crust and the rigid uppermost mantle.

What is the asthenosphere?

A weaker, mostly solid region of the upper mantle that can deform and flow slowly beneath the lithosphere.

What happens at a divergent plate boundary?

Plates move apart. At mid-ocean ridges, magma cools to form new oceanic crust.

What happens at a convergent plate boundary?

Plates move toward each other. One plate may subduct, or two continents may collide.

What happens at a transform plate boundary?

Plates slide horizontally past each other.

What is subduction?

The sinking of one tectonic plate beneath another into the mantle.

What features form when an oceanic plate subducts beneath a continent?

An ocean trench and a volcanic arc on the continent can form, as along the Andes.

What features can form when two oceanic plates converge?

An ocean trench and a volcanic island arc can form.

What happens when two continents collide?

The crust shortens and thickens, building mountains such as the Himalayas.

Where is oceanic crust youngest?

At mid-ocean ridges, where it forms.

How do matching magnetic stripes on either side of a ridge support seafloor spreading?

New crust records Earth's magnetic polarity as it cools. Spreading carries matching bands away on both sides of the ridge.

What evidence supported Wegener's idea of continental drift?

Continents fit together broadly and share fossils, rock formations, and evidence of ancient climates across oceans.

What is slab pull?

The force exerted as a cold, dense oceanic plate sinks and pulls the rest of the plate behind it.

How can a hot-spot island chain show the direction of plate motion?

If the hot spot stays roughly fixed, the plate has moved from the youngest volcano toward the older volcanoes.

A plate boundary has a trench and earthquakes that get deeper inland. What is happening?

An oceanic plate is subducting. The deeper earthquakes trace the sinking plate.