The Changing Crusts Flashcards
Strike-slip
A type of plate movement - two plates moving past each other
Continental Drift Theory
Early 1900s, German scientist noticed similarity between coastlines of S. America and Africa. Postulated they had been joined and had then drifted apart but had no mechanism to generate this motion. Other support included similar fossils and rock types on both continents
Plate Tectonic Theory
Theory states that lithosphere divided into 20 plates. Supported underneath by a fluid, flexible rock medium, they can move and partially overlap. This movement responsible for volcanoes, earthquakes, mountain formation, continent movement and changing shape and spreading of ocean floor. 1960s discovery of magnetic bands mirrored on either side of spreading center supplied evidence for a mechanism to drive plate motion
Divergent plates
Where plates move away from each other. Usually occurs at spreading centers where molten rock from the mantle is reaching the surface. Mid-Atl. ridge, a mountain chain in center of Atl. ocean is the site of upwelling magma. This is driving N. American plate westward and the European plate eastward. The movement between African and S. American plates is also divergent, driven by same spreading center
Convergent plates
Where plates come together. Boundaries are either zones of subduction or strike-slip
Strike-slip: San Andreas fault, CA. North American plate moving south relative to northward-moving Pac. plate.
Subduction: Aleutian Islands, Alasks marks boundary between Pac. and N. American plates still undergoing subduction
Where is the youngest oceanic crust found?
Spreading centers in the middle of the oceans are where crust is being formed. The age increases away from the spreading center on either side. Each side usually a mirror image.
Earthquake
Crust of Earth moving all the time. It is also under stress. When the strength of the rock is less than stress, the crust breaks. This sudden movement called an earthquake.
Fault
Break in earth’s crust where it has cracked from internal pressure. Most strong earthquakes occur along faults. There are 3 types
What are 3 types of faults
normal
reverse
thrust
Uncomformity
A break in rock record. Types:
- Angular unconformity where rocks have been tilted and later other rocks deposited
- Disconformity where there is no apparent break but some geologic time is not represented (usually missing index fossils)
- Nonconformity where sedimentary rock is in contact with non-sedimentary rock
What causes earthquakes?
Internal pressure causes the Earth’s crust to fracture. The vibrations in crust are felt as an earthquake.
The state of ___ in the W. United States has many earthquakes along the ___ fault
California
San Andreas
A building is less likely to be damaged during an earthquake if it rests on _____
solid rock (provides a more stable foundation and vibrates less than loose earth; therefore building may sustain less damage
Tsunami
Powerful and possibly destructive ocean waves produced by earthquake
Shock waves
Vibrations that travel through earth produced by earthquakes
Where do shock waves travel?
Waves travel through crust and also through core and mantle. Some of the waves won’t travel all the way through Earth. This is how we know the outer core of Earth is liquid, not solid.
What determines the speed of a shock wave?
Speed depends on the rock material, density, and temp. the wave is traveling through
Richter Scale
A logarithmic scale from 1-10 that indicates intensity of an earthquake. A value of 3 is 10 times more powerful than value of 2 and 100 times more powerful than value of 1. It is based on amplitude (thus energy) of the waves recorded.
Seismograph
Machine that detects the presence of shock waves and records their magnitude
Seismologist
A scientist who studies earthquakes
Focus
Center and source of earthquake shock waves
Epicenter
Point on the Earth’s surface directly over the focus of an earthquake
Aftershock
Usually a large earthquake followed by many smaller earthquakes (aftershocks). These sometimes result from the moving plates readjusting themselves to new position and sometimes from other continued lesser stresses
3 basic types of earthquake waves
P-waves: primary waves
S-waves: secondary waves
L-waves long (or surface) waves
P-wave
Primary wave - a longitudinal (compressional) wave where rock particles vibrate in direction the earthquake is traveling. Waves can travel through solid and liquid
S-wave
Secondary wave - a transverse wave where rock particles vibrate vertically as the wave energy passes through. These waves can travel ONLY through a solid
L-wave
Long wave (or surface wave) - the wave that travels near the surface of the Earth and causes the most damage. It has a complex, orbital motion that causes major oscillations
What evidence is there that earthquakes existed in the past?
Many faults in the Earth’s crust, indicating that earthquakes have occurred for millions of years
Where do most of the Earth’s earthquakes occur?
At plate boundaries
Many mountains are made of sedimentary rock. How does this suggest that the Earth’s crust moves over time?
Sedimentary rock is formed in basins and low areas at the bottom of ocean. For low areas to become high areas, faulting and compression must take place. The rock making these mountains was once under water.
Define folds. How do they suggest that the Earth’s crust moves?
Large-scale bends of rock exposed when a mountain is cut. They are evidence of compressional pressure on the rock and resulting movement. Usually mountains are made from the low areas, and the high areas are eroded into valleys because the crests crack when they are folded. Erosion focuses there.
4 types of mountains
fault block
fold-belt
domal
volcanic
How is a fold-belt mountain formed?
Pressure at the ends of layers of sed. rock causes them to fold up and down in a wave pattern
Anticline
Formation in a fold-belt mountain where rock is folded up
Syncline
Formation in a fold-belt mountain where rock is folded down
Geosyncline
Zone of weakness in the Earth’s crust. Size: 10+ km wide, 100+ km long, with 10+ km depth of sediment. Generally found at edges of continents
Peneplain
Hills formed in folded mountains may be worn down by erosion. When the area is almost flat, it is called a peneplain.
How is a fault-block mountain formed?
When lg. section of crust fractures and is pushed up relative to its neighbor
How is a domal mountain formed?
Upward pressures from under the surface cause portions of the crust to be lifted and all sides to slope away from center (like Black Hills, SD)
Define plateau. How are they formed
A large flat region that is many feet above sea level. A section of Earth’s crust that rises without bending or breaking may form plateau. The CatSkill mountains in NY are actually a plateau that has undergone non-uniform erosion. The Colorado Plateau is another example
Difference between a plateau and a mountain
Plateau: rock layers are flat
Mountain: rock layers tilted up and down
Define plain. Name 3 basic types.
A flat region that is not as high as a plateau
Types: marine (coastal), glacial, lake
Marine (coastal) plain
Edge of a continent that was underwater when the sea level was higher
Ex: Atlantic Coastal Plain
Lake Plain
When a lake dries out, its former bed becomes a plain
Ex: Great Salt Lake Plain & Bonneville Salt Flats (both in Utah)
Glacial plain
Plain formed from moraine deposits left by glaciers (southern Long Island)
Volcano
Opening in Earth’s crust through which lava emerges
Where do most of the volcanoes occur on Earth?
At plate boundaries. Because molten rock is close to surface where plates are thin (spreading centers), or where they are subducting and melting at depth. The melted plate material rises because it is less dense.
Lava
Magma that has come to the surface of the earth
3 types of volcanoes
shield
composite
cinder cone
How is a shield volcano formed?
This is a broad-shield shaped mountain formed by several slow non violent lava flows over a period of many years
Ex: many of the volcanoes on the Big Island of Hawaii
How is a cinder cone volcano formed?
This is a steep volcano made of ashes resulting from violent volcanic explosions
Composite volcano
Combination of cinder cone and shield
Examples: Mt. Fuji, Mt. Rainier
How is a dome mountain formed?
Magma flows toward the Earth’s crust and applies pressure, pushing up the top layers of sed. rock. Intrusion of magma near the surface of the Earth causes doming around the magma chamber
Ex: Watchung Mountains, NJ
Batholith
Large mass of igneous rock under the Earth’s surface. Formed when molten magma forces its way through layers of rock and hardens. Often form the core of mountains.
Sill
Magma forces its way between layers of rock. Hardens and forms a concordant layer of igneous rock between the other layers of rock.
