Earthquake California: An investigative look at the forces behind continental drift.

Guiding Question: Will part of California eventually, over time break off into the Pacific Ocean? Explain.

Background Research: California

Before you read the article, think about the following:

· How fast are the plates moving?

· Where is the fault located?

· What direction are the plates moving? Is there any evidence about rocks to support your thinking?

The San Andreas Fault is the boundary between the Pacific Plate and the North American Plate. It slices California in two sections from Northern California to the Mexican border. San Diego and Los Angeles are on the Pacific Plate, while San Francisco, Sacramento and the Sierra Nevada are on the North American Plate. Despite San Francisco’s legendary 1906 earthquake, the San Andreas Fault does not go through the city, however communities like San Bernardino, Palmdale, and Bodega Bay lie directly on the fault/

The plates are slowly moving at a couple of inches a year, which is about the same rate that your fingernails grow. This is not a steady motion, it is the average motion. For years the plates will be locked with no movement at all. Suddenly the built-up strain breaks the rock along the fault and the plates slip a few feet all at once. The breaking rock sends out waves in all directions and it is the waves that we feel as earthquakes. 

In many places, the fault is easy to see as a series of scarps and pressure ridges on the surface. In other places, it is more difficult to see because the fault hasn’t moved in many years and is covered with overgrown brush. In San Bernardino and Los Angeles Counties, many of the roads along the fault cut through great mountains of gouge, the powdery, crumbled rock that has been pulverized by the moving plates. 

The hallmark of the San Andreas Fault is the different rocks on either side of it. Being about 28 million years old, rock has traveled great distances and has been found against rocks from very different locations and origins. For example, The Salinian block of granite in central and northern California originated in Southern California.

My Notes:

1. How fast are the plates moving?

2. Where is the fault located?

3. What direction are the plates moving? Is there any evidence about rocks to support your thinking?

University Research: Types of plate boundaries

· Read through the passage, and complete the CDT (Convergent, Divergent, Transform) Table on the following page

Divergent Plate Boundaries:

The most common divergent boundaries are mid-ocean ridges. Shallow earthquakes and minor, basaltic lava flows characterize divergent boundaries at mid-ocean ridges. The seafloor at the ridges is higher than the surrounding plain because the rocks are hot and thus less dense and more buoyant, riding higher in the underlying mantle. As the rocks move away from the spreading center, they cool and become denser and less buoyant. Typically, these types of plate boundaries only produce around 5% of all seismic energy released in a given year and recently determined that they have a moderate amount of volcanic activity.

Convergent Plate Boundaries:

Convergent boundaries are the most geologically active, with different features depending on the type of crust involved. Typically, 90% of all seismic energy released by Earthquakes is resulted from convergent plate boundaries. These earthquakes tend to occur deeper in the crust than at other boundaries. There are different types of convergent plate boundaries, which are explained below.

Oceanic meets continental are subduction zones, where dense oceanic crust is diving beneath more buoyant continental crust. These boundaries are characterized by a very deep ocean trench next to a high continental mountain range, large numbers of earthquakes and large numbers of volcanoes.

Oceanic meets more oceanic occur when two plates converge along a boundary where the crust on both sides is oceanic, a subduction zone also occurs, but the result is slightly different. Since the make-up of the two plates are similar, it is usually the plate with the older oceanic crust that is subducted (goes under) because that crust is colder and denser. Earthquakes progress from shallow to deep moving away from the trench like in the oceanic-continental convergence, and volcanoes form an island arc.

Continental meets continental occurs when two pieces of continental crust converge, the result is a great pileup of continental material. Both pieces of crust are buoyant and are not easily subducted, resulting in large mountain ranges.

Transform Plate Boundaries:

Transform plate boundaries occur when 2 plates move horizontally past one another. This is a very rare occurrence on continents and makes up only about 5% of all seismic energy released in any given year. Not many volcanoes are found in this region due to the movement of the plates and most earthquakes that occur are very shallow. A transform plate boundary is typically marked in some places by linear valleys along the boundary where rock has been ground up by the sliding. In other places, transform boundaries are marked by features like stream beds that have been split in half and the two halves have moved in opposite directions.

While completing the chart below, fill in what features you would find at these types of boundaries

Convergent

Divergent

Transform

USGS Research: Data

You are provided with data for 3 locations, Chili in South America (Pacific Ocean side of South America), Reykjanes Ridge (In the Atlantic Ocean, North to Iceland), and California. All data shown indicates every major earthquake in the region with a magnitude of 6.5 or greater from 1930 – 2014.

Location

Number of Earthquakes

Average Depth

Average Magnitude

Chili, South America

113

54 km

7.0

Reykjanes Ridge, Iceland

4

16 km

6.7

California, United States

13

7 km

6.8

· What conclusions can you make about the data above?

Geological Survey (Map) Data

Figure 1 to the left shows the area of Chili studied for this data set in table 3. Each triangle indicates an active volcano along the fault zone.

· What type of fault line has this many volcanoes?

Figure 2 (below) shows the current active volcanoes in the California region. Each dark triangle indicates an active volcano. Pay attention to the linear valley (Sierra Nevada Mountains) along the location near the San Andreas Fault- it aligns with the volcanoes.

· Are there a lot of volcanoes in the image below? What type of fault line has few volcanoes?

Figure 3 to the right indicates all tectonic plates and their names for reference. The locations of these plates have changed over the years due to continental drift as a result of earthquakes. 200-300 million years ago, the Earth’s surface looked quite different, known as Pangea. All of the continents at this time were combined, forming one super-continent.

· How have the plates location changed over time?

Figure 4 to the left shows the location of Iceland in the middle of the Atlantic Ocean. Running through the middle of the island is the Reykjanes Ridge, which is part of a mid ocean ridge.

· What plate boundary is most typically associated with a mid ocean ridge?

Figure 5 (left) is from the USGS, indicating the average depth of all earthquakes from 1975-1995.

Investigative Reporting from National Geographic

Rift Valley versus Mid-Ocean Ridges

A rift valley forms where the Earth’s crust, or outermost layer, is spreading or splitting apart. This kind of valley is often narrow, with steep sides and a flat floor.

Rift valleys differ from river valleys and glacial valleys because they are created by tectonic activity and not by the process of erosion.

Rift valleys are created by plate tectonics. Tectonic plates are the huge rocky slabs made up of the Earth's crust and upper mantle. Where plates move apart, the Earth’s crust separates, or rifts. Rift valleys can lead to the creation of entirely new continents, or deepen valleys in existing ones.

Mid-ocean ridges are formed as tectonic plates move away from one another at divergent plate boundaries. As the plates separate, molten rock from the Earth’s interior may rise to the surface and harden as it contacts the sea, forming new oceanic crust at the bottom of the ocean. In order to be considered a mid-ocean ridge, there must be a divergent plate boundary at 2 oceanic plates.

This divergent plate boundary occurs along the northern crest of the Mid-Atlantic Ridge (Reykjanes Ridge), where the North American plate and the Eurasian plate are splitting apart, located through the middle of Iceland. The Mid-Atlantic Ridge rifts at an average of 2.5 centimeters (1 inch) per year. Over millions of years, the Mid-Atlantic Ridge has formed rift valleys as wide as 15 kilometers (9 miles).

Two rift valleys on Earth within continental crust include the Baikal Rift Valley and the East African Rift (Right). Tectonic activity splits continental crust much in the same way it does along mid-ocean ridges. As the sides of a rift valley move farther apart, the floor sinks lower.

The deepest continental rift valley on Earth is the Baikal Rift Valley in the Siberian region of northeastern Russia. Lake Baikal, the deepest and oldest freshwater lake in the world, lies in the Baikal Rift Valley. Here, the Amur plate is slowly tearing itself away from the Eurasian plate, and has been doing so for about 25 million years. The deepest part of Lake Baikal is 1,187 meters (3,893 feet), and getting deeper every year. Beneath this is a layer of soft sediment reaching several kilometers. The actual bottom of the rift extends about 10 kilometers (6.2 miles) deep.

· What is the difference between a rift valley and a mid-ocean ridge?

Rift Valley

Mid-Ocean Ridge

Research Expectations

Your group is to furnish a report, answering the guiding question, “Will part of California eventually, over time break off into the Pacific Ocean?” Your report must include only data derived from the previous 6 pages of information including the background research, university research, USGS Data, Geological Survey Data, and Investigative Reporting Data. No other outside sources may be used. All of your statements must be supported by data included on those 4 pages. Speculation and opinion are not allowed. Your report must follow the claim/evidence/justification model as indicated below.

Your Report

Guiding Question: Will part of California eventually, over time break off into the Pacific Ocean?

Our Claim: This is where your answer to the question goes after completion of lab.

Justification:

Why your claim is correct in paragraph form while reflecting on the data. You MUST connect the evidence to your answer!

The following KEY VOCABULARY must be included:

· Convergent

· Divergent

· Transform

· Mid-ocean ridge

· Sea floor spreading

· Volcanoes or Mountain Building

· Continental Drift

Evidence:

All data collected over the course of the lab goes in this box. This data can include (but not limited to)

· Data from data tables (you may not want the entire data table, but you could average the data or make a generalization)

· Data from Maps (You can copy a section of the map and highlight it here)

· Key pieces of data from the readings

· Background research about California