(Brachiopod Shells from Cincinnatian Group, Ohio.)
We began the day talking about fossil shells at the top of mountains.
This is not a hypothetical scenario – these rocks actually exist
On mountains like this
Highest mountain in Western Hemisphere (6,961 meters, 22,837 feet)
Part of the Andes Mountain Range
Mesozoic marine rocks make up this range; rich in gastropods, bivalves (all sorts of shells)
When those fossils are alive, they look like this, and live in warm ocean basins.
So how do they end up as fossils at the top of mountains.
Leonardo Da Vinci wondered this too…
Leonardo made many observations on mountains and rivers, and he grasped the principle that rocks can be formed by deposition of sediments by water, while at the same time the rivers erode rocks and carry their sediments to the sea, in a continuous grand cycle.
Leonardo appear to have grasped the law of superposition, which would later be articulated fully by the Danish scientist Nicolaus Steno in 1669: in any sequence of sedimentary rocks, the oldest rocks are those at the base.
He also appears to have noticed that distinct layers of rocks and fossils could be traced over long distances, and that these layers were formed at different times. Nearly three hundred years later, the rediscovery and elaboration of these principles would make possible modern stratigraphy and geological mapping
How did those shells come to lie at the tops of mountains? Leonardo's answer was remarkably close to the modern one: fossils were once-living organisms that had been buried at a time before the mountains were raised. To Leonardo da Vinci, as to modern paleontologists, fossils indicated the history of the Earth, which extends far beyond human records.
Since the time of Da Vinci, scientists continued to investigate this question.
By gathering evidence from a variety of phenomena, scientists constructed (and continue to construct!) an explanation.
The name of the process is Plate Tectonics and this theory explains that the solid outermost layer of the Earth is broken up into pieces called plates, and that these plates are constantly moving and interacting with one another.
But why do plates move?
Let’s think of this process/phenomenon in terms of a system:
- Input process output
INPUT = Earth’s internal Heat
PROCESS = Convection
OUTPUT = Plates’ movement
We could go on describing each type of plate movement now, but we’ll do that later.
First, we’re going to put on our scientists hats and go through the process of discovering the process of plate tectonics by looking at the evidence it leaves behind.
The following are the scientific specialties that provide the key evidence for plate tectonics
1. VOLCANOLOGISTS: (The distribution of volcanic features) scientists that study volcanoes, and they have uncovered that volcanic features are not randomly distributed throughout the surface of the Earth, but rather tend to be concentrated in zones that extend in lines at certain locations. The most recognized concentration is along the edge of the Pacific Ocean, where the high concentration of volcanoes (everywhere from South America, North America, and Asia) has earned it the nickname The Ring of Fire. Although they are not as well mapped as features on land, scientists continue to gather evidence that there is also a tremendous amount of volcanic activity along the ocean ridges, mainly in the form of small underwater volcanoes and hydrothermal vents.
2. SEISMOLOGISTS: (The distribution and depth of earthquakes) scientists that study earthquakes (sudden releases of energy stored in rocks) by collecting data about both the location of earthquakes as well as the depth at which earthquakes occur. They noticed that, just like volcanoes, earthquakes tend to concentrate in lines. They saw that shallow earthquakes line up along the mid ocean ridges, as well as along some land masses like Southern California. They also noticed that in other places there are earthquakes of many different depths—shallow, intermediate, and deep– and that these depths occurred in bands arranged from shallowest to deepest.
3. GEOCHRONOLOGISTS: (The age of the seafloor) study the age of rocks. The bottom of the ocean is a difficult place to explore, and so much of the work of geochronologists working on the seafloor did not occur until the 1970’s. While studying the rocks that form the crust below the Atlantic Ocean, they realized that the rocks are youngest in the middle (farthest from land on either side) and get progressively older in both directions—towards the Americas on one side, and towards Europe and Africa on the other. At other plate boundaries where oceanic crust meets continental crust, older oceanic rocks butt up to the edges of continents.
4. GEOMORPHOLOGISTS: (The distribution of topographic and bathymetric features) study the origin and evolution of the Earth’s landforms. Before mapping of the seafloor began more than 200 years ago, they did not have a clear picture of what, if any, features existed below the water. Today, with increasingly accurate satellite and sonar technology, we have a much clearer picture of what all of the Earth’s surface looks like: large mountain ranges at the edges of continents, deep ocean trenches along the edges of ocean basins, mid ocean ridges, and large continental masses.
Use map to reference features to whole class
Use map to reference features to whole class
Use map to reference features to whole class
Born in Michigan in 1920, Marie Tharp was a geologist and cartographer during her lifetime. In partnership with her colleague Bruce Heezen, Tharp was the first to scientifically map the ocean floor. Tharp began her work in science as a geologist for Stanolind Oil in the 1940s. After completing a degree in mathematics, Tharp began working for the Lamont Geological Laboratory in 1948 at Columbia University in New York, which is now the Lamont-Doherty Earth Observatory. It was there that she met Bruce Heezen, also a geologist, and two began working on a project to locate downed aircraft from World War II using photographic data. However, the majority of Tharp and Heezen’s work was a research project to map the topography of the ocean floor.
Up until that time, the ocean floor had previously been envisioned as a flat plain of mud. Tharp and Heezen wanted to map the ocean floor in order to understand its geology and hypothetically connect it to the continents. In order to this, Tharp and Heezen collaborated for many decades, from the 1950s into the 1970s, gathering information. Bruce Heezen went out on research vessels to sea and collected data. Much of the raw data came from soundings, or sonar measurements, of the ocean depths. The process involved the sending out of sound signals at regular intervals. The echo of those signals would be picked up by a microphone on a ship and then a stylus would make marks on a recording paper. The technological improvements of the time allowed for the continuous readings of sounding numbers.
Women at the time were not allowed to work aboard ocean vessels so Tharp used the data collected by Heezen to systematically map the ocean floor back at home. Working with only pens, ink, and rulers, Tharp took those thousands of sonar readings and literally drew the underwater details of the ocean floor, longitude degree by latitude degree. Tharp used what is known as the physiographic mapping technique, using light and texture for her diagrams instead of color. Tharp also integrated research data from other sources such as the Woods Hole Oceanographic Institution and data from seismographs of underwater earthquakes.
Use map to reference features to whole class
divergent boundaries: in places where plates move away from each other, new crust is formed as mantle material rises, cools, and is added to an existing plate. If a divergent boundary occurs in oceanic crust, it forms a ridge (like the Mid-Atlantic Ridge); if it occurs in continental crust, it forms a rift (like the Great Rift Valley in Africa). Because the crust is thinnest at divergent plate boundaries, volcanic activity is quite common here, mainly in the form of hydrothermal vents and underwater volcanoes. Most divergent boundaries occur at the ocean floor (Figure 3).
divergent boundaries: in places where plates move away from each other, new crust is formed as mantle material rises, cools, and is added to an existing plate. If a divergent boundary occurs in oceanic crust, it forms a ridge (like the Mid-Atlantic Ridge); if it occurs in continental crust, it forms a rift (like the Great Rift Valley in Africa). Because the crust is thinnest at divergent plate boundaries, volcanic activity is quite common here, mainly in the form of hydrothermal vents and underwater volcanoes. Most divergent boundaries occur at the ocean floor (Figure 3).
convergent boundaries: in places where plates collide into each other, crust is deformed and destroyed as it is folded and fractured into mountain ranges (like the Himalayas) or subducted into the mantle (like in the Aleutian Islands). In subduction zones, where denser plates sink under less dense plates, earthquakes occur as the plates scrape past one another, and volcanoes result as the subducted plate melts and rises to form pools of magma near the surface. Subduction also creates trenches as the subducted plate bends and goes under the overriding plate (like the Marianas Trench). Trenches are the deepest parts of the ocean, and are found at the edges of ocean basins.
transform boundaries: in places where plates slide past one another, crust is neither created nor destroyed. The primary features of these boundaries are earthquakes that result when the sliding plates catch on each other and store energy that is eventually released as seismic waves. The San Andreas fault system is part of the best known transform plate boundary in the Western Hemisphere, but most transform faults are small and form displacements along spreading ridges. (Figure 5)
