History Channel's How the Earth Was Made is a wonderful earth science documentary series. In the episode titled "The Deepest Place on Earth", the program describes the discovery and exploration of the 7-mile deep Marianas Trench, and how it became linked to the theory of plate tectonics. The episode is set up as a type of detective story, with evidence shown to back up a hypothesis. The episode is available on YouTube
The video worksheet is a one-page, double sided handout consisting of 42 multiple choice questions that track the progress of the video. This format enables the students to pay attention to the video while quickly recording their answers. In this way, the students are not bogged down in writing long responses, and they can better enjoy watching the video. A key is included, and the files are provided in both MS Word and PDF formats.
History Channel’s How the Earth Was Made: The Deepest Place on Earth Overview
Oceanic discoveries in the 1950s and 60s led to the development of a powerful new theory of earth science called plate tectonics. The theory states that the earth’s crust consists of about a dozen large, rigid sections called plates. The motions and relative positions of these plates help to determine where earthquakes, volcanoes, and mountain ranges occur. The discovery of the Marianas Trench and other features of the ocean floor were important in the development of the theory.
In 1874, the British ship HMS Challenger undertook an ocean exploration mission. Using ropes and lead weights, the ship members measured ocean depths at regular intervals across the Pacific Ocean. In an area 200 miles off of the coast of Guam, the crewmembers discovered a 7 mi deep chasm in the ocean floor. Now named the Marianas Trench, this feature is so deep that 30 Empire State Buildings would extend from the trench bottom to the ocean surface! The crushing water pressure at the bottom is also the equivalent weight of 50 jumbo jets! In 1960, United States Navy Captain Don Walsh and Jacques Piccard piloted a submersible craft named the Trieste down into the deepest portion of the Marianas Trench, now called the Challenger Deep. After an adventurous descent to a depth of 35,800 ft., they observed a flatfish that was somehow able to survive under these extreme conditions.
In the same decade as the Trieste expedition, a group of geologists led by Harry Hess used sonar to map the ocean floor. Originally developed to detect hidden submarines, sonar works by the transmission and reception of sound waves that bounce off of objects in the deep. Hess and his companions discovered that the Marianas Trench was merely a segment of a planet wide system of underwater canyons that ringed the Pacific Ocean. The trench was also found to be positioned opposite of the East Pacific Rise, an apparent underwater mountain range located nearly on the other side of the Pacific Ocean. Further exploration revealed the rise to be part of another planetwide phenomenon; a giant set of undersea mountains that circumnavigate the ocean bottoms and resemble the stitching on a baseball.
Seismographs set up to monitor the development of nuclear weapons during the Cold War were also used to map the locations of earthquakes. It was discovered that earthquakes are not random occurrences, and that they are concentrated on earth’s underwater ridges and trenches. The identification of these earthquakes led to the recognition that ridges and trenches were likely connected by an overarching planetary mechanism. Ridges and trenches appeared to be the yin and yang of the ocean bottom.
More Cold War technology, a set of magnetic sensors termed Magnetic Anomaly Detectors (MAD), brought new discoveries concerning earth’s magnetic field and the properties of iron-bearing rocks in the earth’s crust. Towed behind ships, the MADs revealed that the ocean floor consists of alternating stripes of magnetic polarity, now called zebra stripes. These stripes act as a tape recorder preserving evidence of changes in earth’s magnetic field. By studying the magnetic properties of igneous rocks, scientists realized that the earth’s magnetic field experiences periodic reversals, where the north pole exchanges places with the south pole (and remarkably a north-pointing compass would instead point south!). The zebra stripes represent a record of earth’s shifting magnetic polarity, with changes occurring on average about every 300,000 years. The facts that the polarity changes are aligned with the length of the ridge, and that they extend in a symmetrical fashion away in opposite directions from the ridge’s center, demonstrate that the ridges are the actual source of the ocean floor crust! Upheavals of magma occurring at the ridges become solidified into the igneous rock basalt. This freshly made rock forms new ocean floor material and, by a then unknown mechanism, is pushed away from the ridges. The crust grows outward from the ridge in opposite directions by about two inches per year, which is the same rate as human fingernails grow.
A set of volcanoes named the Marianas Islands extends in a line parallel to the Marianas Trench. Other ocean trenches are found to be associated with similar volcanic arcs. It seems likely that the ocean trenches might be locations where the earth’s crust becomes recycled back into the mantle. The investigators reasoned that, because the earth doesn’t change its diameter, new crust generated at the ridges must be matched by places where old crust is later destroyed. It was realized that the volcanoes were positioned on the ocean floor directly above places where magma becomes heated in the mantle, and that this was likely due to the friction generated as one piece of ocean floor sinks beneath another (the addition of ocean water helps to facilitate the melting process). This process whereby one plate is forced under another was termed subduction, and the trenches where this occurs were later named subduction zones.
Undersea exploration of the Marianas Islands also revealed gently erupting “mud volcanoes” that were apparently the product of the soft, serpentine-bearing rocks of the Marianas Trench ocean crust. The Marianas Trench was unusual in that it lacked major earthquakes. The soft rocks dampen the sudden, jerking motions of earthquakes. The crust is also the heaviest and oldest part of the ocean floor, and this increased weight causes the Pacific Plate to subduct under the adjacent Philippine Plate (this mechanism of slab pull, where the heavier plate pushes its way under an adjacent plate, likely forms the mechanism for plate growth at the ridges). Rocks drilled from the Marianas Trench date to 170 million years, and this appears to be the oldest that ocean floor rocks can get before being subducted or recycled back into the mantle.
Discoveries such as the planetwide system of ocean ridges and trenches, earthquake patterns, and the ages and magnetic orientations of ocean crust became evidence for the powerful new theory of plate tectonics. In plate tectonics, the earth is broken into roughly a dozen large plates, which move slowly around the earth, growing at the ocean ridges, and disappearing at the trenches. The boundaries between plates are places that generate earthquakes and volcanoes. In this way, plate tectonics is a fundamental theory that explains much of the earth’s features.