Recycling the Seafloor
Lisa Ayers Lawrence, Virginia Sea Grant, Virginia Institute of Marine Science
- Classify the earth's layers and types of plate boundaries.
- Predict locations of plate boundaries.
- Interpret relationships between plate boundaries and seismic activity.
Continental drift, Plate tectonics, ore, Mantle, Crust, Plate boundaries, Divergent, Convergent, Transform, Boundary zones
Map of Major Earthquakes 1900-2017
Outline the location of plate boundaries using the National Oceanic and Atmospheric Administration's Accoustic Monitoring Program's underwater earthquake data.
Since the days of Leonardo da Vinci, people have pondered over the fact
that the continents' edges seem to fit together like p
days of Leonardo da Vinci, people have pondered over
the fact that the continents' edges seem to fit together like puzzle pieces.
Over the centuries, scientists and naturalists speculated on this, and in 1912
German meteorologist Alfred Wegener proposed his theory of continental drift
Supported by the fit of the continents and the presence of matching fossils and
geologic structures on different continents, the continental drift theory
contends that all the continents were connected in one supercontinent called Pangea, which split apart into Laurasia and Gondwanaland 200 million years ago. Over time,
these continents split and drifted apart forming the earth as we know it today.
advances in technology, especially during World War II, more clues to the earth's
geologic history were unearthed. Scientists were better able to map
and study the ocean floor and discovered the presence of an underwater mountain
range chain called the mid-ocean ridge system and deep-sea trenches.
They also discovered that the ocean floor is not as old as expected and that
the magnetic properties of the ocean sediment alternate in a matching pattern on
either side of a mid-ocean ridge. These discoveries led geologist Harry Hess to
develop his seafloor spreading theory. According to this theory, the mid-ocean
ridges are spots where the earth's crust is weak, allowing magma from the
earth's mantle to come up to the surface and create new seafloor. Deep-sea
trenches are areas where old seafloor is being driven back into the mantle.
Further research found that earthquakes and volcanic activity are concentrated
near ridges and trenches. The clues all led to the modern day theory of plate
understand plate tectonics, we must first look at the make-up of the earth. The
earth is divided into three main layers:
- core - made of dense
metals, the inner core is solid and the outer core is liquid
- mantle - a thick layer of
semi-solid molten rock surrounding the core
- crust - the thin, hard
outer layer broken into different slabs or plates
The semi-solid mantle moves in a
circular motion, pulling the overlying crustal plates with it. The mantle's circular motion comes from two sources. First,
heat from within the earth warms the mantle material, causing it to rise toward
the crust. Near the crust, the mantle material cools and sinks down toward the
core. The second force impacts the mantle at deep sea trenches. The heavy crustal plate is pulled by gravity down into the mantle
dragging mantle material with it (this is called "slab
pull"). Due to this convection motion in the mantle, the large
plates covering the earth's surface are constantly moving. The boundaries
between these moving plates are very active areas. The plate
boundaries can be one of four types:
- divergent - moving away from
one another (mid-ocean ridges)
- convergent - moving toward one
- transform - slipping past one
- boundary zones - complex areas with
several interacting plates
activity students will use data from underwater earthquakes to deduce the
location of plate boundaries.
- Divide class into pairs or small
groups. Print a copy of the world map for each group.
- Download the ocean seismicity map based on USGS data showing major earthquakes from 1900-2017.
- Using the location of seismic activity
(underwater earthquakes) from the NOAA data, ask your students to outline
on the world map where they believe the relevant plate boundaries are. When
they are finished, compare to the USGS's map of the plates and
discuss. Did the students' plate boundaries align with those of the USGS? Ask them to finish sketching and label the
relevant plates. Were there areas
with earthquakes but no plate boundaries?