PLATE TECTONIC DBQ
Plate Tectonic Theory
The theory of plate tectonics is the most important geological theory of the last 100 years. The theory
states that the crust of the earth is broken into about twenty large sections called plates and that
these plates move. As the plates move, they can collide, divide or slide by other plates. These interactions
can result in the formation of a variety of geological features such as mountains, volcanoes, island arcs and
rift valleys.
Formation of Plates
About 4.6 billion years ago, the earth
was a molten ball of magma. Over
time, the outer portion of the earth
cooled and hardened forming the crust.
As the crust formed, it shrank and
cracked. The cracking of the crust
resulted in the formation of the tectonic
plates. These plates, which can contain
both oceanic and continental crust,
float on the partially melted portion of
the mantle called the asthenosphere.
As the asthenosphere moves, the plates
move.
Plate Boundaries
A plate boundary is the area where two plates meet. Plates can move in three different directions in
relation to each other, creating three types of plate boundaries. At divergent plate boundaries, the plates
are moving apart. Convergent plate boundaries occur when plates are colliding into each other.
Transform plate boundaries form when plates slide by each other. Plate boundaries that are visible on
land are called faults. A fault is a crack in the earth's crust along which movement occurs. The most
famous fault is San Andreas Fault in California
Divergent Plate Boundaries (create crust)
New ocean floor is created at divergent plate boundaries involving two oceanic plates. When ocean plates
separate, it is called seafloor spreading. This is occurring at the mid ocean ridge. The mid ocean ridge is
an underwater mountain chain that goes the entire way around the earth. If an ocean plate and a continental
plate separate, the ocean will become wider. If two continental plates divide, a rift valley forms. A rift
valley is an area along a divergent boundary where the land has dropped downward. This low area will fill
with water and eventually a new ocean will forms.
Convergent Plate Boundaries (destroy crust)
If two ocean plates collide, one of the plates is destroyed at a subduction zone. A subduction zone is an
area on the ocean floor where ocean crust is forced beneath another plate into the mantle. The subducted
ocean crust is melted and the magma rises forming volcanoes on the ocean floor. When the volcanoes
become large enough to break the ocean's surface, they are called island arcs.
If an ocean plate and a continental plate collide, the ocean plate is subducted and the magma rises to form
volcanoes on land. If the convergent boundary involves two continental plates, a mountain range forms.
Transform Plate Boundaries
At a transform plate boundary, crust is neither created nor destroyed, merely changed. A transform plate
boundary involves any type of crust that slides by another type of crust. When crust slides by one another,
a tremendous amount of heat and pressure occurs forming regional metamorphic rock.
Label each figure by writing the type of plate boundary it shows.
1. _____________________ 2. __________________________ 3. __________________________
4. Describe what happens when
A. Two continental plates collide
B. Two oceanic plates collide
C. A continental plate and an oceanic plate collide.
5. Breaks in the Earth's crust where plates slide by each other are called ________________.
6. The crust is broken into separate sections called _____________ and they are able to move.
7. A _________ is a deep valley on land along a divergent plate boundary.
8. The geological theory that states that large sections of the Earth's crust are slowly but constantly moving
is called ________________