MARS 120
Week 2 Pre-lab exercise
Due at the beginning of lab (10 pts)
Provide answers on a Scantron form
Marine sediments: refer to Chapter 5 and Table 5-1 on page 139 in Thurman and Trujillo
Match the following numbered rock type with the lettered statement
1) Lithogenous sediments
a. derived from continental materials
2) Biogenous sediments
b. space dust
3) Hydrogenous sediments
c. grain size classification of sediments
4) Cosmogenous sediments
d. derived from inorganic chemical reaction with seawater
5) Wentworth scale
e. derived from biological processes
Match the following numbered organism with the lettered statement
6) Coccolithophore
a. siliceous algae
7) Diatom
b. calcareous algae
8) Foraminifera
c. calcareous protozoan
9) Radiolarian
d. calcareous framework
10) Coral
e. siliceous protozoan
MARS 120 Laboratory
Week 2
Marine Rocks
Objective:
1. to become familiar with marine rocks
2. to understand concepts and be able to do calculations of SONAR two-way travel
time, one way travel time, seismic reflection and seismic stratigraphy
3. to be able to read and interpret data shown in a seismic section
Part I Marine Rocks
In this part of the lab exercise, your task is to become familiar with the igneous and sedimentary rocks
that make up the floor of the oceans and continental margins. You will examine hand samples of
typical oceanic rocks and view organisms whose microscopic tests make up the majority of deep ocean
sediments. You will also examine the associations of oceanic rocks, and consider where in the oceans
they are found.
Station 1. IGNEOUS ROCKS
Igneous rocks are formed when silicate magma solidifies to form a rock. Volcanic igneous rocks are
formed when magma erupts at the surface and solidifies quickly. Rapid crystallization produces small
(usually microscopic size) interlocking mineral crystals. Basalt, a volcanic igneous oceanic crustal
rock, is usually black and very fine grained (crystals too small to see without a microscope). Plutonic
igneous rocks are formed when magma cools slowly and solidifies at great depth in the crust. Gabbro,
a plutonic igneous oceanic crustal rock, usually has black, dark green and dark gray crystals that are
big enough to see and that form an interlocking mosaic of crystals. Peridotite, a plutonic igneous
upper mantle rock, is usually has dark green and/or black crystals that are big enough to see and that
form an interlocking mosaic of crystals. In general, oceanic lithosphere is made of a layered sequence
of rocks, listed from sea floor downward: marine sediments; basalt; gabbro; peridotite.
In your lab notebook, describe each of the rocks at Station 1.
1. What is the color?
2. What is the texture (very small crystals; crystals large enough to see)?
3. Is it plutonic or volcanic? How do you know?
4. What is the name of the rock?
5. Arrange the rocks in the order they are likely to be found in oceanic lithosphere.
6. Sketch a columnar section of oceanic crust showing the sequence of igneous rocks
SEDIMENTARY ROCKS:
Sedimentary rocks are formed when materials pile up on the sea floor, continental margin or
continents, and then are buried or cemented, allowing them to harden into rock. Refer to Table 5-1 in
Thurman & Trujillo, page 139.
Station 2 LITHOGENOUS SEDIMENTARY ROCKS
Lithogenous sedimentary rocks (also called clastic sedimentary rocks) are composed of clasts (rock
fragments or single mineral fragments) derived from preexisting rock material. Clastic sedimentary
rocks are usually classified on the basis of the size of the particle which makes them up and are named
on that basis. Thus, a rock formed of sand-sized particles is called a sandstone, and a rock made of silt-
sized particles is called a siltstone. There is one exception to this rule: a rock made of pebble sized
pieces of rock is called a conglomerate if the pieces are rounded, or a breccia if the pieces are angular.
The greatest production of clastic sediments in the world occurs on the edge of the ocean basins, where
sediments brought by rivers to the ocean pile up on the continental shelf, and occasionally slide down
submarine canyons to the ocean floor. These rocks sort themselves out somewhat as the slides, which
are called turbidity currents, settle out. The layered bodies of rock formed in this way are called
turbidites. They include mixed up sandstones, siltstones, conglomerates and breccias. Claystone is
found in the deepest oceans, where a material called red clay builds up very, very slowly. Red clay
forms only if no other sediment can form.
Describe each of the rocks at Station 2.
1. What is the color?
2. What is the texture (size of the clasts: pebble, sand, silt, clay)?
3. Can you see obvious layers or not?
4. Can you see fossils or not? If fossils are present, what is the organism?
5. What is the name of the rock?
6. Where in the oceans is the rock likely to be found (near shore, continental shelf, abyssal plain)?
Station 3 BIOGENOUS SEDIMENTARY ROCKS
Biogenous sedimentary rocks are composed tests (shells) of marine organisms. The sea floor receives
a constant snow of tests from the simple organisms that live near the surface of the ocean. The test of
three of these organism groups are large enough to see under an ordinary microscope. These are the
foraminifera, which make their tests of calcite, the diatoms, which make their tests out of silica, and
the radiolarians, which also make their tests of silica. It is the foraminifera whose tests eventually turn
into chalk, and the diatoms and radiolarians whose tests become chert. Foraminiferal sediments form
anywhere in the oceans that the sea floor lies above the Calcite Compensation Depth. This depth varies
from ocean to ocean, but in general is around 4500 meters. Radiolarian sediments build up
significantly near the equator, where nutrient rich waters well up. Diatoms are abundant in cold polar
waters and also live in warm tropical waters. Diatom sediments build up in polar regions, around 60
degrees north and south latitude.
Station 3A – Foraminifera
There are three families of foraminifera in this slide. Sketch each of them.
Station 3A – Globerigina
Examine the shells and beach made of globerigina cells (made of calcium). Draw a test.
Station 3B - Diatoms
Sketch four types of diatoms. Be sure you are actually seeing different varieties and not the same one
from a different angle.
Station 3B - Radiolarians
These slides contain Radiolarians. Find and sketch two radiolarians in the slide.
Station 3C – Oozes
Examine the two types of oozes. One is chalk, made out of calcite oozes, and the other is dolomite,
made from siliceous oozes. Use the 10% HCl to figure out which is which.
Station 4 HYDROGENOUS SEDIMENTARY ROCKS
Hydrogenous sedimentary rocks (sometimes called Chemical sedimentary rocks) differ from clastic
sediments in the size of the particles that make them up. They precipitate out of sea water, filling up
the spaces between particles in clastic sedimentary rocks, or forming beds entirely of one mineral on
the continental shelves. Thus, they are originally made up of molecule-sized particles rather than siltor clay-sized particles.
Station 4A Limestone and Chert
The most abundant chemical sedimentary rock is limestone, which is made of calcite. Limestone is an
extremely common rock, but it's also tricky to define. Any rock made almost entirely, or entirely of
calcite is a limestone. Limestone in the oceans is formed in a great variety of locations. The most
important and widespread is on the deep sea floor away from the continent, where great thicknesses of
calcite shells (tests) from small marine organisms build up on the ocean floor, forming the variety of
limestone called chalk. Chalk may become recrystallized into limestone. Another form of
biologically produced limestone is coquina, which forms when seashells are cemented together. Reefs
also form a very wide variety of limestones, many from the formation of skeletal structures by calcite
secreting organisms like corals. Limestone may form in restricted basins, which are places where part
of the ocean has been isolated, so that the water in it evaporates, and the materials in the water
precipitate. Chert is made of diatoms and radiolarians whose tests have recrystallized.
Describe each of the limestones and chert
1. What is the color?
2. Do you see layers or not?
3. Do you see fossils or not?
4. Do these rocks effervesce or not in dilute acid?
Station 4B Oolite
Oolites are small calcite spheres that precipitate directly from warm shallow seawater. Describe the
oolite.
1. What is the color?
2. What is the texture?
Distribution of Sediments
You are chief scientist on a research expedition that is conducting piston coring in the Pacific Ocean.
Piston cores are ten meter long samples of ocean bottom sediment. The corer is driven into the ocean
bottom and, when it is pulled up, it contains an undisturbed plug of sediment from the ocean bottom.
As each core arrives on the deck of the ship, you hand it to your graduate student, who is supposed to
write on it the latitude and longitude where you took the core. Your graduate student, however, is
thinking about all the episodes of "Days of Our Lives" he's missing and all he does is stack the cores in
the cooler with no labels. In port, you discover this, and grind your teeth. How will you figure out
where you took the cores? You know that cores were taken:
Core 1) On the continental rise at a depth of 3500 meters.
Core 2) At two locations off the East Pacific Rise, one on relatively young crust at a depth of
3300 meters.
Core 3) The other on older crust, at a depth of 4800 meters.
Core 4) In the center of the North Pacific gyre, at a depth of 6000 meters.
Core 5) In the equatorial Pacific, just beneath the upwelling area.
Core 6) In the far northern waters of the Pacific, around 60 North.
You realize that all you have to do is to describe the sediments, and you'll know where the cores were
taken. Which of the above locations corresponds to each sediment type listed below?
a) Red clay
b) Foraminiferal ooze
c) Turbidite sequences
d) Foraminiferal ooze, topped by a few cm of red clay.
e) Siliceous ooze
f) Foraminiferal ooze with about 40% radiolarians
In one core, you notice a fascinating sequence of sediments. There are, from top to bottom:
1.0 meters of turbidite deposits
5.0 meters of diatomaceous ooze
1.0 meters of red clay
3.0 meters of foraminiferal ooze
In what general direction must the lithospheric plate underlying these cored sediments have moved,
and why do you think so?
Part 2 Water depth and sediment layer thickness determined using sound energy
A. Echo sounding: SONAR technology can be used to measure the depth of the oceans. A pulse
of high frequency sound energy is released by a device on a ship called a transponder. The
sound pulse travels through the water to the ocean floor, bounces off the ocean floor and travels
back to the ship. A device called a transceiver mounted on the hull of the ship receives the
return sound pulse. The length of time between the release of the sound and the reception of
the bounced returned sound is measured on the ship. If you know the speed of sound in water,
you can figure out the depth of the water.
a. The speed of sound in water varies with temperature and salinity. On average, the
speed of sound in seawater is 1500 m/s.
b. How deep is the sea floor if it takes 4.0 seconds for the pulse of sound to make the
round trip from transponder to the seafloor and back to the transiever?
Sea level
Sea floor
Distance = velocity X time
B. Seismic Stratigraphy Seismic energy has been used to study the layers of material that
accumulates on the sea floor, marine sediments. Seismic velocity, the speed of sound energy,
depends on the density of the material. For example, the speed of sound in air is about 350 m/s.
Above, we said that the average speed of sound in seawater is about 1500 m/s. The speed of
sound in rocks depends on the density of the rock. For example:
speed, in m/s
Clay
1000
Sandstone
2000
Basalt
3000
Granite
5000
Therefore, we can use the speed of sound as it travels through the various sea floor materials to
infer what the material is made of.
Not all the sound travels through a layer of rock continues on through to the material on the far
side of the layer. Any time there is an interface between two materials with different physical
properties, some of the sound will be reflected off the interface and will bounce back toward
the place it started from. The sound that returns to the surface is called a reflection or a return.
1. If we generate a sound at time 0.000 and detect a return at 0.003 seconds, what is the
seismic velocity of the rock layer?
transponder
transeiver
3 meters
2. What is the rock type?
3. In the real marine world, we rarely have only one layer of rock to consider. Marine
sediments usually include many layers of rock, all of which lie under a layer of water. The
study of the succession of rock layers in nature is called Stratigraphy. When we study
these layers using sound energy, this is called Seismic Stratigraphy. In Seismic
Stratigraphy, we make a noise just below the sea surface using an air piston or water piston
and measure the two way travel time it takes for the series of reflections to come back to the
transceiver. Each of the reflections is cause by an interface between two different rock
layers. The interfaces between rock layers are called reflectors. Each reflector is measured
in terms of two way travel time.
On the next page is a seismic section from the Central Pacific. Each vertical line wiggle
represents the pulse from one shot. The reflectors show up as nearly horizontal or
somewhat irregular darker lines on the profile. In the margin on the right, the rock units
defined by the reflectors are given a color name and the seismic velocity for each rock unit.
The narrow horizontal lines are 0.1 seconds apart. Using the data in the seismic profile,
answer the following questions:
1. How deep is the water at this location?
2. How long after the shot did the transceiver receive the reflection from the Red
reflector?
3. How thick is the layer between the Red and the Yellow reflectors?
4. What is the total thickness of the sediments? (Remember that each layer has a
different velocity. You must calculate the thickness of each layer individually
and then sum the thickness to find the total thickness.)